Lava Eaters

The aim of this study was to assess the abundance, species richness and phylogenetic diversity of endolithic and epilithic microbial communities inhabiting young, unsedimented ocean crust at the sea floor. Hence, basaltic lavas of various ages and alteration states were sampled by the authors from the East Pacific Rise (EPR) and around Hawaii and then analyzed using quantitative PCR, FISH and microscopy. The PCR measurements of the glassy rinds of lava flows showed that the total bacterial and archaeal cell densities were ranging from 3x10⁶ to 1x10⁹ cells/g and that bacteria were dominating (88–96%) all the rock samples examined. This results were then confirmed by the FISH analysis, which revealed dense populations of Bacteria (significantly more abundant than Archaea) exhibiting cell abundances of 3-4 orders of magnitude greater than in the overlying deep sea waters (8x10³ - 9x10⁴ cells/ml) where half of the cells were instead, Archaea.

Subsequently, in order to evaluate in more detail the community composition, the authors used fulllength 16S ribosomal RNA gene clone libraries constructed from basaltic lavas and surrounding sea water samples. These phylogenetic analyses revealed that both the basalt-hosted biospheres (EPR and Hawaii), were harbouring high-richness bacterial communities and that community membership was shared between these sites. A statistical approach was then used to evaluate the species richness (number of operational taxonomic units) as compared to other oceanic environments analyzed in other studies (e.g. an hydrothermal white smoker, the upper water column of the Sargasso Sea, hydrothermal fluids from the Mid-Atlantic Ridge and deep-subsurface sediments from the Nankai Trough). These comparative analysis, revealed that abundance, phylogenetic diversity and richness of Bacteria in these other deep-sea environments were clearly lower and much different than EPR and Hawaii deep-sea basalts. The 21 taxonomic groups recovered from basalt were dominated by Proteobacteria (68% and 66% of all sequences in EPR and Hawaii respectively), while non-Proteobacteria groups included Plantomycetes (8%/5%), Actinobacteria (7%/8%), Bacteroidetes (4%/1%), Acidobacteria (3%/4%) and Verrucomicrobia (2%/2%). Interestingly, the OTU richness for the two geographically separated basalt communities (EPR and Hawaii) showed considerable overlap in community membership, suggesting that oceanic basalt microbes are widely distributed among this biotope.

These differences in phylogenetic diversity, species richness, and total biomass between the basaltic lavas and overlying sea water raised questions about what energy source fuel this biosphere. Potential energy sources capable of sustaining microbial life in ocean crust include hydrothermal input of manganese and iron (chemolithoautotrophic growth) and dissolved organic carbon in sea water or hydrothermal fluids (heterotrophic growth). However, according to the authors, the most plausible explanation is that oceanic lithosphere exposed at the sea floor undergoes seawater-rock alteration reactions and these reactions are capable of supplying sufficient energy for chemolithoautotrophic microbial growth. Lava surfaces in fact, are composed predominantly of volcanic glass, a highly reactive rock component that contains reduced elemental species such as iron, sulphur and manganese. Oxygen and nitrate in deep sea water oxidize these  constituents and chemolithoautotrophic microorganisms can potentially exploit the free energy changes associated with these redox reactions for their metabolic requirements. Laboratory studies have already demonstrated that iron-oxidizing bacteria isolated from the sea floor are able to use rock and minerals, including glassy basalt, for metabolism and growth. The authors estimated that about 6x10⁷-6x10⁹ cells per g basalt may be supported through these reactions and actually, cell densities in EPR basalts were falling exactly within this range. So in conclusion, alteration reactions in the upper ocean crust may fuel microbial ecosystems at the sea floor, which constitute a trophic base of the basalt biotope, with important implications for deep-sea carbon cycling and chemical exchange between basalt and sea water. This hypothesis supports the understanding of the phylogenetically rich and distinct nature of the basalt biotope. The enrichment of taxa from diverse metabolic groups may result from the establishment of chemical microenvironments within or on rock cavities and surfaces during alteration, mineral precipitation and biofilm formation. This niche creation would allow for a greater variety of redox reactions and metabolic pathways (e.g. heterotrophic, anaerobic, or reductive) including those supporting complex organotrophic and mixotrophic communities. 

Reference:

Santelli, C.M., B.N. Orcutt, E. Banning, W. Bach, C.L. Moyer, M.L. Sogin, H. Staudigel, and K.J. Edwards. (2008). Abundance and diversity of microbial life in ocean crust. Nature 453:653-656.