NASA Astrobiology Institute (NAI)

We have been studying a microbial biofilm (Figure 1) growing at very low light intensities and high temperature and humidity below the entrance of a lava cave in Kilauea Crater, Hawai'i Volcanoes National Park. (Figure 2) The cave presents an oligotrophic environment, but condensation of geothermally heated groundwater that vents at the rear of the cave has promoted the development of a complex microbial community, similar in higher order taxonomic structure to copiotrophic soil environments. Given the existence of lava tubes of similar geologic composition on Mars, geothermal activity there may have allowed the existence, or persistence, of complex microbial communities in similar Martian environments, wherein they would be shielded from the effects of harmful UV radiation.

Monitoring of the cave environment using in situ sensors has revealed diurnal and seasonal cycling of temperature and humidity, with maximum temperatures around 45.5C, near the upper limits for photosynthesis, and maximum relative humidity of 102.5%. We have continued to analyze metagenomic data (randomly sequenced DNA fragments) and have now conducted Bacterial V6-ribosomal tag sequence analysis (mediated by massively parallel 454 sequencing technology) to provide a comprehensive taxonomic inventory of the mat bacterial community. Both these analyses were conducted from a single sample of DNA extracted from the purple-pigmented microbial mat. Initially, this analysis has revealed a previously unknown dominant community member from the phylum Chloroflexi, that was not found using full-length 16S rDNA clone libraries. Therefore, these emerging methods have provided an alternative view of the microbial community structure than that obtained more traditional methods. Of 64 206 Bacterial ribosomal V6 tag sequences that passed our quality control measures, 10,766 were unique. BLAST analysis of unique sequences resulted in 774 unique “phylotypes” (defined as unique Genbank Accession numbers as Best Hit), while 598 unique sequences (accounting for ~8% of total sequences) had no BLAST homology to any sequence in the Genbank nr database and possibly highly novel bacterial sequences. Rarefaction analysis of tag sequence data indicated community coverage was still not fully saturated, however diversity lower bound indices Chao and Ace estimate community diversity at 97% tag sequence identity to be 2,215 and 2,433 phylotypes respectively compared to 2,024 observed phylotypes. The major phylogenetic components of the mat community are illustrated in Figure 3.

In total 40 different phyla (all classes within the Proteobacteria) or candidate divisions were represented in the tag sequence library.

This ongoing project was described as part of a successful joint proposal for the purchase of a set of laser tweezers ($339,561 + $85,700 in kind support) which will be used to aid cultivation of select community members.