Role of Marine Bacteria in Global Carbon Cycle

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In another dimension, marine bacteria that live in the dark depths of the ocean play a newly discovered and significant role in the global carbon cycle.

The “dark ocean” — everything that lies below 200 meters — makes up 90 percent of the ocean. Very little is known about the microscopic life in this realm and its critical role in transforming carbon dioxide to cell material, proteins, carbohydrates and lipids. This freshly produced organic material can then be consumed by other marine organisms enhancing the productivity of the ocean.

Most dark ocean carbon is captured in the mesopelagic zone, which lies between 200 and 1000 meters below the ocean surface. Identities of microorganisms performing this process and the energy sources involved have remained a great mystery.

By analyzing the genomes found in seawater samples from this zone, scientists from Bigelow Laboratory for Ocean Sciences have now identified some of most significant contributors — nitrite-oxidizing bacteria.

These bacteria derive their energy from the oxidation of nitrogen compounds. They account for fewer than 5 percent of the microbial cells in the dark ocean, which previously led scientists to greatly underestimate their contribution.

This study reveals that despite their relatively low abundance, nitrite-oxidizing bacteria capture more than 1.1 gigatons of carbon dioxide in the mesopelagic zone annually. This is comparable to prior estimates of the total carbon captured in the entire dark ocean.

The team analyzed the composition of microbial mesopelagic communities based on genetic information in seawater samples from 40 locations around the world. They then used single cell genomics tools to fully sequence individual cells and examine their biology based on their genetic blueprints.

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