Aquatic Invertebrate Protein Sources for Long-Duration Space Travel

Lara Brown

, Jared Peick

, Melanie Pickett

, Tracy Fanara

Sandra Gilchrist

, Adrienne Smiley

, Luke Roberson

1. Smith College, Northampton MA

2. University of North Dakota, Grand Forks, ND

3. University of South Florida, Tampa, FL

4. Mote Marine Research Lab, Sarasota, FL

5. New College of Florida, Sarasota, FL

6. Fisk University, Nashville, TN

7. NASA, Kennedy Space Center, FL

Abstract

During the summer of 2020, NASA returned to launching astronauts to the International Space

Station (ISS) from American soil. By 2024, NASA’s mission is to return to the Moon, and by 2028

create a sustainable presence. Long duration missions come with obstacles, especially when trying to

create a sustainable environment in a location where “living off the land” is impossible. Some

resources on the Moon can be recovered or resupplied; however, many resources such as those needed

for sustaining life must be recycled or grown to support humans. To achieve sustainability, food and

water must be grown and recycled using elements found within the habitat. NASA’s current work

focuses on food resupply and growing plants as supplemental nutrient content. This paper examines

the possibility for using aquaculture systems to purify water while growing nutrient-rich species as

food sources, which aquatic food sources would be ideal for a habitat environment, and which species

might provide an ideal test case for future studies aboard ISS. The aquatic species should be rapidly

grown with high protein content and low launch mass requirements. Although there are numerous

challenges and unknown technology gaps for maintaining aquaculture systems in reduced gravity

environments, the benefit of employing such systems would be of great advantage towards creating

a sustainable presence beyond Earth’s orbit for sustainable aquaculture.

Introduction

As the Artemis Generation begins planning a sustainable habitat system for long duration visits to

the Moon and Mars

, challenges arise in converting the traditional NASA spaceflight architectures

from a survivable to a sustainable environment.

2-4

Skylab, Mir, and the International Space Station

(ISS) all proved that humans can survive within a Low Earth Orbit (LEO) environment.

In LEO,

rapid and frequent resupply missions are possible to replenish resources such as water, food, and

equipment to maintain crew survivability – their health and livelihood. However, frequent resupply

missions to the Moon and Mars is not practical given today’s launch technology.

This creates a

technology gap at NASA to address how water and food systems can be sustainably integrated to

overcome infrequent resupply.

NASA typically resupplies the ISS on a bimonthly schedule to provide food and water for human

consumption, nitrogen and oxygen for atmospheric resupply, as well as necessary equipment,

supplies, and science experiments. Resupplying the necessary amount of food and water isn’t

challenging in LEO. However, expansion of human exploration to the Moon in 2024 and sustaining

astronauts within a lunar base in 2028 will be much more difficult. Previous missions to the Moon

during the Apollo program relied upon the gear and supplies that traveled with them. Future Artemis

missions are planned to be resupplied using commercial resupply vessels through the Commercial

Lunar Payload Services (CLPS) contract.

7, 8

Reducing the amount of resupplied water and food would

save NASA a great deal of money while providing additional space for more habitation equipment

and science experiments.

9, 10

Therefore, creating a sustainable habitat on the Moon where water and

food could be recycled would have huge advantages towards the longevity of the habitat.

Without frequent resupply missions, NASA must build sustainable food production systems that

address the crew’s caloric and nutrition requirements. In the past, NASA prepared preserved food

choices to last for the entirety of a crew’s mission; however, due to shelf-life issues of some vital

nutrients, in situ production of food will likely be required for longer-duration missions to the Moon

and certainly Mars.

11, 12

NASA successfully designed and tested several plant growth systems aboard

the ISS to provide nutrients to the crew,

13, 14

yet plants grown within the size and volume restrictions

within a crew habitat can only produce so much edible biomass, essential nutrients, and, depending

on species, tend to lack the protein content needed to survive. It is likely that additional food sources

will be required to supplement key nutritional needs for long-term, sustainable crew survival. This

paper seeks to consolidate previous studies on aquatic invertebrate growth systems and to consider

the organisms’ potential to provide animal protein within confined, closed-loop systems for long-

duration spaceflight applications.

In situ Space Food Production: Overview of Demand, Status, and Potential

As crewed missions reach beyond LEO, mission planners must understand how nutrition

requirements can be met as distances between the crew and Earth become greater. To fulfill current

nutrition guidelines, the ISS receives terrestrial-based fresh food on orbit approximately every 90

days. This includes entrees, soups, salads, and desserts, as well as small quantities of fresh fruits,

vegetables, and breads.

Freeze dried, irradiated, retorted, and intermediate moisture foods provide

the vital calories and nutrients stated in NASA’s flight requirements.

