Marine phytoplankton are microscopic organisms from the Greek words for “plant” (phyto) and “wanderer” (plankton). These tiny, drifting beings are not true plants but function similarly, using chlorophyll to perform photosynthesis. They are single-celled bacteria and protists that float in the sunlit upper layers of oceans and other bodies of water to convert sunlight into energy.
Major Types of Marine Phytoplankton
The world of marine phytoplankton is diverse, with organisms varying in size, shape, and function. One common group is diatoms, known for their intricate, glass-like shells made of silica. These rigid, interlocking shells give them unique structures, and they rely on ocean currents for movement.
Dinoflagellates possess whip-like appendages called flagella to propel themselves through the water. Cyanobacteria are ancient photosynthesizing bacteria, and some species can “fix” nitrogen, allowing them to thrive in nutrient-poor waters. Coccolithophores are distinguished by ornate calcium carbonate plates, called coccoliths, that cover their bodies. This plating makes them visually distinct and plays a role in marine chemistry.
How Phytoplankton Survive and Grow
The survival and growth of phytoplankton depend on light, carbon dioxide, and inorganic nutrients. Through photosynthesis, they convert carbon dioxide absorbed from the atmosphere into the organic compounds needed to live and multiply. This reliance on sunlight confines them to the photic zone, the uppermost layer of the ocean where sunlight can penetrate.
Phytoplankton require nutrients from the surrounding water, including nitrates and phosphates for building proteins, fats, and DNA. Some groups have additional needs; for instance, diatoms require silicate to construct their shells. The availability of these nutrients and trace metals like iron determines the size and composition of phytoplankton populations in different ocean regions.
Global Impact of Marine Phytoplankton
Despite their microscopic size, phytoplankton have a significant global impact. Through photosynthesis, they produce an estimated 50% of the Earth’s oxygen, a contribution comparable to all terrestrial plants combined. This oxygen output helps maintain the atmospheric balance that sustains life on the planet.
Phytoplankton form the foundation of nearly all marine food webs. As primary producers, they are the “grass of the sea,” converting sunlight and nutrients into organic matter. This feeds everything from microscopic zooplankton and small invertebrates like shrimp to massive filter-feeding whales. Without phytoplankton, the biodiversity of the oceans would collapse.
These organisms are central to the global carbon cycle, acting as a biological pump that helps regulate Earth’s climate. During photosynthesis, they draw large quantities of carbon dioxide from the atmosphere and store it in their cells. When phytoplankton die, they sink, carrying this captured carbon to the deep ocean, where it can remain sequestered for thousands of years, mitigating the greenhouse effect.
Challenges Affecting Phytoplankton Populations
Phytoplankton populations face threats from pollution and climatic shifts. A visible problem is the formation of harmful algal blooms (HABs), which occur when excess nutrients from sources like agricultural runoff cause a rapid growth of certain phytoplankton species. These dense blooms can discolor the water, creating “red tides.”
Some species of dinoflagellates and cyanobacteria in HABs produce toxins that accumulate in shellfish and fish, harming marine animals and humans who consume them. When the bloom dies and decomposes, the process consumes large amounts of oxygen. This creates hypoxic “dead zones” where other marine life cannot survive.
Climate change threatens phytoplankton through rising sea surface temperatures and increased ocean stratification. This can limit the mixing that brings nutrients from deep water to the sunlit surface. Ocean acidification, caused by the ocean absorbing excess CO2, is another concern as it can interfere with the ability of organisms like coccolithophores to build their calcium carbonate plates. These changes alter the species composition and productivity of phytoplankton, affecting marine ecosystems and global climate.