Microbial biomass and metabolic roles in ocean sediments and soil ecosystems

¶ … Microbes

The Role of Microbes

Whitman and colleagues estimated in 1998 that the microbial population in the ocean's sedimentary layers represented between 55% and 86% of all microbial biomass on the Earth's surface and 27% to 33% of the biomass for all forms of life living on this planet (Kallmeyer, Pockalny, Adhikari, Smith, and D'Hondt, 2012). A more recent estimate, based on a much larger and diverse set of samples, reveals that that the seafloor microbial biomass represents only 0.18% to 3.6% of the global total. The reason for this revision is due to the finding that the highest concentrations of microbial biomass within the ocean's sedimentary layers are in the shallowest water depths close to land. The land-based microbial biomass is therefore the largest source of microbes on the Earth's surface by far.

Aquatic Microbes

The microbes that live in the sedimentary layers of the oceans are amazingly diverse and highly active metabolically (D'Hondt et al., 2004). In samples taken off the coast of Peru, at ocean depths ranging from 150 m to 5300 m, columnar samples revealed abundant microbial habitation in sediment depths ranging from 0 to 420 m and at 1? To 25?C. The concentrations of sulfate and nitrate in these samples were below those expected, suggesting these microbes actively reduce these compounds. The concentrations of other compounds or elements were unexpectedly high, such as inorganic carbon, ammonia, sulfide, methane, manganese, and iron, consistent with the active oxidation of these compounds or elements. The authors of this study also found evidence for the production and consumption of formate, acetate, lactate, hydrogen, ethane, and propane, in addition to the reduction of iron.

The chemical turnover of these compounds is not uniform, but depends on depth and temperature, with deeper sedimentary layers showing the greatest redox activity. The overall effect is the oxidation of organic matter and the removal of dissolved sulfate and nitrate from land-based organisms. The primary metabolic product of this process is methane.

Gut Microbes

The bodies of animals likewise serve as viable habitats for bacteria, fungi, viruses, and archaea, from the skin to the digestive tract (reviewed by Wang and Yang, 2013). Currents estimates suggest that there is 10 times the number of microbes inhabiting an adult human body than there are human cells. The overall effect is to increase the genetic material at the disposal of the human host, up to 150-fold more genes than is encoded within the human genome. Most of these organisms are commensal and help to maintain human health through energy metabolism, nutrient absorption, immune regulation, and combating disease-producing microbes. Any disruption in this mutually-beneficial relationship, however, can result in disease.