It is not practical to resupply

and stow multiple years’ worth of dry food requirements (0.64 kg/individual-day) while constrained

by the payload mass and volume limitations of present-day spacecraft and launch vehicles.

Additionally, current technologies associated with food preparation, packaging, and storage are not

capable of maintaining all nutritional components of food for long duration missions exceeding 18

months.

Studies show that even with modern-day food systems, long-term storage of food leads to

considerable losses of Vitamin C, B, and A.

15, 16, 19

Long-duration crewed missions will experience

insufficient nutrient access if the ISS food system is carried beyond LEO.

To mitigate the issue of reduced availability of nutrients, new methods to generate food containing

self-sustaining calories and nutritional elements will be necessary. To date, in situ food production

has primarily focused on plant growth systems. NASA grows plants on the ISS through several plant

growth chambers including the Vegetable Production System (Veggie) and Advanced Plant Habitat

(APH), both of which are intended to reduce reliance on Earth-based foods.

20-22

These plant growth

systems grew peas, radishes, lettuce, sunflowers, zinnias, and others.

In 2015, the first crop of red

romaine lettuce was consumed from the Veggie system, initiating a new chapter for in situ food

generation in space that lessens the reliance on terrestrially-supplied resources.

Establishing

technologies to grow plants in microgravity has enabled further optimization research to improve

overall crop yields and increase nutrient availability for astronauts.

The nutrients stored in plants are important for maintaining human health, but current plant growth

capabilities aboard the ISS do not wholly address the dietary needs of astronauts.

Model diets for

astronauts that incorporate plant growth supplementation still require resupply of products with a

high protein content.

Even though plant-based protein sources exist, spaceflight nutritional

guidelines suggest 60-66% of total protein should derive from an animal source.

12, 18

Animal protein

provides access to all essential amino acids that are otherwise missing from most plant sources.

Consuming adequate protein is imperative for maintaining a balanced protein turnover rate,

sustaining energy levels, balancing nitrogen concentrations, and reducing the loss of body protein in

reduced gravity environments.

12, 25-29

For these purposes, protein should constitute 12-15% of total

energy consumed per person for long duration missions.

There remains a technology gap in growing

a sustainable source of protein for long duration spaceflight applications. Just as with plants, animals

have the potential to be grown as a source of nutrition. This will aid in the transition from a reliance

on terrestrial resupply to self-sustainable spaceflight operations.

Animal Protein

Many challenges exist in growing animals as a protein source for long-duration space applications.

Elevated levels of radiation and reduced gravity are environmental conditions inherent to

interplanetary transit and on planetary surfaces.

Animals, just as plants, evolved on Earth with the

effects of 1 G gravitational force. Developmental, biological, chemical, and structural functions and

processes all have the potential to be affected by changing gravitational forces.

In order to maintain

an organism’s health in a reduced gravity environment, engineers must develop habitat standards that

accommodate the organism in these unfamiliar gravitational conditions. From a technology

development side, engineers must address habitat volume and mass, resource balancing, and energy

sources required to achieve homeostatic equilibria.

Just as physiochemical systems fail, biological

systems may face inadvertent death and population level failures. Designing a system that can quickly

return to equilibrium after significant biological failures will be important in maintaining the animal

population.

Identifying organisms that would be suited for growth in a confined, closed-loop system while also

providing an adequate protein source presents an additional challenge. Certain phyla are inherently

better suited for spaceflight environments than others due to their adaptability and their natural

conditions for growth. Biological factors to consider include disease susceptibility, reproduction and

growth rates, life cycle stages, and the predictability of metabolic activity.

17, 30

An animal must also

survive within a limited volume, must not require high energy input per calorie of food produced,

must not require substantial crew time to maintain, and must be palatable for human consumption.

These factors are all identified in Table 1. Closing the loop of a biological system for protein

production while balancing the variables previously mentioned remains a challenge.

Table 1. Relevant factors for the selection of animal protein sources for long duration spaceflight.

Organismal Factors

Crew Considerations

Maintenance Factors

Minimal disease susceptibility

Minimal crew time required

Minimal habitat volume

Advanced understanding of life

cycle stages

Palatability

Predictable metabolic rates

Easily controlled reproduction

Minimal caloric intake per

unit of biomass produced

Maximal growth rate

Easily treatable waste

Adaptation to reduced gravity

Animal protein production on Earth can be broadly categorized into red meats, poultry, and seafood.

Red meats currently account for 36% of calories available in the American food supply, whereas

poultry and seafood account for 3.5% and 0.9% respectively.

