Key factor for marine fish fry stocking
Microalgae cultures have been an established part of modern aquaculture for more than four decades. Due to their high nutritional value, photoautotrophic strains of microalgae play an important role in the cultivation of marine fish larvae as well as in the early stages of development of other aquatic organisms. The demand for microalgae is already considerable and could increase further, because the cell biomass also has the potential to replace a proportion of fishmeal.
Almost 35.1 million tonnes of algae and aquatic plants with a value of USD 16.5 billion were produced in aquaculture worldwide in the year 2020. However, this was almost all macroalgae such as kelp, wakame and gracilaria, which are almost exclusively used to manufacture agar and carrageenan. The production of microalgae, microscopically small organisms that occur in both fresh and salt water and that float freely in the water or grow benthically, is a significantly smaller industry in comparison. But they do have enormous biological significance in aquatic ecosystems, as they are located right at the bottom of aquatic food chains and therefore represent the basis for zooplankton as well as the different stages of development of other life forms in the water. Nauplii and the larvae of shellfish, bristle worms, snails, crustaceans, echinoderms and also some fish larvae – even those of species which later become carnivorous – can rely on microalgae to feed and develop in a healthy way, either permanently or temporarily during specific stages of development. Anyone wishing to successfully cultivate young organisms to stock aquaculture farms, such as shrimp, oysters or fish, therefore cannot really get around cultivating microalgae for feeding purposes in the hatcheries. This explains why, despite the comparably small quantities in which they are produced, microalgae are almost indispensable and play a large part in the success of global aquaculture.
In recent decades, several hundred species of microalgae have been tested for their suitability for aquaculture purposes, but only about twenty are used widely. The requirements are high, as the algae must meet the nutritional needs of the bivalves, crustaceans or fish as well as being as easy as possible to cultivate. First, what is important is rapid growth rates, suitability for mass cultures and sensitivity to light and temperature fluctuations. Second, it is also about nutritional composition, taste, digestibility or toxins that may be present. In addition, the size of the algae must be suitable for the width of the mouth and the consumption capacity of the target species. Generally, 1 to 15 μm for filter feeders and 10 to 100 μm for grazing species that graze the microalgae from hard structures is seen as suitable.
Foundation of aquatic food chains
The explanation for the enormous value of the microalgae diet, which is practically indispensable for initial or permanent feeding in marine food chains, is obvious. As photoautotrophic organisms, they are able, in the presence of sunlight, to convert carbon dioxide into sugar compounds and other organic molecules that are important to life, including lipids with multiple unsaturated long-chain fatty acids (omega-3 type PUFAs such as EPA and DHA). With these ingredients, microalgae provide the larvae of fish and those of other groups of species with a balanced nutritional profile that meets their needs. They promote the development of microflora in the larval intestinal tract and have antibacterial properties that support the health of the fish larvae. Microalgae contain a variety of bioactive substances, from essential amino acids to vitamins and minerals, to pigments, some of which have antioxidant effects. Depending on the species and age, however, the nutritional value of microalgae can vary significantly. The decisive factor for this is, in addition to age-related changes in the biochemical makeup, primarily the cell wall structures. In the logarithmic growth phase, when the number of algae cells in the culture increases very quickly, microalgae typically contain 30 to 40% protein, 10 to 20% lipids and 5 to 15% carbohydrate. As soon as the stationary phase has been reached, however, the proportion of carbohydrate can double at the expense of the protein.
The main application of microalgae is therefore in the area of nutrition, where the cells can be used in their whole form or in the form of certain cell fragments. An example of this is the yellow or red pigments (carotenoids) contained in some microalgae that are mixed into fish feed to give the fish an attractive colour, which makes many customers more willing to buy them. Natural carotenoid pigment from the Haematococcus pluvialis algae, for example, serves to colour salmon fillets, the skin of the red porgy (Pagrus pagrus) or Litopenaeus vannamei shrimp an intense red. Salmon with algal biomass mixed into their feed were livelier, healthier and more active than conventionally fed fish. The taste and texture of their fillets was said to be significantly improved, which experts claim to be the result of the bromophenol, vitamins and minerals contained in the algae. Generally, microalgae are good sources for natural, highly bioavailable vitamins, the quantity of which can however vary significantly. The greatest deviations are shown for ascorbic acid (vitamin C), the concentration of which, depending on the species of algae, can vary over the course of the year by about 16 times (between 1 and 16 mg/g dry weight). For other substances such as niacin, thiamin, riboflavin, pyridoxin, cobalamin or biotin, the extent of the fluctuation is significantly lower, at two to four times. Algae are, after all, natural products, the composition of which is influenced by a variety of internal and external factors that result in changes in their contents.
