Phytoplankton are microscopic organisms that inhabit the sunlit surface layers of oceans, lakes, and other bodies of water. These tiny aquatic organisms form the foundation of nearly all marine food webs. They are also significant producers of the oxygen that sustains life on Earth. Understanding their feeding mechanisms is fundamental to comprehending marine ecosystems.
How Phytoplankton “Eat”: Photosynthesis
Phytoplankton are categorized as autotrophs, meaning they create their own sustenance, much like plants on land. This process is primarily achieved through photosynthesis, where they convert light energy into chemical energy. They absorb carbon dioxide dissolved in the water and, with the help of sunlight and chlorophyll, transform it into sugars. These sugars serve as their energy source and building blocks for growth, releasing oxygen as a byproduct.
Sunlight penetration into the water column limits phytoplankton to the euphotic zone, the upper layer where light is sufficient for photosynthesis. Different species, such as diatoms, dinoflagellates, and cyanobacteria, all use this photosynthetic mechanism. This process underpins their survival and proliferation across vast aquatic environments.
Key Nutrients for Phytoplankton Growth
Beyond sunlight and carbon dioxide, phytoplankton require inorganic nutrients from the surrounding water to grow and reproduce. Macronutrients are needed in larger quantities. Nitrogen, typically as nitrates, is used for building proteins, nucleic acids, and other cellular components. Phosphorus, often as phosphates, is important for energy transfer within cells and for forming DNA. Silicates are important for diatoms, which use silicon to construct their intricate glass-like cell walls.
Micronutrients, though needed in smaller amounts, are equally important for various cellular processes. Iron is a prominent example, serving as a cofactor in many enzymes, including those involved in photosynthesis and nitrogen fixation. The availability of these elements directly influences phytoplankton growth rates and abundance.
Factors Affecting Nutrient Availability
The supply of dissolved nutrients to phytoplankton in surface waters is influenced by several natural physical processes. Ocean upwelling is a significant mechanism where deep, cold, nutrient-rich water rises to the surface, bringing a fresh supply of nitrates, phosphates, and silicates to the sunlit zone. Ocean currents also distribute these nutrients across vast expanses.
Water column stratification, the layering of water masses by density due to temperature or salinity, can limit the vertical mixing of nutrients from deeper waters to the surface. Warmer surface waters can lead to stronger, more persistent stratification, which may restrict nutrient availability. Human activities also influence nutrient availability, with agricultural runoff introducing excess nitrogen and phosphorus into coastal waters, potentially leading to harmful algal blooms.
The Global Impact of Phytoplankton’s Diet
The ability of phytoplankton to produce their own food from sunlight and inorganic nutrients has far-reaching implications for global ecosystems. They form the base of nearly all marine food webs, supporting zooplankton, which are consumed by fish, and ultimately by larger marine animals like whales and seabirds. Without this primary production, most marine life would not exist.
Phytoplankton contribute significantly to Earth’s oxygen supply, producing an estimated 50% to 80% of the oxygen in the atmosphere through photosynthesis. Their “diet” also plays an important role in the global carbon cycle. By absorbing massive amounts of atmospheric carbon dioxide for photosynthesis, they help regulate Earth’s climate. When phytoplankton die, their bodies and waste products sink to the ocean floor, sequestering carbon in deep waters, a process known as the biological pump.