Insects are eaten globally as a protein

source; however, this is not common practice in the United States. Despite the dominance of red

meats in the supply chain, these terrestrial options pose great challenges when considering the

development of animal protein in enclosed partial gravity environments. These terrestrial organisms

require prohibitive amounts of land area

, will likely have a more challenging adjustment to

microgravity,

and will also produce solid, liquid, and gaseous waste streams that will need to be

managed with additional life support infrastructure. Protein from insect sources, though not

customary in the United States, are consumed regularly in some areas of the world and may be well

suited for space cultivation due to high yields. Aquatic organisms have the potential to serve as more

well-adapted organisms for food production in micro- or partial- gravity; as compared to terrestrial

organisms, they are less dependent on gravity for normal orientation and movement on Earth.

32, 33

Insect Protein

Protein from insects is another viable option for future space missions.

With high population

densities, insects could provide substantial protein in a small volume while feeding on spoiled food

or inedible plant biomass.

Protein content varies from 20-76% of dry matter depending on the type

and development stage of the insect.

Insects also are high in omega-3 fatty acids, calcium, iron,

selenium, zinc and Vitamin B.

Additionally, certain insects were found to have antimicrobial,

antioxidant, and antihypertensive qualities in the protein peptides.

Though many cultures regularly

consume insect protein (entomophagy) using over 2100 different insect species globally

38, 39

, it is not

common in most developed nations. However, based on a United Nations (FAO) report, Edible

Insects: Future Prospects for Food and Food Security, several commercial ventures were launched

in Europe.

Thailand has been a leader in insect farming for many years and their culturing

techniques may be easily adaptable for use on future space missions. Tong et. al. assessed the

nutritional content and respiration rates of silkworm larvae and found variation depending on food

source and composition. When ground into silkworm powder, eliminating any texture issues with

human consumption, 359 kcal/100g of powder was achieved.

Another likely candidate is mealworm

larvae. Stoops and colleagues (2017) showed that a minced meat-like product can be produced to

mimic other types of minced meats.

Facilities for rearing insects could be integrated into a terrarium

system using design parameters currently available through the insect rearing for pet industry and

human consumption models. If insect farming was established, oils could be extracted from larvae

and used in cooking as a healthy substitute for other types of oils.

Further research and testing would

be needed to confirm space applicability, but recent studies prove promising.

Aquatic Protein

Similar to insect cultivation, aquaculture provides another mechanism for cultivating small, high-

level organisms in dense systems to produce stable protein sources. Prior investigations into

aquaculture for space applications identified fish as an efficient source of animal protein production,

citing their reduced energy requirements and ability to adjust to altered gravity scenarios.

As an

additional bonus, maintaining aquaria has also been shown to alleviate psychological stress and

reduce heartrate, providing potential emotional benefits to crew members.

However, bony fish

struggle to properly inflate their swim bladders (an organ critical to regulating buoyancy) in

microgravity.

Fish grown in microgravity hence require the addition of air chambers in their

aquaculture system to enable them to properly maintain neutral buoyancy.

Despite this challenge,

fish still serve as potential candidates for space-based protein production. Fish are frequently raised

in high-density aquaculture systems on Earth.

While many species currently grown for food

production, such as salmonids, are carnivorous,

tilapia and other herbivorous fish can be integrated

with hydroponic plant production to consume plant biomass that is inedible for humans. This provides

an efficient mechanism of elemental conservation in a closed aquaponics food web.

43, 46

Aquatic invertebrates, particularly organisms of the phylum Mollusca, are expected to easily adjust

to micro- or partial-gravity, since some species consumed by humans are sedentary after their larval

stage and do not require the ability to move throughout the water column to seek out food.

Many

mollusks are filter feeders, assimilating nutrients from the flow of surrounding water to build

biomass.

Some mollusk species, such as mussels, can be grown in brackish water in integrated

systems with phytoplankton and crustaceans. These mussels are already under commercial

cultivation, valued for their high nutritional content including proteins, vitamins (A, B1, B2, B6, B12,

and C), and PUFAs

. Organisms of the order Decapoda, including certain shrimp species, possess

the ability to filter feed or scavenge particles of various sizes (including bacteria) and are commonly

grown for protein production on Earth.

49, 50

Studies demonstrated that shrimp production under

aquaponic conditions were possible and can even exert a stabilizing effect on a closed-loop system.

Organisms like shrimp could be consumed whole or can be used as a nutritious condiment (shrimp

paste).

Such species produced more concentrated waste streams consisting primarily of soluble

ammonia and minimal solid waste,

reducing the concern of direct biogas excretion associated with

terrestrial mammals.

Research on terrestrial systems for both phyla investigated the potential to incorporate invertebrates

into water treatment processes combining resource recovery with food production.

