Phytoplankton are microscopic, plant-like organisms that drift in the oceans. Despite their tiny size, these organisms are abundant in marine environments. They are fundamental to marine ecosystems.
What Are Phytoplankton?
Phytoplankton are photosynthetic organisms, converting sunlight into energy. They contain chlorophyll, which allows them to capture light, and they utilize carbon dioxide to create chemical energy. These single-celled organisms are found in both saltwater and freshwater environments, predominantly inhabiting the sunlit upper layers of the ocean, known as the euphotic zone, where light can penetrate. This zone extends to about 200 to 300 meters deep.
Phytoplankton exhibit significant diversity, encompassing various types of microorganisms. Some are photosynthetic bacteria, such as cyanobacteria, while others are single-celled algae, classified as protists. Common examples include diatoms, which are encased in silica shells, dinoflagellates, and coccolithophores, which are covered in chalky plates. This diversity contributes to their roles in marine ecosystems.
The Foundation of Marine Life
Phytoplankton serve as the primary producers in the ocean, the base of marine food webs. They convert inorganic carbon from the atmosphere and seawater into organic compounds through photosynthesis, making energy available to other organisms. This process initiates the flow of energy that sustains marine life, from the smallest zooplankton to the largest whales.
Zooplankton graze on phytoplankton, transferring energy up the food chain. These zooplankton, in turn, become food for larger organisms, including small fish and invertebrates, which are then consumed by even larger predators. This sequence of consumption illustrates how energy moves through the aquatic ecosystem, with phytoplankton at the initial step.
Phytoplankton produce a substantial portion of Earth’s oxygen. Through photosynthesis, they release oxygen as a byproduct, contributing 50% to 80% of atmospheric oxygen. This amount is comparable to, or even exceeds, the oxygen produced by all land plants annually.
Phytoplankton’s Climate Connection
Phytoplankton absorb large amounts of carbon dioxide (CO2) from the atmosphere, playing a role in the global carbon cycle. During photosynthesis, they take in dissolved CO2 from the surface waters, incorporating it into their cellular structures. This process helps to lower the partial pressure of CO2 in the upper ocean, facilitating the absorption of more CO2 from the atmosphere.
The transfer of carbon to deeper ocean layers is known as the “biological carbon pump.” When phytoplankton die, their carbon-rich organic matter can sink to the ocean floor. Similarly, when zooplankton consume phytoplankton, their waste products, in the form of fecal pellets or aggregates, can also sink, carrying carbon to the deep ocean.
A portion of this carbon can be sequestered from the atmosphere for hundreds to thousands of years, reaching depths of 500 meters or more. This process helps regulate Earth’s climate by removing atmospheric CO2. The global biological carbon pump is estimated to transfer about 10 gigatonnes of carbon from the atmosphere to the deep ocean each year.
Threats to Phytoplankton and Their Impact
Phytoplankton face threats from environmental changes, including rising ocean temperatures, ocean acidification, and nutrient pollution. Ocean warming can alter phytoplankton productivity, sometimes causing declines in surface waters while deep-living phytoplankton may thrive. This shifts biomass distribution. These temperature changes can also affect nutrient availability by increasing water column stratification, reducing the mixing that brings nutrients from deeper waters to the surface.
Ocean acidification, caused by increased atmospheric CO2 absorption, lowers seawater pH. This change affects phytoplankton; some species experience reduced growth or die, while others flourish. For instance, it reduces iron availability, a necessary nutrient, which can decrease productivity. Such shifts in species composition can disrupt the delicate balance of marine food webs.
Nutrient pollution from agricultural runoff and wastewater introduces excess nitrogen and phosphorus into coastal waters. While nutrients are necessary, overabundance can lead to rapid, uncontrolled growth known as harmful algal blooms (HABs). These blooms can produce toxins harmful to marine life and humans. Even non-toxic blooms can deplete oxygen in the water when they decompose, creating “dead zones” that suffocate other aquatic organisms. A decline in phytoplankton abundance or significant changes in their community structure due to these threats could disrupt the marine food web, reduce oxygen production, and diminish the ocean’s capacity to absorb CO2, ultimately impacting global climate regulation.