Marine microbes living beneath seabed resort to cannibalism

In order to survive, marine microbes living buried beneath seabed sediments must resort to cannibalism, according to a new study.

In an effort to better understand how microorganisms break down carbon trapped in the sediments at the bottom of the ocean, a team of researchers at the Ludwig-Maximilian University of Munich conducted a genetic analysis of the enzymes secreted by resident microbial communities.

Extracellular enzymes are secreted by microbes to accelerate the breakdown of surrounding organic detritus. The byproducts of the breakdown process are reabsorbed as proteins that serve as fuel for the microbes.

Enzymes secreted into the surrounding sediment contain signature strings of amino acids that mark its extracellular destiny. The microbe's secretory mechanism recognizes this signature and allows the enzyme access to the outside world. These RNA fragments can be sequenced in the lab and used to study the microbes that released them.

"Using a novel bioinformatic method, we searched for evolutionarily conserved, and hence functionally important, amino-acid sequence motifs within these recognition sequences," William Orsi, professor of geomicrobiology at LMU, said in a news release. "In this way, we were able, for the first time, not only to use genetic data to deduce enzyme functions, but also to specifically identify those enzymes that are secreted by cells that live in these sediments."

Researchers sequenced the genetic material found in sediments recovered from deep-sea drilling sites situated off the coast of Peru. Some of the sediments were 2.8 million years old.

The researchers' analysis -- detailed this week in the journal Nature Microbiology -- showed different communities of bacteria, archaea and fungi buried in seabed sediments use an array of enzymes to breakdown carbohydrates, lipids and proteins into usable sources of energy.

The enzymes can also breakdown dead cells, allowing microbes to eat their own kind for additional sustenance.

"Many of the enzymes synthesized and secreted by fungal cells specifically attack the cell walls of archaea, while many of the extracellular enzymes released by bacteria can degrade the cell walls of fungi," Orsi said. "In other words, different classes of microbes apparently cannibalize one another's 'carcasses.'"

Marine sediments are the planet's largest carbon reserve. Scientists hope an improved understanding of the microbial communities living in the seabed will help them more accurately model how carbon and other organic molecules are recycled.

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