Bacterioplankton

Bacterioplankton refers to the bacterial component of the plankton that drifts in the water column. The name comes from the Ancient Greek word πλανκτος (planktos), meaning 'wanderer' or 'drifter', and bacterium, a Latin term coined in the 19th century by Christian Gottfried Ehrenberg. They are found in both seawater and freshwater. Bacterioplankton refers to the bacterial component of the plankton that drifts in the water column. The name comes from the Ancient Greek word πλανκτος (planktos), meaning 'wanderer' or 'drifter', and bacterium, a Latin term coined in the 19th century by Christian Gottfried Ehrenberg. They are found in both seawater and freshwater. Bacterioplankton occupy a range of ecological niches in marine and aquatic ecosystems. They are both primary producers and primary consumers in these ecosystems and drive global biogeochemical cycling of elements essential for life (e.g., carbon and nitrogen). Many bacterioplankton species are autotrophic, and derive energy from either photosynthesis or chemosynthesis. Photosynthetic bacterioplankton are often categorized as picophytoplankton, and include major cyanobacterial groups such as Prochlorococcus and Synechococcus. Other heterotrophic bacterioplankton are saprotrophic, and obtain energy by consuming organic material produced by other organisms. This material may be dissolved in the medium and taken directly from there, or bacteria may live and grow in association with particulate material such as marine snow. Bacterioplankton play critical roles in global nitrogen fixation, nitrification, denitrification, remineralisation and methanogenesis. Bacterioplankton abundance depends on environmental variables like temperature, nutrient availability and predation. Like other small plankton, the bacterioplankton are preyed upon by zooplankton (usually protozoans), and their numbers are also controlled through infection by bacteriophages. Photosynthetic bacterioplankton are responsible for a large proportion of the total primary production of aquatic food webs, supplying organic compounds to higher trophic levels. These bacteria undergo oxygenic and anoxygenic photosynthesis. Differences between these processes can be seen in the byproducts produced, the primary electron donor, and the light harvesting pigments used for energy capture. Cyanobacteria are a large group of photosynthetic bacterioplankton, often growing as cells or in filamentous colonies. These organisms are the dominant group of bacterioplankton using oxygenic photosynthesis in aquatic ecosystems. Cyanobacteria, along with photosynthetic eukaryotes, are responsible for approximately half of the total global primary production making them key players in the food web. They use photosynthesis to generate energy in the form of organic compounds and produce oxygen as a byproduct. Major light harvesting pigments include chlorophylls, phycoerytherin, phycocyanin and carotenoids. The majority of cyanobacteria found in marine environments are represented by the genera Synechococcus and Prochlorococcus. Synechococcus is cosmopolitan, having been reported across temperate and tropical waters. Prochlorococcus is a very small in size and is found mainly in the euphotic zone of tropical waters. Factors including light, nutrients, and temperature can cause cyanobacteria to proliferate and form harmful blooms. Cyanobacteria blooms can cause hypoxia and produce high levels of toxins, impacting other aquatic organisms as well as causing illnesses in humans. Some Cyanobacteria are capable of nitrogen fixation.The genus Anabaena uses specialized cells called heterocysts to physically separate nitrogen fixation and photosynthesis. Trichodesmium is an example of cyanobacteria that is capable of fixing nitrogen through an alternative photosynthetic pathway. Other photosynthetic bacterioplankton, including purple and green bacteria, undergo anoxygenic photosynthesis in anaerobic conditions. The pigments synthesized in these organisms are sensitive to oxygen. In purple bacteria the major pigments include bacteriochlorophyll a and b and carotenoids. Green bacteria have different light harvesting pigments consisting of bacteriochlorophyll c, d and e. These organisms do not produce oxygen through photosynthesis or use water as a reducing agent. Many of these organisms use sulfur, hydrogen or other compounds as an energy source to drive photosynthesis. Most of these bacterioplankton are found in anoxic waters, including stagnant and hypersaline environments. Heterotrophic bacterioplankton rely on the available concentration of dissolved organic matter in the water column. Usually these organisms are saprophytic, absorbing nutrients from their surroundings. These heterotrophs also play a key role in the microbial loop and the remineralization of organic compounds like carbon and nitrogen. Pelagibacterales, also known as members of an alphaproteobacteria clade, are the most abundant bacterioplankton in the oceans. Members of this group are found in waters with low nutrient availability and are preyed on by protists. Atmospheric carbon is sequestered into the ocean by three main pumps which have been known for 30 years: the solubility pump, the carbonate pump, and the biological carbon pump (BCP). The biological carbon pump is a vertical transmission pump driven mainly by the sinking of organic rich particles. Bacterial phytoplankton near the surface incorporate atmospheric CO2 and other nutrients into their biomass during photosynthesis. At the time of their death these phytoplankton, along with their incorporated carbon, sink to the bottom of the ocean where the carbon remains for thousands of years. The other biologically mediated sequestration of carbon in the ocean occurs through the microbial pump. The microbial pump is responsible for the production of old recalcitrant dissolved organic carbon (DOC) which is >100 years old. Plankton in the ocean are incapable of breaking down this recalcitrant DOC and thus it remains in the oceans for 1000's years without being respired. The two pumps work simultaneously, and the balance between them is believed to vary based on the availability of nutrients. Overall, the oceans act as a sink for atmospheric CO2 but also release some carbon back into the atmosphere. This occurs when bacterioplankton and other organisms in the ocean consume organic matter and respire CO2, and as a result of the solubility equilibrium between the ocean and the atmosphere.