Zooplankton are generally heterotrophic, meaning they cannot produce their own food through photosynthesis. These microscopic animals drift through the water, relying on consuming other organisms for energy. The question of whether zooplankton photosynthesize, however, has a nuanced answer due to some exceptions in the marine environment. While the majority are consumers, certain single-celled zooplankton have evolved unique ways to utilize sunlight for nutrition.
Zooplankton’s Primary Role: Consumers in the Marine Food Web
Zooplankton are the animal component of the plankton community, fundamentally different from phytoplankton, which are the plant-like component. Phytoplankton are autotrophs, using chlorophyll to convert sunlight, water, and carbon dioxide into energy through photosynthesis. Zooplankton are heterotrophs, requiring them to acquire energy by feeding on others.
The primary function of zooplankton is to act as the crucial middle link in the ocean’s food web. They are the primary consumers, grazing on the vast biomass of phytoplankton in the sunlit surface waters. This consumption transfers the solar energy captured by phytoplankton up to higher trophic levels.
Zooplankton populations are diverse, ranging from microscopic protozoans (ciliates and foraminiferans) to small crustaceans (copepods and krill). Copepods are a major food source and make up a significant percentage of zooplankton species. They obtain energy through various feeding methods, including filter feeding on phytoplankton or actively preying on smaller zooplankton.
Microzooplankton are particularly important grazers, consuming an estimated 59 to 75% of the daily marine primary production. This grazing pressure regulates the population of primary producers, directly impacting the entire marine ecosystem. Their role as energy conduits supports the survival of larger marine life, including various fish species.
The energy gained from consuming other organisms fuels their growth, reproduction, and movement. This process establishes zooplankton as the second trophic level, directly supporting the next level of consumers in the food chain.
When Zooplankton Utilize Light: Symbiosis and Kleptoplasty
Although most zooplankton are strictly consumers, some single-celled groups have developed mechanisms to benefit from light energy. These organisms are called mixotrophs because they combine feeding on other organisms with light utilization.
One method involves establishing a symbiotic relationship, where the zooplankton hosts photosynthetic algae inside its cell or shell. Certain radiolarians and foraminifera host dinoflagellates or other algae as endosymbionts. The host provides protection and nutrients, while the internal algae produce sugars through photosynthesis for the zooplankton’s energy use.
The second mechanism is called kleptoplasty, derived from the Greek word for “thief.” In this process, the zooplankton consumes an alga but selectively retains the chloroplasts—the organelles responsible for photosynthesis—from the prey. The retained chloroplasts, known as kleptoplasts, remain functional within the host cell, continuing to perform photosynthesis and provide a nutritional subsidy.
Kleptoplasty is observed in several groups of single-celled zooplankton, including certain dinoflagellates and ciliates. For example, some species of the dinoflagellate Dinophysis can maintain functional kleptoplasts for up to two months after feeding. This principle allows some planktonic protozoans to temporarily become part-time producers.
The Impact of Their Feeding Habits on Ocean Ecosystems
Zooplankton’s role as consumers has profound consequences for global ocean processes, particularly in regulating the planet’s climate. Their feeding habits are central to the biological carbon pump, a process that moves carbon from the surface ocean to the deep sea.
Zooplankton consume carbon-rich phytoplankton in the sunlit surface layer. When they excrete waste, they produce dense fecal pellets that sink rapidly to the deep ocean. This sinking of particulate organic carbon transports carbon dioxide, originally absorbed from the atmosphere, down to depths where it can be stored for centuries.
Zooplankton also actively contribute to carbon transport through diel vertical migration. This daily pattern involves migrating to deeper, darker waters during the day and ascending to the surface at night to feed. By respiring, or releasing carbon dioxide, at depth, they effectively pump carbon away from the surface layer, regulating the ocean’s carbon cycle.
The population dynamics of zooplankton are directly tied to the productivity of commercial fisheries. As primary consumers, they are the main food source for forage fish like sardines and anchovies, which are then consumed by larger predatory fish. Changes in zooplankton abundance or species composition can have a cascading effect, impacting the availability of fish stocks that support human economies.