Latent value potential only partially exploited
Globally there are thousands of hatcheries that grow microalgae strains in cultures to supply stock animals for aquaculture farms with nutritionally rich feed. The algae grow in special systems and equipment, the design of which ranges from open ponds or tanks with or without ventilation, to bioreactors, which are very demanding in terms of their control technology and work in closed circuits. Algae production can be done in batches (batch culture) as well as in continuous permanent operation (steady-state cultures), which allows for algal biomass to be regularly extracted. If larger quantities of algae are required, outdoor basins or ponds are often used. This is relatively economical, but is also prone to challenges, because the cultures can be inhibited by adverse environmental impacts or contamination. For this reason, many hatcheries turn to closed photobioreactors instead, which are frequently designed in the form of ventilated vertical tubes or as flat plates. Balloon bottles (10 to 20 l), polyethylene sacks (100 to 500 l) and plastic basins (1,000 to 5,000 l) are particularly popular for microalgae mass cultures. Such systems are productive, but also relatively expensive. The energy requirements alone for permanent lighting, ventilation and motion of the body of water with the algae result in significant costs.
Technically complex and correspondingly expensive
Depending on the size, hatcheries produce several hundred to a thousand litres of algal mass per day, whereby the cell densities in standard systems are usually between 105 to 107 algal cells/ml of water. The production costs usually vary between 50 and 200 USD/kg of algal dry weight and can constitute 20 to 50% of the total operating costs of a hatchery. There are, however, considerable economies of scale: the highest costs almost always occur in smaller hatcheries. As an alternative to cost-intensive algae production that requires light, researchers are currently intensively searching for microorganisms such as bacteria or yeasts that can grow heterotrophically without light in “fermenters”. Lipid emulsions are also being considered as economical alternatives to microalgae, although they have a lower nutritional value than microalgae mixtures. Full equivalent replacements for microalgae have still not been found, so microbial biomass is in practice always used as part of a mixed feed.
Microalgae are required in aquaculture to cultivate different groups of species. All development stages of mussels, for example, can use microalgae as their food source, because they are filter feeders. Every mussel hatchery therefore has sufficient capacity for microalgae production. The larvae of abalones (gastropods, snails) and some sea urchin species (Echinoidea) do not need any microalgae during their plankton phase, as they are still nourished by internal reserves of yolk. As soon as they leave the plankton stage and transition to life on the sea floor, however, they need benthic microalgae as their initial food. At least until they are large enough to feed on macroalgae. Therefore, benthic diatoms such as Navicula, Nitzschia and Amphora algae are mass-cultivated for these species. The plankton larval stages of commercially important crustaceans, which primarily include penaeid shrimp, initially feed on microalgae, which are later replaced with living zooplankton.
Enrichment improves the nutritional profile of zooplankton
For the larvae of some fish species, particularly from the marine environment, microalgae are practically indispensable as an initial and supplemental feed to ensure that they receive balanced nutrition. Only a few species, such as salmon and other salmonids, are able to immediately consume and digest dry feed diets immediately after exhausting their extensive reserves of yolk. In contrast to mussels, snails and crabs, however, only a few fish larvae consume microalgae as food directly, as they prefer live zooplankton, the motion activity of which stimulates their predator reflex. Since the 1960s, almost all fish hatcheries therefore use rotifers (usually Brachionus species) and brine shrimp nauplii (Artemia sp.) as feed for the fish larvae, which resemble their natural zooplanktonic live prey. The advantages of rotifers and Artemia are their easy availability and simple culture in high stocking densities. Rotifers also have rapid reproduction rates in the cultures. The disadvantage of both groups of species is their sub-optimal nutritional profile, which can trigger nutritional and developmental disorders in the fry. The lack of n-3 PUFAs has particularly serious consequences. In order to remedy this issue, rotifers and Artemia nauplii are frequently fed on microalgae before being fed to fish larvae. The zooplankton organisms are, as it were, used as live transport vehicles to get the nutritious microalgae into the bodies of the fish larvae. This step, which improves the nutritional profile of the live prey, is called enrichment. Fish larvae thus receive the extra portions of proteins, amino and fatty acids, vitamins, minerals and biologically active secondary plant materials that they need for their development.
What sounds simple is often somewhat tricky in practice, however, because enrichment depends on dosage and the correct timing, particularly for rotifers. It is important that sufficient algal food is available to them at all times. If the algae concentration in the water becomes too low, the number of rotifer eggs produced reduces and the culture yield then decreases. An important criterion for the density of the algal soup is the green colouring of the culture medium, the assessment of which requires a certain amount of experience. The only thing that is certain is that the density of microalgae is not sufficient if the culture water is too clear. The larvae of some tropical snappers and groupers are so small, however, that even Brachionus rotifers, which measure only 200-350 μm, are too large to use as an initial food. Such species are often fed with powdered food, sieved offspring (copepodite stages) of copepods. Others, such as the larvae of the Atlantic halibut, have very special nutritional needs and require diverse wild plankton as feed to prevent developmental disorders. These zooplankton organisms are also often enriched with microalgae.
Although the production of microalgae lags well behind macroalgae in terms of quantity, their significance for global aquaculture is already considerable. For the moment, this is only in specialised areas such as stock production or the manufacture of pigments, but when success has actually been achieved in future in exploiting microalgal biomass for supplying protein to aquaculture farms, these tiny green cells will likely become even more indispensable.