Life Beneath the Seafloor

Our knowledge about endolithic microorganisms associated with igneous rocks in the oceanic crust is very limited and to date, the gabbroic (deep intermediate layer between basalt and peridotite) microbial communities, have not yet been investigated. That's why in this study, 22 igneous rock samples were collected from 0.45-1391 meters below seafloor (mbsf) during the 'Integrated Ocean Drilling Program' and analyzed using microscopy, cloning, restriction fragment polymorphism (T-RFLP), DGGE, sequencing and gene microarray, in order to assess abundance, phylogenetic and metabolic diversity of the microbial population associated with the central dome of the Atlantis Massif (30°N – 42°W). An ocean core complex composed of deep crustal (gabbro) and upper mantle rocks (peridotite) that have been exposed at the surface as a result of faulting, providing thus, a rare opportunity to sample gabbroic microbes, generally beyond the reach of modern drilling technologies.

The gabbroic microbial community resulted to be relatively depauperate, consisting of a low diversity of proteobacterial lineages closely related to known hydrocarbon degraders, while little or no evidence of Archaea in either rock or seawater samples was found by the authors, despite their numerous attempts to amplify archaeal 16S rRNA genes. Prokaryotic cell densities of interior sections of core samples over the entire 1400 m interval were below the level of detection (<10³ cells cm^-3) and much lower than those normally reported for basalts and carbonates in other studies. These low cell densities were congruent with the low species diversity obtained by the authors with DGGE and T-RFLP analysis. Microbial diversity however, was found to vary with depth from 400–1400 mbsf and interestingly, this depth variance was related to rock alteration, with the most altered rock supporting the greatest microbial diversity. Probably because alteration could result in changing permeability and oxidation state of the rocks, providing thus, additional niches in the gabbroic rocks.

Subsequently, in order to provide insight into the potential metabolic diversity of the microbial community, the authors analyzed conserved regions of functional genes using a 'GeoChip' microarray. Results revealed that genes coding for hydrocarbon degradation (methane and toluene oxidation) and anaerobic respirations (nitrate, sulfate and iron reduction) were present in rock samples, as well as genes for carbon and nitrogen fixation, denitrification, ammonium-oxidation, organic contaminant degradation and metal toxicity/resistance. The predominant genes were found to code for organic contaminant degradation, carbon degradation, carbon fixation, methane oxidation and methane generation. For all these reasons, the authors suggested that the gabbroic layer hosts a complex microbial community able to degrade hydrocarbons, fix carbon and nitrogen and employ various non-oxygen electron acceptors in both aerobic and anaerobic conditions. 

In this case, the presence of seawater circulation processes in the upper 800 meters of oceanic crust, would provide the limited amount of oxygen required for aerobic processes, with a transition to anaerobic processes following oxygen depletion below 800 mbsf, where reducing conditions tend to prevail.

Since, marine basalts and gabbros are nearly identical in chemical composition, the authors initially hypothesized that similar organisms specialized for growth in subsurface igneous rocks would have been recovered also from gabbros. However, none of the clades endemic to basalts were found in gabbros and the high similarity of gabbroic microorganisms to hydrocarbon degrading bacteria, suggested instead, that gabbroic microflora are not ocean crust endemic specialists, but rather they are transient generalist microbes, able to survive in a variety of hydrocarbon-rich environments, including deep subsurface igneous rocks, such as those analyzed in this study. Abiotic production of unbranched alkanes (including methane) through serpentinization reactions in the Earth’s crust was suggested by the authors, raising the intriguing possibility that these deep hydrocarbons could provide carbon and energy to extant microbial communities and support complex micro-endolithic populations in the interior of the oceanic crust. In conclusion, since ocean crust covers 70% of the earth’s surface, endolithic microbial processes in this subseafloor environment, do really have the potential to significantly influence oceanic and atmospheric biogeochemistry and so, future efforts should be directed towards quantifying the role and the importance of these processes on a global scale.

Reference:

Mason O. U., Di Meo-Savoie C., Van Nostrand J. D., Zhou J., Fisk M. R., Giovannoni S. J. (2010). First investigation of the microbiology of the deepest layer of ocean crust. PLoS ONE5, e15399.