Oceans Eat CO2… Photosynthesis Regulates Carbon Cycle

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Posted By : ScienceIndia Administrator

Carbon fixation by oceanic phytoplankton plays a key role in the global carbon cycle. Oceans comprise of a very large reservoir of carbon. Almost half of the ocean’s carbon fixation is by phytoplankton and more than half of the earth’s oxygen production is via phytoplankton comprising of algae, cyanobacteria and small planktonic organisms.

Blue-green algae, or cyanobacteria, grow in vast numbers in sunlit surface waters of the oceans, known as photic zone. They use sunlight to ‘fix’ carbon by converting carbon dioxide into sugars and other organic compounds through photosynthesis. Cyanobacteria belong to the ‘picophytoplankton’, the tiniest phytoplankton. Until now they have been thought to dominate carbon fixation in the open ocean, with species belonging to the genera Prochlorococcus and Synechococcus being particularly abundant.

Like all bacteria, cyanobacteria are prokaryotes are distinguished from eukaryotes by the absence of a nucleus. However, although much less abundant than cyanobacteria, the photic zone also has a high biomass of small eukaryotic phytoplankton capable of carbon fixation.

It is estimated that 258 Gigatonnes of global CO2 is fixed annually via photosynthesis taking place on both terrestrial and aquatic levels. Significant amounts of aquatic carbon fixation is done by eukaryotic phytoplankton  contributing up to 44% of the totalCO2, compared to 33% by terrestrial plants. This is most likely because eukaryotic phytoplankton cells are bigger than cyanobacteria allowing them to assimilate more fixed carbon. Small eukaryotic phytoplankton can obtain carbon by feeding on bacteria, supplementing carbon fixed through photosynthesis.

It is possible that some of the organic carbon from eukaryotic phytoplankton is eventually exported from the photic zone to the deep ocean, rather than being returned to the atmosphere in  the form of carbon dioxide.

Given their clear importance, now scientists are understanding the factors controlling growth of small eukaryotes in the oceans. In this way, oceanic carbon fixation can contribute as a major source of biological CO2 sequestration. They play a key role in oceanic carbon fixation and exploring the phytoplankton. Widespread sample collection across the world oceans can give more data for predicting which group is responsible for higher carbon fixation. 

Authors: M V Rohit (UGC-Senior Research Fellow) and K Rajesh (CSIR-Senior Research Fellow), Bioengineering and Environmental Science Lab, EEFF Department CSIR-Indian Institute of Chemical Technology

(Image source: https://www.nature.com/articles/483S17a)

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