Ipsa Tripathy
Bhubaneswar: When we think about the organisms responsible for producing oxygen, forests and trees are usually the first things that come to mind. But a large part of Earth’s oxygen production happens somewhere far less visible: the upper layers of the world’s oceans. Floating in these sunlit waters are microscopic organisms called phytoplankton. They are too small to be seen individually, yet collectively they perform one of the most important biological processes on Earth. Through photosynthesis, marine phytoplankton produce roughly half of the oxygen generated on Earth.
Their importance, however, goes far beyond oxygen. Phytoplankton form the foundation of most marine food webs and play a major role in the movement of carbon between the atmosphere and the ocean.
What Exactly Are Phytoplankton?
The word phytoplankton broadly refers to microscopic organisms that drift with water currents and can carry out photosynthesis. They are not a single species or even a single biological group. Phytoplankton include cyanobacteria, diatoms, dinoflagellates, coccolithophores and other microscopic algae. Some are bacteria, while others are protists or plant-like organisms.
Like terrestrial plants, photosynthetic phytoplankton use sunlight to convert carbon dioxide and water into organic matter, releasing oxygen as a by-product. They are therefore classified as primary producers organisms that convert inorganic materials and energy from sunlight into organic matter that can support other organisms.
The Ocean’s Invisible Oxygen Factories
Scientists estimate that approximately half of Earth’s oxygen production occurs in the ocean, with the majority coming from marine plankton. This does not mean that half of every breath we take consists of oxygen newly produced by phytoplankton.
Oxygen is continuously produced and consumed throughout the planet. Marine organisms also use oxygen for respiration, and microorganisms consume oxygen while decomposing dead organic matter. Much of the atmospheric oxygen we breathe has accumulated over hundreds of millions of years.
The important scientific point is that marine photosynthesis contributes an enormous share of Earth’s ongoing oxygen production. One particularly important organism is Prochlorococcus, a tiny photosynthetic cyanobacterium. NOAA estimates that it may account for up to 20% of the oxygen production in the entire biosphere. An organism invisible to the naked eye can therefore have a planetary-scale influence.
The Foundation of the Ocean Food Web
Phytoplankton are also the starting point of most marine food chains. Small animals known as zooplankton feed on them. Fish and other marine organisms then consume zooplankton, creating a food pathway that extends upward to larger fish, seabirds and marine mammals.
In this way, the microscopic organisms floating near the ocean surface support organisms thousands of times larger than themselves. NASA describes phytoplankton as the foundation of the oceanic food web, with their abundance and distribution influencing organisms ranging from zooplankton and shellfish to fish and whales. A change in phytoplankton populations can therefore affect much larger parts of the marine ecosystem.
Phytoplankton and the Carbon Cycle
Their role in the climate system is equally important. During photosynthesis, phytoplankton remove carbon dioxide from surrounding seawater and incorporate carbon into their biomass. When phytoplankton are eaten, some of that carbon moves through the marine food web. When they die or produce organic waste, a portion of the material sinks into deeper waters.
Some carbon eventually reaches the deep ocean and seafloor, where it can remain isolated from the atmosphere for extended periods. This process forms part of the biological carbon pump. NASA estimates that this biological process transfers approximately 10 billion tonnes of carbon from the atmosphere to the deep ocean each year. However, not all carbon captured by phytoplankton remains permanently in the deep ocean. Much of it is returned to surface waters and the atmosphere through respiration, decomposition and other processes.
Why Phytoplankton Bloom
Phytoplankton require sunlight and nutrients to grow. Their abundance changes naturally with seasons and environmental conditions. Nutrients such as nitrogen and phosphorus are essential, while other elements including iron in some ocean regions can limit growth. When conditions become favourable, phytoplankton populations can increase rapidly, producing what scientists call a phytoplankton bloom.
These blooms can sometimes become large enough to be detected from space because the chlorophyll inside phytoplankton changes the colour of surface waters. Satellites therefore provide scientists with an important method for monitoring marine ecosystems across enormous areas. NASA’s satellite observations use measurements of chlorophyll as an indicator of phytoplankton biomass and distribution.
Not Every Bloom Is Harmful
An important distinction is often missed when discussing phytoplankton. Phytoplankton blooms are natural and are not automatically dangerous. Most algal species are not harmful. However, some species can produce toxins or cause ecological problems when they occur in extremely high concentrations. These events are known as harmful algal blooms.
When large quantities of algae die and decompose, microorganisms can consume substantial amounts of dissolved oxygen. This can produce hypoxic or oxygen-depleted conditions, sometimes creating areas known as “dead zones” where many marine organisms cannot survive. Therefore, phytoplankton can simultaneously be essential to healthy marine ecosystems and, under particular conditions, contribute to ecological problems.
A Changing Ocean
Phytoplankton populations are sensitive to changes in their environment. Ocean temperature, nutrient availability, light conditions and water-column mixing all influence where different phytoplankton communities can thrive. Warming can also increase ocean stratification, making it more difficult for nutrient-rich deep water to reach sunlit surface waters in some regions.
Because phytoplankton respond rapidly to environmental changes, scientists study them as important indicators of changing ocean conditions. Modern satellite missions are making this research increasingly sophisticated. NASA’s PACE mission, for example, uses hyperspectral observations to study ocean colour and improve understanding of phytoplankton communities and their ecological functions.
The Smallest Organisms, the Largest Role
Phytoplankton may be microscopic, but their influence reaches across the entire planet. They produce a substantial share of Earth’s oxygen, form the foundation of marine food webs and help move carbon from the atmosphere into the ocean. They also remind us that the most important components of an ecosystem are not always the most visible.
A forest can be seen from kilometres away. A whale can be spotted from a boat. But the microscopic organisms supporting much of the ocean’s biological productivity remain largely invisible to us. Yet every day, in the sunlit surface waters of the world’s oceans, billions upon billions of these tiny organisms continue to photosynthesise. Their world is hidden. Their influence is not.