54, 55

This potential

integration of life support processes makes aquatic invertebrates particularly compelling subjects for

future research regarding animal protein production for space habitats. However, few studies have

examined the potential for mollusk or decapod production of small-scale, closed-loop, recirculating

systems that will be necessary for such applications.

56, 57

Understanding Invertebrate Protein Production

Role in the Natural Environment

In their natural ecosystem on Earth, filter-feeding organisms are often keystone species with a major

influence on the dynamics of aquatic ecosystems.

Aquatic filter feeders and scavengers are part of

a complex cycle and balance of nutrients, energy, and other organisms. This dynamic role is important

to consider when designing a closed-loop habitat for the culturing of such organisms. As with all

other organisms, filter feeders require energy inputs and generate various outputs. They remove

inorganic material, carbon, phosphorus, and nitrogen from the water column, transporting nutrients

and inorganics to the sediment.

Kellogg

described the nitrogen cycling pathway by beginning with

atmospheric nitrogen that dissolves into a water column as inorganic nitrogen. Algae in the water

column consume nitrogen as a nutrient for growth. Invertebrates feed on algae in the water column,

assimilating organic matter and nitrogen from the algae into tissue and shell material. Organic matter

and nitrogen that go unused by the organisms are excreted as feces. Nitrogen is expelled in the form

of ammonia which becomes available to nitrifying bacteria in aerobic sediments. Nitrifying bacteria

convert ammonia to nitrate, which is then available to other life forms, such as plants. Some ammonia

is buried long term in anaerobic sediments where denitrifying bacteria convert the ammonia to nitrous

oxide and nitrogen gas which returns back to the atmosphere.

This cycling is represented in Figure

1. In a closed aquaponic system, it is important to understand that nitrogen transformations are

affected by many different factors that should be well-understood and optimized for the particular

system that will be employed.

A robust system balancing nutrients, energy, and organisms for

sustainability at a micro scale will be required.

While mollusks are highly efficient at filtering the water column and consolidating particles and

nutrients, they only facilitate nitrogen transformation rather than acting as a removal system.

53, 59, 60

The same is true for filter feeding and scavenging decapods. The greater food web that mollusks and

other filter feeders are within is responsible for nitrogen removal.

These removal systems can be

replicated by a balanced, multi-species ecosystem providing nutrient bioremediation and mutual

benefits to the co-cultured organisms.

Incorporating specific plant and bacterial species to close the

nitrogen cycle within a closed system will help facilitate this process. Many aquatic plant species

(such as the giant duckweed Spirodela oligorrhiza) prefer uptake of ammonium species as a nitrogen

source rather than nitrates.

63, 64

In addition to aquatic plants’ ability to play a major role in water

purification, nitrification can also occur via biofilters, which encourage nitrifying bacteria,

Nitrosomonas and Nitrobacter, to aerobically convert toxic ammonium species first to nitrite and

then to nontoxic nitrate.

To this point, nitrogen can be removed from the boundaries of a closed-

loop system through long-term burial, denitrification, or physical transport.

66, 67

Harvesting filter

feeding organisms out of a system is an effective method of physical removal of nutrients because

the shell and tissue that contain nitrogen are removed.

In designing a system to harbor mollusks

and/or decapods, one must consider how to include or mimic the various biological components that

are imperative to the balance and flow of nutrients.

Figure 1. Nitrogen Cycling within an aquaculture system. Dashed lines represent N forms that leave the

water column; Commodities outlined in white are directly useable by crew, those outlined in blue provide

support roles.

Habitat Maintenance

When considering an aquatic species for space travel, down-selection criterion is necessary for

comparing system resource requirements to sustain the organisms. Table 2 outlines basic water

quality requirements for aquarium settings to maintain a variety of common invertebrate species.

Table 3 outlines the resulting outputs for those organisms.

The primary requirement to sustain these different organisms is their shared ability to feed on various

types of particulate (algae, phytoplankton, plant) matter.

68-76

Freshwater species tend to have a

narrower range in temperature and salinity tolerances when compared to their saltwater

counterparts.

69, 70, 73, 74, 77-81

Shrimp often consume their shed skeleton to recover calcium; adult shrimp

such as Neocaridina and Halocaridina only molt to replace lost limbs, Macrobrachium will continue

to molt throughout the lifecycle but slows dramatically with age. Conversely, shellfish leave behind

a shell requiring some processing before repurposing such significant mass. The drastic differences

in population densities for each organism is noteworthy. The tank size requirement is the volume

recommended by literature for an individual species to thrive, whereas the density requirement of the

species is the number of individuals that may reside in a given volume. These two numbers, although

they may intuitively seem related, are different and species specific. Depending on species, water

quality sensitivity requirements may play a role on the tank volume required. A larger tank size

assumes increased stability in water quality. Tank size required will increase due to sensitivity of an

organism, independent of how many organisms are present. Therefore, the tank volume may differ

from the volume calculated by recommended species density.

