Marine microorganisms are single-celled or cell-cluster organisms in the ocean, too small to be viewed with the naked eye. This diverse group includes bacteria, archaea, viruses, and eukaryotes like protists and fungi. A single drop of seawater can harbor millions of individual microbes, and they collectively account for the vast majority of the ocean’s biomass.
Major Groups of Marine Microbes
The most abundant cellular microbes in the ocean are bacteria. These prokaryotic organisms are found in every marine habitat and exhibit a diversity of metabolic strategies. They are involved in nearly every chemical process in the ocean, from decomposition to primary production. Their functional variety makes them a significant component of marine ecosystems.
Archaea represent another domain of single-celled life, distinct from bacteria. While visually similar, their genetic makeup reveals a closer relationship to more complex organisms. Many archaea are known as extremophiles, thriving in inhospitable environments like the scorching waters of hydrothermal vents or highly saline brine pools. They are not limited to these zones and are also found throughout the water column.
The most numerous biological entities in the ocean are not cellular but are marine viruses. Scientists estimate there are about 10 billion viruses in a single liter of seawater. The primary role of these viruses is to infect other microorganisms, particularly bacteria. By causing their host cells to burst, they regulate microbial populations and release organic matter that other organisms can use.
Microbial eukaryotes are single-celled organisms with a more complex cellular structure, including a nucleus. This group includes marine fungi and a wide array of protists. Among the most notable marine protists are phytoplankton, microscopic, plant-like organisms that perform photosynthesis. Important phytoplankton groups include diatoms, which build intricate shells from silica, and dinoflagellates.
Where Marine Microorganisms Live
Marine microbes colonize every habitat the ocean offers, from the sun-drenched surface to the deepest trenches. In the sunlit photic zone, which extends down about 200 meters, photosynthetic microbes flourish. Here, phytoplankton use sunlight to convert carbon dioxide into organic matter, forming the base of the food web in this upper layer.
Below the sunlit zone lies the aphotic zone, a realm of perpetual darkness, immense pressure, and cold temperatures. Microbes in the deep sea cannot rely on sunlight for energy. Instead, many survive by consuming “marine snow,” a continuous shower of sinking organic detritus from the productive waters above.
Some of the most unique microbial habitats are extreme environments on the seafloor. At hydrothermal vents, cracks in the ocean floor release superheated, mineral-rich fluids. Here, chemosynthetic microbes harness chemical energy from compounds like hydrogen sulfide to produce food, supporting entire ecosystems. A similar process occurs at cold seeps, where hydrocarbons and sulfide-rich fluids emerge from sediments.
The seafloor itself, composed of layers of sediment, is an enormous microbial habitat. Trillions of microbes live buried within these layers, extending hundreds of meters below the seabed. In this environment, they slowly decompose organic material that has settled over millennia, participating in long-term nutrient cycles.
The Role of Microbes in Ocean Ecosystems
As primary producers, microorganisms form the foundation of the marine food web. Photosynthetic microbes like phytoplankton use solar energy to create organic carbon, while chemosynthetic microbes use chemical energy in dark environments. This production supports all marine life, from zooplankton to whales.
These organisms also function as the ocean’s recyclers. As decomposers, bacteria and archaea break down dead organic material and waste products. This decomposition releases nutrients, such as nitrogen and phosphorus, back into the water in a form that phytoplankton can use. This recycling is a part of keeping marine ecosystems productive.
This nutrient regeneration is part of a concept known as the “microbial loop.” In this process, dissolved organic matter, too small for most larger organisms to consume, is taken up by bacteria. These bacteria are then eaten by slightly larger microbes, which are in turn consumed by other plankton. The loop recovers energy and nutrients that would otherwise be lost, reintroducing them into the main food web.
Global Impact of Marine Microorganisms
The collective activities of marine microorganisms have a profound impact on the planet. A significant portion of the oxygen in Earth’s atmosphere is generated by marine phytoplankton. Through photosynthesis, these microbes absorb carbon dioxide and release oxygen. The cyanobacterium Prochlorococcus, one of the most abundant photosynthetic organisms, is a major contributor to this global oxygen supply.
Marine microbes also play a part in regulating Earth’s climate through the biological carbon pump. As phytoplankton photosynthesize, they incorporate atmospheric carbon dioxide into their cells. When these microbes die, a portion of them sink to the deep ocean, carrying their captured carbon with them. This process transports carbon from the atmosphere to the deep sea, where it can be sequestered for hundreds or thousands of years.
The influence of these organisms extends to weather patterns. Certain marine bacteria can produce particles that get aerosolized into the atmosphere and act as cloud condensation nuclei. Water vapor condenses on these microbial particles, contributing to the formation of clouds. These clouds can influence regional weather by reflecting sunlight and producing precipitation.
Human Applications and Interactions
The unique biochemistry of marine microbes offers a rich source for biotechnology and medicine. Scientists have isolated compounds from marine bacteria and fungi that have led to new drugs, including antibiotics and anti-cancer agents. Enzymes from microbes that thrive in extreme temperatures or pressures are used in industrial processes, such as in detergents, food production, and biofuels.
Certain marine bacteria have a natural ability to break down harmful substances, a capability harnessed for bioremediation. Following oil spills, specific hydrocarbon-degrading bacteria are used to help clean up the damage. These microbes consume the oil as a food source, converting toxic petroleum compounds into less harmful substances.
Interactions with marine microbes can sometimes be harmful. Under certain conditions, some phytoplankton species can grow explosively, creating harmful algal blooms (HABs), also called “red tides.” These blooms can produce toxins that accumulate in shellfish, harming marine life and posing a health risk to humans who consume contaminated seafood.