Microbial community diversity in seafloor basalt from the Arctic spreading ridges

Abstract

Microbial communities inhabiting recent (< or =1 million years old; Ma) seafloor basalts from the Arctic spreading ridges were analyzed using traditional enrichment culturing methods in combination with culture-independent molecular phylogenetic techniques. Fragments of 16S rDNA were amplified from the basalt samples by polymerase chain reaction, and fingerprints of the bacterial and archaeal communities were generated using denaturing gradient gel electrophoresis. This analysis indicates a substantial degree of complexity in the samples studied, showing 20-40 dominating bands per profile for the bacterial assemblages. For the archaeal assemblages, a much lower number of bands (6-12) were detected. The phylogenetic affiliations of the predominant electrophoretic bands were inferred by performing a comparative 16S rRNA gene sequence analysis. Sequences obtained from basalts affiliated with eight main phylogenetic groups of Bacteria, but were limited to only one group of the Archaea. The most frequently retrieved bacterial sequences affiliated with the gamma-proteobacteria, alpha-proteobacteria, Chloroflexi, Firmicutes, and Actinobacteria. The archaeal sequences were restricted to the marine Group 1: Crenarchaeota. Our results indicate that the basalt harbors a distinctive microbial community, as the majority of the sequences differed from those retrieved from the surrounding seawater as well as from sequences previously reported from seawater and deep-sea sediments. Most of the sequences did not match precisely any sequences in the database, indicating that the indigenous Arctic ridge basalt microbial community is yet uncharacterized. Results from enrichment cultures showed that autolithotrophic methanogens and iron reducing bacteria were present in the seafloor basalts. We suggest that microbial catalyzed cycling of iron may be important in low-temperature alteration of ocean crust basalt. The phylogenetic and physiological diversity of the seafloor basalt microorganisms differed from those previously reported from deep-sea hydrothermal systems.

Prokaryotic diversity, distribution, and insights into their role in biogeochemical cycling in marine basalts

“…This pattern suggests that basalt microflora are largely associated with rocks of varying ages, but that a minority may reside in, for example, younger, less-altered rocks to the exclusion of older, more-weathered rocks. This finding is consistent with that of Lysnes et al (2004), who reported that specific bacterial species are found only in rocks of a certain age.…”

“…This is the first report of this clade in this environment, as previous studies that examined the archaeal communities in basalts revealed only Marine Group I Crenarchaeota Fisk et al, 2003;Lysnes et al, 2004;Mason et al, 2007). Recently, quantitative analyses of the microbial communities in basalts revealed that Archaea comprise 4-12% and 0.02% or less of the prokaryotic communities (Santelli et al, 2008;Einen et al, 2008), respectively.…”

“…Although these estimates are disparate, they do reveal that Archaea are a minor component in the overall microbial communities that reside in basalt. Although Archaea are less prevalent, they are ubiquitous in basalts and have been reported in all studies that assayed for their presence Fisk et al, 2003;Lysnes et al, 2004;Mason et al, 2007;Einen et al, 2008;Santelli et al, 2008). Further, as discussed previously, a clade of Marine Group I Archaea appears to be endemic to basalt (Mason et al, 2007).…”

“…The ubiquity of certain clades, such as the a-Marine Group I ocean crust clade IX delineated by Mason et al (2007), regardless of the age of the host rock, suggests that overall basalt microflora do not change on a temporal scale. However, Lysnes et al (2004) reported that basalts of varying ages support different microbial phyla and subphyla. For example, the Actinobacteria were associated with older basalts, but were absent in recently erupted material.…”

“…Certain microbial taxa may be associated with rocks of varying ages, as suggested by Lysnes et al (2004); however, it is unclear what factors contribute to this habitat specificity. Fresh basalts are B8% wt FeO and 2% wt Fe 2 O 3 .…”