Table 2. Comparison of basic aquarium input requirements for each organism.

Organism

Fresh, Salt,

Brackish

Tank

Size

(gal/#)

Temp

(°C)

Salinity

(ppt)

Diet

Bulinus

australianus

68, 77,

82, 83

Freshwater

0.05

10-35

Algae, plant matter,

detritus

Crassostrea

gigas

Saltwater

4-35

10-42

Phytoplankton,

bacteria, protozoa,

detritus (org & inorg)

Crassostrea

virginica

Saltwater

20-30

5-40

Phytoplankton,

microalgae

Mercenaria

mercenaria

70, 71,

Saltwater

5-35

4-35

Algae, POC

Hyriopsis

(Limnoscapha)

myersiana

72, 79

Freshwater

2-10

10-16

N.D.

Phytoplankton, org

particulate

Unionidae

73, 79, 80

Freshwater

3-10

10-16

<6-12

Detritus (org & inorg),

bacteria phytoplankton

Mytilus edulis

74,

Saltwater

N.D.

5-20

20-35

>15

Phytoplankton, algae,

detritus

Macrobrachium

rosenbergii

Fresh/Brackish

14-35

0-25

Algae, aquatic plants,

insects, mollusks

Neocaridina

heteropoda

75, 86

Fresh

2-5

18-

29.5

N.D.

Algae, plant material

Halocaridina

rubra

76, 87

Brackish

N.D.

2-36

Plant material, aquatic

insects

*Note: N.D. = No Data; 1 = Snail, 2 = Oyster, 3 = Clam, 4 = Mussel, 5 = Shrimp;

salinity is species-

dependent;

for larvae;

for adults.

Beyond the general cultivation parameters outlined in Tables 2 and 3, filter feeder species have

differing metabolic demands, requiring fine-tuning of species inclusion and water quality

maintenance to maintain homeostasis in a particular environment. Still, general rules apply with

regard to the needs and environments of different species. As illustrated in Table 4, studies on habitat

requirements and metabolic parameters for aquatic invertebrates in small-scale systems are limited

in number, and few have standardized methodologies or reporting units. This table offers a baseline

of water quality parameters for a range of aquatic organisms, providing species selection for future

studies based on anticipated water conditions. Notably, food consumption is relatively consistent

across all organisms considered.

55, 88, 89

Table 3. Comparison of basic aquarium outputs for each organism.

Organism

Typical Density

(#/m

)

Protein

Byproducts

Bulinus australianus

68, 77,

82, 83

Seasonal variation

800-1300;5000 max

16.5g/100g

16-18% weight

Feces, pseudofeces,

shell

Crassostrea gigas

78, 90, 91

1000-2000

39.1-53.1 as %

dry weight

Feces, pseudofeces,

shell

Crassostrea virginica

50-131

7.1g/100g of

wet body tissue

Feces, pseudofeces,

shell

Mercenaria mercenaria

59,

70, 71, 84

322-3,875

9-26

0.73 g (dry)

Feces, pseudofeces,

shell

Hyriopsis (Limnoscapha)

myersiana

72, 79

26-80

N.D.

Mucous, pearl

material, feces, shell

Unionidae

73, 79, 80, 93

32-36

N.D.

Pearl

, feces, shell

Mytilus edulis

74, 81

N.D.

1.4-6.5g

Shell

Macrobrachium

rosenbergii

1,000 post larvae

200 small juveniles

17.6g/100g of

wet body tissue

Feces, shed

exoskeleton

Neocaridina heteropoda

75,

86, 94

N.D.

Shed exoskeleton

Halocaridina rubra

76, 87

100’s

N.D.

Shed exoskeleton

*Note: N.D. = No Data, 1 = Snail, 2 = Oyster, 3 = Clam, 4 = Mussel, 5 = Shrimp;

some species

Heavy metal toxicity levels within the water source are a relevant factor to consider when designing

an aquatic invertebrate production system. Heavy metal tolerances are less frequently reported for

adult organisms but are an important consideration when designing a closed-loop life support system

for growing aquatic invertebrates. Copper (Cu), for example, is increasingly used in health care

settings as an antimicrobial coating

; similar applications might exist as part of environmental

control and life support systems (ECLSS) aboard a spacecraft. Although at low concentrations copper

is essential for human health (~900 µg/day

), the tolerable upper intake levels are around 10,000

µg/day

and chronic exposure to high levels of copper can result in liver damage and gastrointestinal

symptoms.

