Nutrient resorption in algae and mussel cultures

by Thomas Jensen

Positive environmental impacts of farming extractive species

This article was featured in Eurofish Magazine 2 2024.

Aquaculture makes a significant contribution to the global food supply. As the demand for healthy protein increases, its importance is likely to continue to grow. Unfortunately, this also exacerbates some environmental problems, as more intensive aquaculture introduces additional nutrients into aquatic ecosystems. Algae and mussel cultures, which extract many nutrients from water, offer a solution.

Global aquaculture produces more than fish, shellfish and algae for direct human consumption. Numerous products are also used as raw materials for animal feed, fuel, cosmetics, nutraceuticals and pharmaceuticals, as well as other products ranging from enzymes to fish leather. Conventional aquaculture also produces various ‘wastes’ that can contaminate adjacent aquatic ecosystems. These are mainly uneaten feed and fish excrement, which are still rich in nutrients, especially nitrogen and phosphorus compounds. Compared to the amount of agricultural nutrients that enter rivers from arable land and are washed out into the sea, the amount of nutrients from aquaculture is much lower, but still contributes to pollution or even eutrophication of coastal waters. According to conservative estimates, nutrient inputs from Chinese aquaculture alone amounted to nearly 99,100 tonnes of total nitrogen and 16,100 tonnes of total phosphorus in 2017.

The aquaculture industry is paying increasing attention on this issue as it seeks to become more sustainable. Environmental laws and regulations are being tightened almost everywhere due to growing global concern about the potential environmental impacts of aquaculture activities. Major research projects are looking at ways to protect coastal ecosystems and the marine environment more effectively. At present, we know relatively little about what technologies can be used to reduce or, even better, prevent the environmental damage caused by open aquaculture. However, there are some possible interventions that could potentially help curb nutrient emissions. Dissolved inorganic nutrients produced during metabolism and excreted by fish through the gills, kidneys or intestines (NH4 and PO4) can hardly be removed by technical means. While this is partly possible with particulate organic nutrients in faeces and unused feed, it would be very time-consuming and correspondingly expensive. Reduction in nutrient inputs can be achieved much more cost-effectively through optimised feed management Although this does not significantly reduce environmental pollution, it should still be pursued. Especially as it also benefits the fish farmers themselves – every gram of feed not eaten is an economic loss for the business.

Nutrient inputs are ­usually greater than nutrient removals

Reducing nutrient inputs into water lessens some problems, but does not solve them. Perhaps nature can serve as an example because it uses two strategies at once. One is the dilution effect caused by hydrodynamic turbulence in the water. Wind, waves and currents tend to disperse the nutrients quickly in the surrounding area. Although this does not eliminate them, their negative effects are less pronounced because the concentration has been reduced. In comparison, the second strategy is much more effective as it relies on the introduction of nutrients into the marine food chains. The solid particles of nutrients sediment are deposited on the bottom, where they are usually eaten by benthic animals. This works relatively well as long as the amount of sediment does not exceed the uptake capacity of bottom-dwelling organisms. The dissolved nutrients also quickly find consumers in nature. These are primarily algae, which can be roughly categorised into microalgae and macroalgae according to their size. While microalgae, which mostly float freely in the water like phytoplankton, are tiny and can only be detected under a microscope, macroalgae are much larger. They grow on the sea floor and solid structures down to the water depths where light is able to penetrate. What both groups of algae have in common is the ability to photosynthesise and convert carbon into energy-rich compounds. This ability puts micro- and macroalgae at the very beginning of aquatic food chains. They are the ‘primary producers’ on which almost all aquatic life relies, from zooplankton to apex predators. However, algae also need nutrients from aquaculture for photosynthesis. This realisation closes the circle: algae ­cultures can help extract some nutrients from the water and use them for their own development. They act as natural ‘treatment plants’ that reduce the nutrients in the water, thereby purifying it.

