Deep ocean bacteria discovered to play large role in carbon capture
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.
“We knew these bacteria were there and involved in the global carbon cycle, but their role is so much larger than what scientists previously thought,” said Maria Pachiadaki, postdoctoral scientist at Bigelow Laboratory.
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.
“Before genomics techniques, the dark ocean was a black box, because microorganisms from this environment refuse to grow in research laboratories,” said Ramunas Stepanauskas, a senior research scientist at Bigelow Laboratory and director of the Single Cell Genomics Center. “Now, using contemporary tools that were developed by our group, we can open this black box and understand who is living there, what they are doing, and how they are doing it.”
The research behind this Science paper began with an effort to identify new groups of microscopic organisms in the mesopelagic zone. According to Pachiadaki, the previously accepted explanation for carbon capture in the dark ocean did not seem correct under close inspection. Archaea, a much more abundant group of microorganisms in the dark ocean, had been credited with doing most of the work, but the math just didn’t add up. The team set out to discover the unidentified organisms that were the true champions of carbon dioxide capture in the vast dark ocean.