Metals (cadmium (Cd), zinc (Zn), lead (Pb), and Cu) can be found in terrestrial tap

water or wastewater due to the leaching (or corrosion) of Cu, Pb, or galvanized steel plumbing

systems. Mercury (Hg) is likely not a concern for space applications, since most sources of Hg

originate from terrestrial system processes such as combustion of coal, incineration of waste, and

mining.

The design issue is compounded with the concern over heavy metal bioaccumulation in

filter feeders’ tissues

that could end up consumed by human crew members. Risk is minimized

substantially when preventing corrosion through maintaining piping and fittings

100

and avoiding the

use of lead-brass taps and fittings.

101

As illustrated in Table 3, saltwater oysters exhibit a higher

copper tolerance than freshwater mussels. Larval organisms typically are more sensitive to heavy

metals than adults and caution should be taken to prevent lethal and sublethal accumulaton.

102

Differences in heavy metal tolerances across saltwater and freshwater species will need to be studied

further considering level of risk associated with a proposed design.

Dissolved oxygen (DO) is another necessary parameter to track in an invertebrate aquarium, as

mollusks and decapods require oxygen to carry out cellular respiration. While reported DO

requirements vary within the same family of organisms, DO levels are of lesser concern than the

other listed parameters, because most bivalves can tolerate anoxic conditions for up to several weeks

by closing their shells.

103

Notably, oyster ammonia tolerance is significantly greater than that of clams

or freshwater snails, while ammonia generation remains within a restricted range across all species.

Crustaceans, especially larval forms, experience stress and death from low oxygen in combination

with other water conditions. Gastropods may be able to survive periods of hypoxia depending on

their developmental stage, but are often stressed by this condition.

104

Ammonia generation

consistently increases with increased water temperature

105

, and the increased accumulation of

ammonia can cause cellular stress

, disrupt filtration

106

, and inhibit growth

107

for all organisms.

However, nitrifying bacteria in water systems act to convert ammonia to nitrite and then to nitrate,

and these bacteria can inhabit oyster shells and internal tissues

108

, reducing the accumulation of toxic

ammonia in the invertebrates’ habitat. Understanding ammonia production and tolerance of a specific

species is critical to establishing a balanced closed-loop system. In addition to temperature, pH plays

a role in nitrogen formation. Ammonia-nitrogen (NH

-N) has a more toxic form at high pH and a less

toxic form at low pH, un-ionized ammonia (NH

) and ionized ammonia (NH

), respectively.

109

Factors for Integrating Aquaculture into Long-term Space Missions

Water quality parameters are critical to maintaining aquariums on Earth and during spaceflight. The

introduction of aquatic organisms beyond LEO brings an additional set of long-term challenges that

might hinder system self-sustainability. A complete aquaculture system integrated with life support

architecture for a long duration space flight mission would include crewmembers as part of the

biosphere; their caloric intakes and waste generation would be calculated in the overall system mass

balance. At this point, a challenge arises in having crew acting as a component of the system while

attempting to manage it from within.

110

The capacity to understand and manage complex engineered

ecosystems while simultaneously existing within those systems will need to be defined.

Challenges for Future Closed-loop System Design

Designers must consider the biological hurdles faced when creating a closed-loop aquaculture

system. These hurdles will likely include the natural shift in nutrients caused by the organisms. A

prime example occurs when the death of an invertebrate results in a disturbance of water quality,

including spikes in ammonia levels, which can be especially toxic to the remaining organisms in a

closed system. As shellfish filter water, they absorb nitrates into their flesh and shells. Upon death a

portion of those stored nutrients get released back into the water in the form of ammonia. This spike

in ammonia was shown to increase short term aquarium mortality rates.

In general, precautions must

be taken to mitigate any disturbances in water quality, whether this be through daily health monitoring

or by implementing a post-casualty organism removal and water treatment procedure. While there is

research being performed regarding ammonia production and accumulation rates for invertebrate

species,

99, 105-107

additional research needs to be conducted to determine the influence of individual

mortality on local tank inhabitants.

Table 4. Advanced water quality factors for closed-loop aquaculture systems.

Organism

Food

Consumption

(µg C/L/g C of

organism)

Heavy Metal

Tolerance

(ppb)

Reqs

(ppm)

Ammonia

Tolerance

(ppm)

Ammonia

Generation

(µmol/g dry

biomass/hr)

1- Snail,

freshwater

111

N.D.

0.74

N.D.

2- Oyster

59, 60,

88, 89, 105, 112-114

4.0

300*10

cells/day/

oyster

2:1 dried: wet

biomass

Cu: 560

2.4-4.8

19.1

0.4-0.8

1.1e

-6

µmol/ g C

biomass

3- Clam

55, 70

3.0

N.D.

4.2

N.D.

