Marine ecosystems, complex and finely tuned, rely on planktonic organisms such as copepods and phytoplankton to drive their fundamental processes. These microorganisms are critical not only for marine food webs but also for global biogeochemical cycles. Copepods, belonging to the zooplankton category, serve as intermediaries between primary producers and higher trophic levels, while phytoplankton, as primary producers, form the foundation of the marine food web.

The significance of these organisms extends beyond their trophic roles. Phytoplankton contribute to global oxygen production and carbon sequestration, helping to regulate atmospheric CO₂ levels. Copepods, on the other hand, not only serve as food for marine larvae but also regulate microbial populations and nutrient recycling. This review delves into the biology, ecological roles, and the challenges of culturing these organisms, focusing on their relevance to both natural ecosystems and aquaculture. We will also critically analyze the risks associated with in-home plankton culture and the emerging research into plankton-associated microbiomes.

Copepods: Keystone Organisms in Marine Food Webs

Taxonomy and Distribution

Copepods are classified under the subclass Copepoda, within the phylum Arthropoda and subphylum Crustacea. This group comprises approximately 13 orders, with the three most significant to marine ecosystems being Calanoida, Cyclopoida, and Harpacticoida. Their distribution is global, with copepods found in almost every aquatic habitat, from shallow coastal waters to deep-sea hydrothermal vents, and even within moist terrestrial environments.

Copepods exhibit diverse ecological strategies, with some species living freely in the water column (pelagic), while others are benthic, inhabiting the ocean floor or even associating with other organisms as parasites. They are integral to marine and freshwater ecosystems, and their presence is so ubiquitous that they are often referred to as the most numerous multicellular organisms on Earth.

Ecological Roles of Copepods

Copepods perform several crucial functions in marine ecosystems, particularly within the food web. As primary consumers, they primarily feed on phytoplankton, small algae, and detritus. In turn, they serve as prey for a wide range of marine organisms, including fish larvae, which are highly dependent on copepods for survival in the early stages of development.

Copepods are key players in the marine microbial loop, a critical process for recycling organic material. Through their feeding and excretion activities, copepods contribute to the cycling of dissolved organic carbon (DOC) and particulate organic carbon (POC). This process facilitates the transfer of energy from microbial communities to higher trophic levels, supporting the productivity and efficiency of the marine food web. Copepods also influence the carbon cycle through their fecal pellet production. These pellets sink rapidly, transporting carbon to the deep ocean, a mechanism essential for long-term carbon sequestration.

Furthermore, copepods participate in the nitrogen cycle. By excreting nitrogenous waste products like ammonium, they provide a vital source of nitrogen to phytoplankton, which require it for photosynthesis and growth. In nitrogen-limited environments, the role of copepods in nitrogen regeneration can significantly influence primary productivity.

Nutritional Composition and Importance in Aquaculture

The nutritional composition of copepods makes them a highly desirable live feed in marine aquaculture. Compared to other live feeds, such as rotifers and Artemia, copepods offer superior levels of essential fatty acids, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These long-chain polyunsaturated fatty acids are critical for the development of marine fish larvae, influencing their growth, immune function, and survival rates. The fatty acid profile of copepods is closely linked to their diet of microalgae, which is rich in these essential lipids.

In addition to their lipid content, copepods are rich in proteins, amino acids, pigments (such as astaxanthin), and vitamins, which are necessary for the proper development of fish larvae. Their small size and high digestibility make copepods particularly effective as a first feed for fish larvae that may struggle to consume larger, more robust prey.

Studies have demonstrated that fish larvae fed on copepods exhibit significantly higher survival rates, faster growth, better pigmentation, and improved stress tolerance compared to those fed on rotifers or Artemia. For example, copepods are known to reduce deformities in fish larvae by providing a more nutritionally complete diet, which is especially important during the critical early stages of larval development.

Microbiome of Copepods and Its Role in Marine Ecosystems

Beyond their role as nutrient providers, copepods harbor a diverse and dynamic microbiome that plays a crucial role in their physiology and in broader ecological processes. The copepod microbiome is composed of a variety of bacterial and fungal species, many of which are involved in the digestion of organic matter, nutrient cycling, and even the mitigation of harmful environmental conditions.

Recent research has shown that the copepod microbiome can influence the cycling of carbon, nitrogen, and other essential nutrients within marine ecosystems. For example, certain bacteria within the copepod microbiome are capable of breaking down complex organic compounds, such as polysaccharides and proteins, making these nutrients more accessible to the host. Additionally, the microbiome can play a role in detoxifying harmful substances, such as cyanotoxins produced during harmful algal blooms.

In aquaculture, the copepod microbiome offers promising potential for biotechnological applications. The microbiome-associated bacteria can suppress the growth of harmful pathogens, acting as natural probiotics that enhance the health and resilience of fish larvae. This probiotic effect is of particular interest in hatcheries, where the use of antibiotics and chemical treatments is often necessary to control bacterial infections. By incorporating copepod cultures into aquaculture systems, it may be possible to reduce the reliance on these treatments and promote more sustainable farming practices.

Challenges in Copepod Cultivation for Aquaculture

Despite the numerous benefits that copepods provide as live feed, the large-scale cultivation of copepods presents several technical and economic challenges. The most significant barrier to the mass production of copepods is the difficulty of achieving high population densities under controlled conditions. While rotifers can be cultured at densities exceeding 2,000 individuals per milliliter, copepod cultures rarely reach densities above 10 individuals per milliliter. This limitation makes copepod production far less efficient than that of other live feeds.

