Earth’s hidden carbon recycling: Sulfur bacteria work together to break down organic matter from the seabed

Sulfate-reducing bacteria break down most of the earth’s oxygen-free zone, especially the organic carbon at the seabed. Among these important microorganisms, the bacterial Desulfobacteria family stands out. This is because its members can decompose a wide variety of compounds, including those with low degradability from the final product, carbon dioxide (CO2).

A team of researchers led by Dr. Lars Wehrbrand and Dr. Ralph Lavas from the University of Oldenburg in Germany have investigated the role of these microorganisms in detail and published the results of a comprehensive study in advances in scientific journals.

The team reports that bacteria are distributed worldwide and have complex metabolism that exhibits modular function. For example, all the strains studied have the same central metabolic architecture for harvesting energy.

However, some strains have additional strain-specific molecular modules that allow for the use of a variety of organic materials. Researchers attribute this group of environmental successes to this multi-purpose modular system. They also explain that their research provides new analytical tools to further promote our understanding of the role of sulfate-reducing microorganisms in the global carbon cycle and their association with climate.

Living at thermodynamic limits

“These sulfate reducing agents live on thermodynamic limits,” explains Labus, who heads the general and molecular microbiology working group at the Institute for Marine Environmental Biology (ICBM) at the University of Oldenburg. These bacteria use sulfates rather than oxygen for respiration, harvesting only a small fraction of the energy they can extract from the decomposition of aerobic substances. However, they are very active and play an important role in the failure of organic matter at the seabed.

“It is estimated that sulfate-reducing bacteria account for more than half of the decomposition of the seabed, especially in coastal waters and shelves, where large amounts of organic matter are deposited,” Lavas said.

He states that the dominant members of bacterial communities often belong to the family Desulfobacterice, and the activity of these microorganisms is clearly visible in environments such as mud-blowing. “This forms dirty hydrogen sulfide and a unique black iron sulfide precipitate,” he explains.

However, little is known about the role played by members of the Desulfubacteriaceae in the degradation of organic matter at a global level or regarding the underlying molecular mechanisms. To get a more detailed overview, the team first analyzed the global prevalence of these sulfate-reducing bacteria. Research in related literature reveals that they are distributed worldwide and occur in all marine regions between the Arctic and Antarctic. Especially in hypoxia or oxygen-free environments, as expected.

Similar molecular strategies to decompose organic compounds

In the next step, the researchers cultivated six very different strains of the Desulfobacteria family.

“Some experts can only break down certain compounds, while others can utilize a wide range of substances. Some are small and spherical, while others are elongated or even thread-like.”

To decipher their metabolism, researchers provided microorganisms with a total of 35 substances (substrates) ranging from simple fermentation products to long chain fatty acids and inadequately degradable aromatic compounds. A total of 80 test conditions were used for the six strains studied. The team then analyzed which genes were activated during the degradation of these substances and which proteins used in this process. It has been revealed that different strains use very similar molecular strategies to break down substances, and all six strains also use the same energy-efficient pathway for central metabolism.

Researchers conclude that the Desulfobacteriaceae are working together like a team, and consequently, they can degrade large pools of different substrates under various geochemical conditions and at a wide range of different geographical locations.

“There is no single, dominant important species,” Lavas emphasizes. Instead, bacteria act as a cooperative community similar to a soccer team.

“Every team has goalkeepers and strikers, but each team does things in its own way,” adds Wöhlbrand. This versatility may explain why the Desulfobacteriaceae is one of the most widespread sulfate reducing agents in the world.

Along with Dr. Michael Schrother from the Institute of Technology in Munich, Germany, researchers investigated whether genetic blueprints for certain key modules of metabolic networks could be detected in sediment samples. In fact, they discovered selected genes in almost all analyzed samples taken from marine regions ranging from shallow to deep seas, including nutrient-rich estuaries, nutrient-rich deep-sea springs from the oxygen-rich Black Sea, and sediments.

The team first concluded that all analyses highlight the important role that Desulfobacteriaceae plays in carbon decay at a global level, and secondly, it shows that the genes investigated can be used as analytical tools to study microbial activity directly on the seabed.

“The importance of sulfate reducing agents in the carbon cycle has probably been underestimated up until now,” said Dr. Michael Wincrofer, Ph.D. of the Institute of Biological and Environmental Sciences at the University of Oldenburg, who was involved in the analysis.

Geophysicists add that the role of these anaerobic microorganisms in the carbon decomposition process in coastal regions may increase in the future.