Microbial Behavior in Ancient Seafloor

At sedimentation rates of 1 millimeter per 1000 years, low rates of microbial community metabolism allow red clay sediments beneath the North Pacific Gyre to remain oxygenated tens of meters below the seafloor.

Micrograph of ancient microbesAlthough microbe communities can subsist deep in ancient seafloor clays for millions of years without fresh supplies of organic matter, low available energy may ultimately control community size. At sedimentation rates of 1 millimeter per 1000 years, low rates of microbial community metabolism allow red clay sediments beneath the North Pacific Gyre to remain oxygenated tens of meters below the seafloor. This subtropical gyre is one of the most oligotrophic regions in the ocean—low in plant nutrients but high in dissolved oxygen—and subsequently an ideal area for studying life under extreme energy limitations.

A recent study [1] found that the oxygen respiration rates dropped from 10 micromoles of oxygen per liter per year near the sediment-water interface to 0.001 micromoles of oxygen per liter per year at 30 meters of depth in clays that are 86 million years old. The cell-specific respiration rate also decreased with depth, but stabilized about 10 meters below the seafloor. Typically oxygen penetrates many meters into the seabed below gyres—indicating extremely low rates of microbial community respiration—compared to other areas where oxygen only penetrates millimeters to decimeters into the seabed.

As fully discussed in the Science article [1], the study found a similarity in metabolic rate per cell between sampling sites with very different organic matter mineralization rates and different terminal electron acceptors. The results suggest that the microbial communities studied may be living at the minimum energy flux needed for prokaryotic cells to subsist and that the total available energy flux ultimately controls deep biosphere community size.

Photo:  Micrograph of ancient microbes collected on R/V Knorr voyage 195 courtesy of Jens Kallmeyer.

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