Cryptic metabolisms in anoxic subseafloor sediment

Abstract

Microbial gene expression in anoxic subseafloor sediment was recently explored in the Baltic Sea and the Peru Margin. Our analysis of these data reveals diverse transcripts encoding proteins associated with neutralization of reactive oxygen species, including catalase, which may provide an in situ source of oxygen. We also detect transcripts associated with oxidation of iron and sulfur, and with reduction of arsenate, selenate and nitrate. Given limited input of electron acceptors from outside the system, these results suggest that the microbial communities use an unexpectedly diverse variety of electron acceptors. Products of water radiolysis and their interactions with sediment continuously provide diverse electron acceptors and hydrogen. Cryptic microbial utilization of these oxidized substrates and H₂ may be an important mechanism for multi-million-year survival under the extreme energy limitation in subseafloor sediment.

Fig 1

Sulfate (cyan), methane (red) and cell counts (black) profiles for sites (A) M0059E, (B) M0063E and (C) 1229D. Data from Expedition 347 Science Party (2015a), Expedition 347 Science Party (2015b) and Shipboard Scientific Party (2003).

Fig 2

Dot plots showing lineage‐specific expression levels of PETs related to catabolic metabolisms and ROS neutralization. Transcripts from different taxonomic groups are colour‐coded at the phylum level (Class level for Proteobacteria). Sites are depicted by depth in meters below the seafloor (mbsf) and colour coded by sulfate presence (orange) or depletion (purple). The dot sizes represent transcript copies, which represent the number of gene variants of each gene detected in the metatranscriptomes.