Phytoplankton are microscopic organisms that float in the water column and form the foundational layer of aquatic food webs. They are responsible for a large portion of the world’s oxygen production. When considering their feeding habits, the traditional classifications of herbivore, carnivore, or omnivore—terms usually reserved for animals—do not easily apply to these diverse single-celled organisms. The methods phytoplankton use to acquire the necessary energy and nutrients are far more flexible and complex than simple grazing or predation.
The Foundation: Primary Production
Phytoplankton are best known for their role as primary producers. Like terrestrial vegetation, these organisms contain chlorophyll, allowing them to capture energy from sunlight. They use this captured light energy to convert water and dissolved carbon dioxide into carbohydrates through a process known as photosynthesis. This biological mechanism forms the base of virtually every marine food web.
The energy they create sustains their growth, which in turn provides the food source for true herbivores in the ocean, such as microscopic zooplankton. They also require dissolved inorganic nutrients like nitrate and phosphate, which they take directly from the surrounding water. Phytoplankton populations can respond rapidly to changing conditions, quickly multiplying when sunlight and nutrients are abundant.
Acquiring Nutrients Through Consumption
While many phytoplankton rely solely on light and dissolved nutrients, some species have evolved the ability to consume external organic matter, exhibiting a behavior that resembles a carnivore. This feeding strategy, known as phagotrophy, involves the cell engulfing prey, such as bacteria or smaller plankton, to obtain nourishment. This consumption acts as a supplementary method for acquiring carbon and essential nutrients like nitrogen or phosphorus.
This consumption of other organisms is often triggered when conditions are suboptimal for photosynthesis. For instance, when light levels are low, or when specific inorganic nutrients are scarce in the water, consuming prey can provide the necessary elements for survival. In these scenarios, the trophic role of phytoplankton can shift depending on the environmental pressures they face.
The Combined Strategy: Mixotrophy
The most accurate classification for a large number of phytoplankton species is neither strictly herbivore nor carnivore, but rather a mixotroph. Mixotrophy is a strategy where a single organism combines both photosynthesis (light capture) and the consumption of prey (phagotrophy). This combined approach offers significant ecological flexibility, allowing the organism to thrive across a wider range of environmental conditions.
In nutrient-poor, well-lit waters, the mixotroph can rely more heavily on its photosynthetic abilities to generate energy. If the mixotroph moves into deeper areas with less light or nutrient-depleted water, it can switch to consuming bacteria and other small plankton to supplement its needs. This ability provides a significant competitive advantage over organisms that are restricted to only one feeding mode.