4- Mussel,

saltwater

55, 115

5.0

N.D.

0.5

>4.2

N.D.

0.9

4- Mussel,

freshwater

80,

103, 116-118

N.D.

Cu: 25; Hg:

5; Cd: 90;

Zn: 3000

1.0-7.8

2.6-8.9

117,

118

N.D.

5- Shrimp

119-121

N.D.

Cu: 452

(96 hr

LC50),

juveniles;

Cu: 32; Zn:

525; Cd: 7

Pb: 35

N.D.

36.6

N.D.

*Note: LC50 is the concentration that lethal for 50% of a population;

minimum,

optimal

Providing a Reliable Food Source

Invertebrate feeding regimens will need to be carefully regulated. Invertebrates species can filter

through 5-25 liters of water and detritus per gram of biomass in an hour

and will require an

autonomous, regimented feeding program if they are to feasibly be raised aboard spacecraft. A strict

feeding schedule must be established when cultivating higher order organisms to maintain a balance

between overfeeding and undernourishment. The selection of an appropriate food source is another

potential challenge. Invertebrates may selectively feed only on species of algae and phytoplankton

native to their natural environment,

for example Chlorella sp. and Isochrysis sp. are common

species for freshwater and marine, respectively. However, Spirulina (Arthrospira sp.) and yeast may

be added periodically as universal supplements for many invertebrates, potentially boosting vitamins,

minerals, protein content, growth, and immune responses.

121, 122

The overall immediate challenge to

overcome will be filling in the gap in data relating to small-scale aquaculture food supplies for

invertebrates. If the food source for the aquaculture system must be resupplied from Earth, the

justification for mass-volume would not constitute having a fresh protein source. However, if the

food source can be created in situ, then a mass-volume balance could be achieved.

Social Concerns

Another concern arises from the understudied effects of having invertebrates in a closed environment

with simulated biological symbiosis. There are many micro-interactions that occur between all living

organisms that cannot be truly replaced with current technologies, complicating attempts to replicate

small-scale ecosystems. Fish grown in the Closed Equilibrated Biological Aquatic System (CEBAS)

mini-module demonstrated signs of social stress when grown in a 10 L tank, with Xiphophorus helleri

fish killing each other in close quarters.

The Unionidae mussel larvae typically spend 6-160 days

attached to a fish host until they reach the juvenile stage.

123

This species may still be considered

depending on duration of flight. Depending on species, Unionidae may live 10 to over 50 years.

124-

130

Not all species of invertebrates, or mussels, require hosts for reproduction; in a space environment it

would be preferred to select species not directly dependent on other species. For Crassostrea gigas

oysters, their sex will change depending on the phytoplankton food supply. When food is plentiful

males will change sex to become female. When food is scarce, the inverse will occur.

These

examples illustrate the nuances in organismal and ecosystem-level health. Numerous experiments

need to be conducted to determine the possible effects of social stress and absentee organisms on

animal well-being. The viability of each species for long-term spaceflight will largely depend on their

abilities to survive in the presence of synthetic hosts and environmental changes.

Genetic Diversity Concerns

Genetic diversity and generational continuity remain significant challenges to overcome regarding

long-term, multi-generational biospheres. In an enclosed aquaculture system, inbreeding will

certainly occur over time, reducing species’ genetic diversity. Lazaridou-Dimitriadou and colleagues

found that the terrestrial snail Helix aspersa demonstrated decreased size in subsequent generations

when cultured in a closed-loop system, resulting in a cessation of reproduction in the seventh

generation.

131

Conversely, Noland and Carriker found that the freshwater snail Lymnaea stagnalis

appressa demonstrated no apparent loss of fitness after twenty generations of culture.

132

Many species

of invertebrates are hermaphrodites and inbreeding is natural, though there could be a loss of fitness

over time without intervention; these species may be preferential to avoid genetic issues long-term

but are more difficult to control with respect to reproduction.

133

Cryopreservation of sperm was

successful for shrimp and may prove useful for maintaining diversity over time.

134, 135

New genetic

engineering technologies such as CRSPR may allow modification of organisms to ameliorate the

impacts of inbreeding.

136

The exact effects of closed, extended invertebrate culture on reproductive

fitness will require further study. Though this may present a long-term challenge, the issue of genetic

diversity can be solved in near-future scenarios (i.e., an early lunar base) via the introduction of new

organisms brought to the habitat from Earth on an infrequent schedule. For setups in which semi-

regular resupplies are unavailable, a tissue culture library or sperm bank can be maintained within

the habitat to introduce genetic diversity to the aquaculture system as needed, enabling enhanced

reproductive fitness over longer timescales. However, introducing materials into an already closed

system will throw off the mass balance already established, perhaps hindering long-term stability.

