Phytoplankton and Cyanobacteria: Their Key Differences in Freshwater Ecosystems

While phytoplankton and cyanobacteria often coexist in freshwater environments and play crucial roles in aquatic ecosystems, they represent fundamentally different forms of life with distinct characteristics, ecological impacts, and evolutionary histories.

Understanding the Basics

Phytoplankton is a broad umbrella term referring to microscopic, photosynthetic organisms that drift in aquatic environments. Phytoplankton are eukaryotes meaning they have a nucleus (cyanobacteria do not).  This diverse group includes various algae such as green algae (Chlorophyta), diatoms (Bacillariophyta), and dinoflagellates, among others. The defining feature of phytoplankton is not their taxonomic classification but rather their ecological role as free-floating, photosynthetic microorganisms.

A microscopic view of Spirulina, showcasing the distinct spiral shapes of this cyanobacterium species.

Cyanobacteria, often called blue-green algae, are actually prokaryotic bacteria, not algae at all. A prokaryote lacks a nucleus.  Despite it  misleading common name, blue-green algae, cyanobacteria are true bacteria that happen to perform photosynthesis. This distinction is critical because it places them in an entirely different domain of life compared to eukaryotic algae.

Cellular Organization: The Fundamental Divide

Close-up view of the green chlorophyte Scenedesmus, a type of phytoplankton, showcasing its vibrant structure under a microscope.

The most fundamental difference between cyanobacteria and other freshwater phytoplankton lies in their cellular structure. Cyanobacteria are prokaryotes, meaning their cells lack a membrane-bound nucleus and other organelles. Their genetic material floats freely in the cytoplasm, and their photosynthetic machinery is embedded directly in internal membrane systems called thylakoids.

In contrast, eukaryotic phytoplankton like diatoms and green algae possess complex cellular organization. They have a defined nucleus containing their DNA, along with specialized organelles including chloroplasts (which house their photosynthetic apparatus), mitochondria, and often other structures like flagella for movement. This cellular complexity reflects billions of years of evolutionary divergence.

Photosynthetic Pigments and Color

While both groups perform photosynthesis, they utilize different pigment combinations that give them characteristic colors. Cyanobacteria contain chlorophyll-a and unique pigments called phycobiliproteins, including phycocyanin (blue) and phycoerythrin (red). These accessory pigments give cyanobacteria their distinctive blue-green coloration and allow them to capture light wavelengths that chlorophyll alone cannot efficiently absorb.

Other phytoplankton groups display varied pigment profiles. Green algae contain chlorophyll-a and chlorophyll-b, similar to land plants, giving them a bright green appearance. Diatoms possess chlorophyll-a and chlorophyll-c along with fucoxanthin, a brown pigment that gives them a golden-brown color. These pigment differences not only affect appearance but also influence which light conditions favor each group.

Ecological Roles and Environmental Impacts

Both cyanobacteria and eukaryotic phytoplankton form the base of freshwater food webs, converting sunlight into organic matter through photosynthesis. They produce oxygen and serve as food for zooplankton, which in turn feed fish and other aquatic animals. However, their ecological impacts can differ dramatically.

Many cyanobacteria species produce toxins (cyanotoxins) that can be harmful to animals and humans. When conditions favor explosive cyanobacterial growth—forming what are commonly called "harmful algal blooms"—these toxins can accumulate to dangerous levels, contaminating drinking water supplies and killing fish, livestock, and pets. Species like Microcystis, Anabaena, and Cylindrospermopsis are notorious bloom-formers.

Eukaryotic phytoplankton generally pose fewer toxicity concerns and Hydralife Solutions only distributes beneficial non-toxic producing phytoplankton. Their ecological impact tends to be more predictable and not associated with toxin production, making them more desirable components of healthy aquatic ecosystems.

Nitrogen Fixation: A Unique Capability

One remarkable ability that sets many cyanobacteria apart is nitrogen fixation. Certain cyanobacteria species possess specialized cells called heterocysts where they convert atmospheric nitrogen gas into ammonia, making this essential nutrient available to other organisms. This capability is virtually absent in eukaryotic freshwater phytoplankton and gives cyanobacteria a competitive advantage in nitrogen-poor waters.

This nitrogen-fixing ability explains why cyanobacteria often dominate in lakes affected by phosphorus pollution but limited nitrogen availability. Once phosphorus levels rise (often from agricultural runoff or wastewater), nitrogen-fixing cyanobacteria can thrive even when nitrogen is scarce, leading to persistent bloom problems.

Conclusion

Understanding the differences between cyanobacteria and other freshwater phytoplankton is essential for managing water quality, predicting harmful blooms, and maintaining healthy aquatic ecosystems. While both groups are microscopic photosynthesizers that drift in the water column, cyanobacteria are prokaryotic bacteria with unique pigments, potential toxicity, and nitrogen-fixing capabilities. Other phytoplankton are eukaryotic organisms with complex cellular organization and generally less problematic ecological impacts. Recognizing these distinctions helps scientists, water managers, and environmental professionals understand and analyze their waters more effectively.