Culturing Techniques and System Design

Several methods have been developed to cultivate copepods, ranging from extensive outdoor pond systems to intensive indoor tank systems. Extensive systems rely on natural phytoplankton blooms to provide food for copepods, while semi-intensive and intensive systems use controlled environments and prepared microalgal cultures to optimize copepod growth and reproduction.

Extensive systems, while simpler and less expensive to operate, are highly susceptible to environmental fluctuations and often produce inconsistent yields. Semi-intensive systems offer more control over environmental conditions and nutrient supply but are still vulnerable to contamination by unwanted species and pathogens. Intensive systems, which are typically housed indoors and rely on artificial lighting, controlled temperature, and high-quality microalgal feeds, provide the greatest level of control but are also the most costly to establish and maintain.

Water Quality and Microbial Control

Maintaining water quality is one of the most critical factors in successful copepod cultivation. Copepods are highly sensitive to changes in temperature, salinity, pH, and dissolved oxygen levels. Even slight deviations from optimal conditions can result in significant mortality or reduced reproductive success. In addition to physical water quality parameters, microbial contamination poses a constant threat to copepod cultures.

Microalgal feed used in copepod culture must be free from contaminants such as harmful bacteria, fungi, and protozoans. Contaminated feed can introduce pathogens into the culture system, leading to population crashes. Moreover, the overgrowth of certain bacteria can create anoxic conditions in culture tanks, further compromising water quality.

To mitigate these risks, strict biosecurity protocols must be followed, including regular monitoring of water quality parameters, sterilization of equipment, and the use of filtered and ozonated seawater. Despite these precautions, the maintenance of stable copepod cultures over extended periods remains a challenge, particularly in large-scale operations.

Feeding Requirements and Algal Cultures

The diet of copepods is another critical factor in their cultivation. Copepods feed primarily on microalgae, and the nutritional quality of the algae directly influences the health and reproductive success of the copepods. Microalgal species such as Tetraselmis, Nannochloropsis, and Isochrysis are commonly used in copepod cultures due to their high content of essential fatty acids and other nutrients.

The production of high-quality microalgal cultures is a challenging task in itself, requiring precise control over light, temperature, and nutrient levels to prevent contamination and optimize growth. Furthermore, different species of copepods have different dietary requirements. For example, calanoid copepods, which are pelagic, require different species of algae than benthic harpacticoid copepods, which feed on detritus and benthic diatoms.

Phytoplankton: The Foundation of Marine Productivity

Phytoplankton are the primary producers of the marine environment, responsible for approximately half of the world’s oxygen production through photosynthesis. They also form the base of the marine food web, providing energy and nutrients to herbivorous zooplankton like copepods, which in turn support higher trophic levels.

Role in Primary Production

Phytoplankton perform the critical function of converting solar energy into chemical energy through the process of photosynthesis. This process not only generates oxygen but also produces organic carbon compounds that are consumed by herbivorous zooplankton, initiating the flow of energy through the marine food web.

In addition to their role in primary production, phytoplankton are integral to the marine carbon cycle. They absorb carbon dioxide from the atmosphere, which is then transferred through the food web and ultimately sequestered in deep ocean sediments as organic carbon. This process, known as the "biological carbon pump," is essential for regulating atmospheric carbon levels and mitigating the impacts of climate change.

Species Diversity and Nutritional Value

Phytoplankton are an incredibly diverse group of organisms, with species ranging from single-celled diatoms to colonial cyanobacteria. Common species used in aquaculture include Chaetoceros, Tetraselmis, and Nannochloropsis, all of which are valued for their high lipid and nutrient content. The choice of phytoplankton species for aquaculture depends on the specific nutritional needs of the target zooplankton or fish species.

Risks and Challenges in Culturing Plankton for Home Aquaria

While the idea of culturing phytoplankton and copepods at home may appeal to marine hobbyists, the practical challenges and risks involved are significant. One of the primary risks is contamination, which can occur from harmful bacteria, parasites, or undesirable plankton species. These contaminants can rapidly take over cultures, introducing pathogens into reef tanks and destabilizing the tank ecosystem.

In mixed plankton cultures, the dominant species can quickly outcompete others, leading to an imbalance in nutrient cycling and microbial diversity. In extreme cases, certain species of phytoplankton may lyse, releasing toxins into the water and causing widespread mortality among tank inhabitants.

For these reasons, purchasing high-quality, contaminant-free cultures from reputable suppliers like Pod Your Reef is highly recommended. These commercial cultures are carefully controlled to ensure that they contain only the desired species and are free from harmful pathogens.

Conclusion

Copepods and phytoplankton play indispensable roles in marine ecosystems and aquaculture. Their contributions to primary production, nutrient cycling, and the marine food web make them critical components of both natural and artificial marine environments. However, the challenges associated with their cultivation, particularly at large scales, require ongoing research and innovation.

This review has highlighted not only the ecological importance of these organisms but also the emerging opportunities in aquaculture and biotechnology, particularly in the context of microbiome research. While the use of copepods and phytoplankton in home aquaria offers potential benefits, it is clear that careful management and sourcing from reputable providers are essential to avoid the risks of contamination and imbalance. As research continues to advance, our understanding of these planktonic organisms will undoubtedly expand, offering new insights into their applications in sustainable aquaculture and marine conservation.