110

Replicating a Dynamic Ecosystem

More technical challenges arise with the attempt to replicate a multi-trophic ecosystem in a closed-

loop manner, specifically regarding system stability. Mass-conservative aquatic ecosystems increase

resistance to population die off as the complexity of the food web implicated in the ecosystem

increases.

137

This increase in resistance strongly correlates with overall ecosystem stability. While

increasing a closed system’s complexity necessitates increased maintenance upon startup and

complicates mass and energy balances, the resulting dynamic biosphere provides long-term stability

and protection against oscillations in system parameters. Conversely, ecosystem resilience, i.e., the

ability of the system to return to equilibrium after a significant perturbation, is negatively correlated

with food web complexity.

137

The challenge in establishing a multi-trophic aquaculture system for

space applications evolves into a challenge of ensuring that the system is not disturbed past its

resistance point, as recovery past this point would likely be very difficult. Precise control of

environmental conditions within the aquaculture system will be necessary to ensure that the complex

biosphere maintains its stability. This is further complicated by the need for contingency plans that

rely on breaking open the closed-loop system, such as re-supply missions or in situ resource

utilization

110

, but that ultimately enables the system to be closed and restored to equilibrium after

disruption.

Potential for Improved ECLSS Efficiency with Space Aquaculture

Despite the inefficiencies inherent to growing higher-level organisms, aquatic invertebrates can be

integrated with other ECLSS components to act as a nutrient sink while producing a palatable protein

source for a more diverse diet. An organism’s ecological efficiency, calculated as the percentage of

its production of biomass relative to the quantity of food it consumes, generally ranges from 6-25

percent.

Herbivores, including filter-feeding invertebrates, are particularly inefficient at feeding;

gross food chain efficiency, a measure of net biomass production relative to food supplied, remains

significantly lower than ecological efficiency for such organisms.

As such, there is an inherent

energy loss between trophic levels when growing animal protein sources. However, this loss is not

entirely prohibitive. If this inefficiency is planned for, the invertebrate growth aquarium can be

integrated with other life support systems to maximize overall resource usage. For example, these

aquatic organisms could be placed in line with partially treated wastewater to act as a nutrient

dampener, or they can be grown in saltwater created with excess salt removed from waste streams.

Additionally, they can consume excess oxygen generated by plants. In this way the overall efficiency

of life support systems can increase with the addition of aquatic protein sources despite the reduction

in ecological efficiency presented by filter feeding herbivores.

Future Study Needs for System Design

To gain a better understanding for growing invertebrate species in space, several parameters and

requirements need to be better established. This can be accomplished via scientific experiments that

monitor the growth, inputs, and outputs of various species in a small-scale system. The input

measurements should consist of food and nutrient uptake, while the output should consist of waste

generation and nutrient output. Depending on the amount and type of waste produced, an additional

waste treatment system may need to be implemented. Water quality parameters will need to be

established and monitored. Without the presence of a natural symbiotic ecosystem, key nutrients will

need to be constantly monitored to determine their indispensability to invertebrate longevity and

growth. A study should be executed to compare precise filtration rates (g/m

*hr) of choice

invertebrates. The supply and cost of consumables required to maintain the system should be

calculated. This list would include variables such as electricity usage (g/kW*hr), cost of water

transportation aboard a spacecraft, cost and volume of food/nutrient supplements required, and

energy required to dispose of invertebrate byproducts (e.g., feces, pseudofeces, empty shells, dead

organisms). Lastly, more in-depth analyses should be conducted regarding the potential to sustain

invertebrate life using nutrients commonly found in wastewater streams. This will become

particularly useful when a closed-loop system is required for astronaut survival.

Conclusion

Aquatic invertebrates are a potential protein source for long duration spaceflight, despite the current

gap in literature and technology regarding small-scale, recirculating aquaculture systems. Raising

animal species in situ for human consumption will provide access to certain essential nutrients that

are otherwise absent from current spaceflight technology. While challenges remain in addressing

animal growth requirements and integrating the necessary technology into spaceflight hardware, the

requirement for animal-based protein drives the need to generate sustainable solutions. In situ animal

protein production systems will alleviate the crew’s reliance on Earth-based resources and enable a

transition from survivable to sustainable life support systems. Long-term space exploration needs to

advance the development of protein sources and in situ animal protein-generating is an area needing

additional investigation. The basic and applied science along with associated technologies needs

focused attention now, so systems can support the Artemis Generation achieving a sustained presence

on the lunar and Mars surface.

Acknowledgement

The authors would like to thank the NASA Human Exploration and Operations Program, the NASA

Office of Education, and the NASA Minority Undergraduate Research Education Program (MUREP)

for the funding of this summer project.

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