Due to their purification function, algae play a vital role in aquatic ecosystems. But it is not that simple, because algae, especially microscopic microalgae, can be both a curse and a blessing. On the one hand, as primary producers, they are the foundation of aquatic life and even offer us the opportunity to reduce environmental problems due to their ‘nutrient hunger’. On the other hand, they can themselves become a danger if masses of algae, known as algal blooms, develop in overfertilised and therefore extremely nutrient-rich (eutrophic) marine areas. Dense carpets of algae then often float on the surface, depriving plants at the bottom of light for photosynthesis and threatening the lives of many bottom-dwelling animals. Particularly dangerous are blooms of toxic algae, which produce poisonous substances and can sicken or even kill many aquatic animals, including fish. It is just as dangerous when the microalgae die at some point and the bloom collapses and sinks to the sea floor. The putrefactive bacteria then immediately set to work on the nutrient-rich ‘algae graveyard’. In their decomposing activity, they draw so much oxygen out of the water that anoxic, oxygen-free zones can form near the bottom.

The potential of algae is far from exhausted

Almost 40 species of algae, mostly macroalgae, are cultivated in large quantities in aquaculture worldwide. Almost all of them are used directly or indirectly for human consumption. For example, edible algae such as the Porphyra species (Nori), which we know as the wrapping for sushi rolls, are used directly. Indirect use means that valuable ingredients such as agar-agar or carrageenan are extracted from the algae and used for many products in the food industry and other applications.
Unlike macroalgae cultures, microalgae production has only recently attracted greater interest. This trend is driven by the demand for the valuable ingredients in microalgae, containing protein, carbohydrates and fats, micronutrients and bioactive or functional substances. These include the essential omega-3 fatty acids EPA and DHA, as well as carotenoids, which are essential for animal and human nutrition. Algae, algae extracts and algae ingredients are used as feed additives in animal nutrition. They are said to strengthen the immune system and improve resistance to disease, increase growth performance and have antioxidant and anti-inflammatory effects. Almost all of the projections also assume that microalgae will be able to replace fishmeal and fish oil in aquaculture feeds in the future. Microalgae in suspension are already used extensively as a supplement to ‘spice up’ the poor nutritional value of Artemia nauplii, which hatcheries often use to feed fish and shrimp larvae. This method, known as ‘enrichment’, provides the larvae with an extra portion of vitamins, fatty acids and trace elements, which ensures a significantly higher survival rate.

Utilising ecological ‘­services’ in a more targeted way


Although algal cultures, with their nutrient resorption, make a significant contribution to water purification and the health of aquatic ecosystems, this free ecosystem service has hardly been utilised until now. This environmentally friendly option lends itself to use, as the cultivation of algae requires relatively little effort and is comparatively inexpensive. Moreover, the beneficial effects of algae go far beyond nutrient resorption, as they also produce high levels of oxygen, stabilise pH levels, inhibit harmful bacteria, act as natural biofilters and serve as cover and food for fish. These benefits are already being used in a targeted manner in the rearing of fish and shrimp larvae in green water, which contains high concentrations of microalgae. Compared to almost clinically pure clear water, rearing success under these conditions is significantly higher.

While the potential of algae as an ‘environmental service provider’ has been recognised, it has not yet been fully exploited. However, work is already underway to develop suitable technologies to utilise microalgae and macroalgae for wastewater treatment. The natural method used by algae to remove nutrients from water is nothing short of ingenious: nitrogen compounds and phosphates are converted into biomass, which is relatively easy to remove from the system and can often even be economically used. It could hardly be more sustainable. In addition, algae can be grown at sea or in space-saving bioreactors, so it does not compete with the very limited arable land available.

However, it is not only algae that have nutrient-absorbing and water-purifying abilities, but also other species groups. These include filter-feeding fish and numerous molluscs such as oysters and other Bivalvia. While algae only absorb dissolved inorganic nutrients, mussels and filter-feeding fish use their gill filters to sieve nutrient-rich organic particles out of the water. Mostly plankton, but also detritus. And because they do this non-selectively, rather than by food types, they are also known as ‘suspension feeders’. With their filter-feeding behaviour, farmed suspension feeders significantly reduce the feed balance in aquaculture, as they grow without supplementary feed. Although the relative share of non-fed species in global aquaculture production has fallen from more than 40% before 2000 to 27.8% in 2020, this is only a mathematical effect, because while the absolute production has remained at almost the same level, global aquaculture production has more than tripled. In 2020, 24.3 million tonnes of non-fed animal species were produced in aquaculture worldwide. Of these, 8.2 million tonnes were filter-feeding fish (mainly silver and bighead carp) and 16.2 million tonnes were aquatic invertebrates, mainly marine mussel species.

