Nanocalorimetric Characterization of Microbial Activity in Deep Subsurface Oceanic Crustal Fluids.
Although fluids within the upper oceanic basaltic crust harbor a substantial fraction of the total prokaryotic cells on Earth, the energy needs of this microbial population are unknown. In this study, a nanocalorimeter (sensitivity down to 1.2 nW ml(-1)) was used to measure the enthalpy of microbially catalyzed reactions as a function of temperature in samples from two distinct crustal fluid aquifers. Microorganisms in unamended, warm (63°C) and geochemically altered anoxic fluids taken from 292 meters sub-basement (msb) near the Juan de Fuca Ridge produced 267.3 mJ of heat over the course of 97 h during a step-wise isothermal scan from 35.5 to 85.0°C. Most of this heat signal likely stems from the germination of thermophilic endospores (6.66 × 10(4) cells ml(-1) FLUID) and their subsequent metabolic activity at temperatures greater than 50°C. The average cellular energy consumption (5.68 pW cell(-1)) reveals the high metabolic potential of a dormant community transported by fluids circulating through the ocean crust. By contrast, samples taken from 293 msb from cooler (3.8°C), relatively unaltered oxic fluids, produced 12.8 mJ of heat over the course of 14 h as temperature ramped from 34.8 to 43.0°C. Corresponding cell-specific energy turnover rates (0.18 pW cell(-1)) were converted to oxygen uptake rates of 24.5 nmol O2 ml(-1) FLUID d(-1), validating previous model predictions of microbial activity in this environment. Given that the investigated fluids are characteristic of expansive areas of the upper oceanic crust, the measured metabolic heat rates can be used to constrain boundaries of habitability and microbial activity in the oceanic crust.
Center for Dark Energy Biosphere Investigations, NASA Astrobiology Institute, University of Southern California, Los Angeles CA, USA.
Alberto
A
Robador
Department of Earth Sciences, University of Southern California, Los Angeles CA, USA.
Douglas E
DE
LaRowe
Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, HonoluluHI, USA; Hawaii Institute of Marine Biology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, KaneoheHI, USA.
Sean P
SP
Jungbluth
Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu HI, USA.
Huei-Ting
HT
Lin
Hawaii Institute of Marine Biology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Kaneohe HI, USA.
Michael S
MS
Rappé
Department of Earth Sciences, University of Southern California, Los AngelesCA, USA; Department of Biological Sciences, University of Southern California, Los AngelesCA, USA.
Kenneth H
KH
Nealson
Department of Earth Sciences, University of Southern California, Los AngelesCA, USA; Department of Biological Sciences, University of Southern California, Los AngelesCA, USA.
Jan P
JP
Amend
calorimetry; cell-specific metabolic rates; crustal fluids; microbial energy turnover; subsurface biosphere