Phytoplankton

Phytoplankton are microscopic, self-feeding autotrophs that constitute the primary producers within marine and freshwater ecosystems. Their name derives from the Greek words for “plant” and “wanderer”, a reference to their drifting movement with currents. As photosynthetic organisms, phytoplankton occupy the sunlit, upper layers of water bodies and form the foundation of aquatic food webs. Despite representing only a small fraction of the planet’s plant biomass, they contribute roughly half of global photosynthetic activity and oxygen production. Their rapid turnover and sensitivity to environmental change make them essential indicators of oceanic and freshwater health.

Characteristics and Ecological Role

Phytoplankton convert sunlight into chemical energy through photosynthesis, using carbon dioxide dissolved in water to produce organic compounds. This process sustains the majority of primary production in the oceans, providing energy for organisms ranging from zooplankton to large marine mammals. Their distribution is largely confined to the euphotic zone, where sufficient light penetrates to support photosynthetic activity. Compared with terrestrial flora, phytoplankton are exposed to less seasonality, occupy a vast surface area, and reproduce far more rapidly, enabling swift responses at regional and global scales to climatic fluctuations.
These organisms are highly diverse, comprising photosynthetic bacteria such as cyanobacteria and small unicellular protists including diatoms. The presence of chlorophyll and accessory pigments, such as xanthophylls and phycobiliproteins, can cause visible colouration of surface waters when populations reach high densities. While most phytoplankton are obligate photoautotrophs, some taxa exhibit mixotrophy or full heterotrophy, acquiring organic material by ingestion. Dinoflagellates, for example, may ingest particulate organic matter, blurring traditional distinctions between phytoplankton and zooplankton.
As the base of aquatic food chains, phytoplankton support a broad range of species, from microscopic grazers to krill and baleen whales. The link between phytoplankton and higher trophic levels underscores their significance in maintaining biodiversity and ecosystem stability.

Diversity and Classification

Phytoplankton encompass several major groups distinguished by morphology, pigmentation, and nutritional strategies. Key groups include:

• Cyanobacteria: Photosynthetic bacteria such as Prochlorococcus and Synechococcus, which dominate nutrient-poor tropical and subtropical waters. Their small cell size and efficiency in low-light and oligotrophic conditions make them ecologically significant.
• Diatoms: Silica-requiring protists that thrive in nutrient-rich waters. They have intricate silica cell walls and are responsible for substantial seasonal blooms, particularly in temperate and polar regions.
• Flagellates: A diverse assemblage of motile protists that include both autotrophic and mixotrophic species. Many occupy niches where light and nutrient conditions vary vertically.
• Coccolithophores: Calcifying phytoplankton covered in calcium carbonate plates known as coccoliths. They play a notable role in biogeochemical cycling and export of carbon to deeper waters.
• Mixoplankton: Taxa formerly considered purely phytoplankton but now acknowledged to combine photosynthesis with ingestion. This trophic plasticity influences nutrient cycling and food-web dynamics.

Such diversity contributes to resilience in phytoplankton communities, enabling adaptation to varied light conditions, nutrient regimes, and water-column structures.

Nutrient Requirements and Environmental Controls

Phytoplankton growth depends on the availability of several key nutrients. Macronutrients such as nitrate, phosphate, and silicic acid are essential for cellular processes and biosynthesis. Their distribution in surface waters reflects the balance between biological uptake, remineralisation, and physical processes such as upwelling. The stoichiometric ratios of these macronutrients often align with the Redfield ratio, a widespread benchmark in ocean biogeochemistry.
Micronutrients, particularly trace metals, also exert strong control over phytoplankton physiology. Iron, manganese, cobalt, zinc, cadmium, and copper are required in minute quantities but their scarcity can restrict growth. In high-nutrient, low-chlorophyll regions such as the Southern Ocean, iron limitation is a common feature and has encouraged scientific interest in iron fertilisation as a means of enhancing productivity and potentially moderating atmospheric carbon dioxide. However, ecological uncertainties and debates over environmental risk have curtailed widespread experimentation.
Phytoplankton also rely on B-vitamins, and deficiencies in these compounds can regulate community composition. Variability in river discharge, glacial meltwater, and weathering on land influences nutrient supply, creating dynamic and spatially heterogeneous growth conditions.

Light, Pigmentation, and Photosynthetic Adaptations

Because phytoplankton are confined to the photic zone, light availability is a dominant factor shaping their distribution. Species possess distinct suites of pigments that absorb different portions of the light spectrum, enabling partitioning of light resources among taxa. This spectral adaptation allows coexistence of diverse species within the same water column. Photodegradation may occur under excessive solar radiation, prompting some phytoplankton to migrate vertically within water layers. Nonetheless, they lack the ability to move against currents and ultimately sink, contributing to deep-ocean carbon sequestration through the deposition of organic matter.
Changes in light quality, rather than quantity alone, can significantly alter community structure. Experiments have demonstrated that shifts in spectral composition influence species composition even when total light intensity remains constant.

Global Biogeochemical Significance

Through photosynthesis and subsequent carbon fixation, phytoplankton drive the oceanic component of the global carbon cycle. They assimilate carbon dioxide and release oxygen, while the export of dead cells and detritus to the seafloor forms a key element of the biological pump, transporting carbon out of the surface ocean. The release of dissolved organic carbon sustains microbial ecosystems and influences nutrient regeneration.
Their sensitivity to temperature, stratification, and nutrient supply renders phytoplankton particularly vulnerable to anthropogenic climate change. Warming alters water-column stability, potentially reducing nutrient replenishment in surface layers. Ocean acidification affects calcifying groups such as coccolithophores, although short generation times allow some degree of rapid adaptation to changing pH. Altered grazing pressure by zooplankton may further shift mortality rates and population structure under future climatic conditions.

Environmental Variability and Indicators of Change

Phytoplankton populations fluctuate at multiple scales in response to physical and chemical variations. Major climate phenomena such as the El Niño–Southern Oscillation influence upwelling, nutrient availability, and community composition in equatorial waters. During El Niño phases, reductions in biomass and significant shifts in species distribution are commonly observed.
Because of their rapid response to environmental change, phytoplankton are widely used as indicators of estuarine and coastal ecosystem health. Modern satellite observations of ocean colour allow continuous monitoring of chlorophyll concentrations and distribution patterns, supporting research into global biogeochemical cycles and climate interactions.