Endospore abundance, microbial growth and necromass turnover in deep sub-seafloor sediment

A new approach, the d : l -amino-acid model, is used to quantify the distributions and turnover times of living microbial biomass, endospores and microbial necromass, and to determine their role in the sub-seafloor carbon budget. Sedimentary microbes play the long game Since the discovery of the deep marine biosphere, which includes microbial communities in deep sub-floor sediments that contribute perhaps one-tenth of all living biomass on Earth, microbiologists have been trying to explain how microorganisms utilize the extremely low supply of carbon and energy in this unpromising habitat. Lomstein et al . have quantified diagnostic microbial cell components in a deep-sea sediment drilling core from the continental shelf off Peru, and have used these data to calculate microbial biomass, the mass of dead microbes and the mass of bacterial spores. They estimate that microbial biomass turnover occurs on a timescale of hundreds to thousands of years. Two decades of scientific ocean drilling have demonstrated widespread microbial life in deep sub-seafloor sediment, and surprisingly high microbial-cell numbers. Despite the ubiquity of life in the deep biosphere, the large community sizes and the low energy fluxes in this vast buried ecosystem are not yet understood 1 , 2 . It is not known whether organisms of the deep biosphere are specifically adapted to extremely low energy fluxes or whether most of the observed cells are in a dormant, spore-like state 3 . Here we apply a new approach — the d : l -amino-acid model — to quantify the distributions and turnover times of living microbial biomass, endospores and microbial necromass, as well as to determine their role in the sub-seafloor carbon budget. The approach combines sensitive analyses of unique bacterial markers (muramic acid and D -amino acids) and the bacterial endospore marker, dipicolinic acid, with racemization dynamics of stereo-isomeric amino acids. Endospores are as abundant as vegetative cells and microbial activity is extremely low, leading to microbial biomass turnover times of hundreds to thousands of years. We infer from model calculations that biomass production is sustained by organic carbon deposited from the surface photosynthetic world millions of years ago and that microbial necromass is recycled over timescales of hundreds of thousands of years.