Water treatment with mussel cultures?

Mussels are particularly efficient as suspension feeders. Some species filter even tiny particles that are only 0.004 millimetres in size out of the water with their gill sieve, thus significantly reducing the turbidity of the water body. The collected nutrient-rich algae mush is then converted into body tissues and shells. This is why mussel beds are often referred to as ‘self-emptying vacuums’ or ‘the guts of the aquatic ecosystem’. However, in individual cases, their cleaning performance depends on how much water passes through the gills per hour or day. This value may vary from animal to animal and depends, among other things, on the size of the animal and its position in the mussel bed. Literature data for individual adult oysters range from 75 to 120 litres per day. The filtration capacity of mussels is about 2 litres per day (but also between 1 and 5 litres per hour). Although the figures vary greatly, it is clear how important a role these ‘natural filters’ can play in keeping local ecosystems clean. This is compounded by their economic value, as many species, from oysters and clams to scallops and mussels, can be used as high-quality food for human consumption or in processed form as animal feed.

Provided, of course, that the shellfish do not contain any heavy metals, pathogenic germs or toxic substances. This is the essential issue of their non-selective filtration: they filter out the valuable and nutrient-rich substances from the water as well as useless waste and toxins. Mussels are efficient purification systems and at the same time robust survival artists that can cope with almost all environmental conditions and rarely become ill even in polluted water. This discovery has led some scientists to believe that mussels can be used not only to remove nutrients from aquatic systems, but also to filter out pollutants and microplastics from wastewater, i.e. for complete purification. An obvious idea. Because mussels filter all kinds of environmental pollutants out of the water and accumulate them in their body tissues, they are often used as indicators of water quality at their growing sites. Contaminated mussel beds are an early warning sign of pollution problems. Initial trials of the biological treatment of water with mussels have shown that this approach works and is feasible. Of course, such contaminated mussels are completely unfit for consumption and must be disposed of. However, the purification effect and clean water achieved are worth losing them as an edible resource.

Trophic level ­networking and climate protection

Fish production in aquaculture is one of the most favourable sources of animal protein production and arguably the only way to meet the demand of the growing world population. In 2020, aquaculture already accounted for 46% of global fish supply of 179 million tonnes and is expected to increase to 53% by 2030. This also puts more pressure on fish farmers to address the problem of nutrient inputs into water bodies. This is because with each additional fish produced, more feed residues and fish faeces, dissolved and solid waste particles end up in the water. One option is Integrated Multitrophic Aquaculture (IMTA), which combines the farming of different animal and plant species belonging to different trophic levels. For example, salmon, sea cucumbers and sea urchins, mussels and macroalgae. The solid waste that sinks to the seabed from the salmon pens is processed by benthic sediment eaters. On the other hand, particles that drift sideways from the salmon pens with the current are utilised by mussels in suspension cultures, which act as a wall to protect the salmon pens from the outside. Some of the dissolved nutrients in the water are absorbed by adjacent algal cultures, stimulating their growth. IMTA systems have already proven their practical suitability in some locations. IMTA is considered a successful example of ecological aquaculture. Despite measurable successes, they currently play hardly any role in global aquaculture, mainly due to the lack or inadequacy of governance structures and disputes over the use of space in the immediate coastal area.

However, algae cultures and mussel-macroalgae IMTA concepts offer further advantages, as algae also absorb enormous amounts of CO2 from the atmosphere. Carbon dioxide, which seeps from the atmosphere into near-surface water layers, upsets the original carbon balance and contributes to ocean acidification. Algae cultures can mitigate these processes, at least at a local level, and counteract the drop in pH value. Similar effects are caused by mussels and other crustaceans, which also play a central role in local carbon cycles. They absorb some of the carbon and bind it as calcium carbonate (CaCO3) as they grow. Both algae and mussels can affect the carbon cycle of coastal ecosystems and improve the carbon absorption capacity of shelf seas. Seasonally, this effect is temporarily reversed (during the summer growth phase, large mussel cultures release more CO2 into the atmosphere than they absorb), but the overall balance is still positive. The oceans are essential as CO2 sinks for climate protection.

Manfred Klinkhardt

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