Phytoplankton are microscopic organisms that drift in the sunlit layers of the ocean and freshwater bodies. They are traditionally categorized as producers, serving as the base of the aquatic food web. However, their feeding strategy is complex, challenging this simple classification. This involves a dual-feeding approach known as mixotrophy, where many species blur the boundary between plant-like and animal-like life forms.
The Primary Role of Phytoplankton
Phytoplankton are fundamentally autotrophs, meaning they create their own food using inorganic materials. This process, called photosynthesis, involves capturing sunlight and converting carbon dioxide and water into organic compounds like sugars. Because of this ability, they are the primary producers of the aquatic world, forming the first trophic level in nearly every ocean and lake ecosystem.
They are responsible for generating approximately half of the oxygen in Earth’s atmosphere, making their photosynthetic activity globally significant. This production occurs in the euphotic zone, the upper layer of water where sufficient sunlight penetrates. Organisms that feed on them, such as zooplankton, are classified as heterotrophs because they must consume other organisms for energy and carbon.
The Reality of Dual Feeding Mixotrophy
The term “mixotrophy” describes organisms that combine both autotrophic (self-feeding) and heterotrophic (consuming others) nutritional modes within a single cell. This strategy makes certain phytoplankton species functional omnivores, though the accurate classification is mixotroph or mixoplankton. This flexibility is widespread in marine plankton, contrasting sharply with the strict plant/animal division seen in most terrestrial life.
Mixotrophs can dynamically adjust their feeding behavior based on environmental conditions, optimizing their growth by switching between or simultaneously using both energy sources. For instance, in nutrient-poor waters, a mixotroph may increase its predatory consumption to obtain essential elements like nitrogen and phosphorus. Conversely, when light is abundant, the same organism may rely more heavily on photosynthesis for its carbon and energy requirements.
This ability to switch feeding modes is a powerful evolutionary advantage, allowing these species to thrive in fluctuating environments. Many members of the dinoflagellate and chrysophyte groups exhibit this behavior. They possess chloroplasts for photosynthesis but also retain the cellular machinery necessary for consuming external food sources.
How Phytoplankton Consume Other Organisms
The heterotrophic strategy involves two main mechanisms for acquiring organic matter. The most dramatic is phagotrophy, the process of physically engulfing prey, such as bacteria or smaller protists. Some dinoflagellates use specialized structures like a peduncle, a tube-like appendage that sucks the internal contents out of a prey cell.
Other mixotrophs employ a less direct method called osmotrophy, which involves absorbing dissolved organic compounds directly from the surrounding water. This absorption supplements the cell’s nutritional needs, particularly when prey is scarce. Some mixotrophs, known as non-constitutive mixotrophs, acquire the ability to photosynthesize by consuming and retaining the functional chloroplasts from their prey, a process called kleptochloroplastidy.
Why Mixotrophy Changes the Marine Food Web
The widespread presence of mixotrophs significantly alters the traditional structure of the marine food web by dissolving the boundary between producers and consumers. They introduce complexity, allowing organisms to occupy multiple trophic levels simultaneously, which complicates ecological modeling efforts. This flexibility enhances the transfer of biomass and energy through the food chain, leading to a higher overall trophic transfer efficiency.
Functioning as both a producer and a consumer, mixotrophy allows carbon and energy to move more efficiently to larger organisms at higher trophic levels. This dynamic also impacts global biogeochemical cycles, particularly the carbon pump, which moves carbon from the surface ocean to the deep sea. Mixotrophs use prey-derived nutrients to fuel photosynthesis, meaning they can fix more carbon in nutrient-limited regions and increase sequestration efficiency.