NASA Astrobiology Institute (NAI)

This project aims to understand how microbial communities thrive under extreme conditions. Our study is based in the Tinto River located in southwestern Spain. The river is a high acid/metal environment with pH values between 2.0-2.5 and iron concentrations as high as 20 g/l. Algae contribute up to 65% of the total biomass of the community. This eukaryotic microbial community is understudied. Our project explores molecular eukaryotic diversity by looking at SSU rRNA genes and considers physiological adaptations allowing survival under pH extremes. Sequencing efforts have confirmed a strong algal component in the microbial community. Our molecular data reveal that certain biofilm-communities are dominated by phototrophs such as Chlamydomonad and Chrysophyte populations. Others are a complex assemblage of fungi, ciliate and flagellate populations. Still others are a combination of ciliate and diatom assemblages. Future work will focus on obtaining sequences from sediment samples from the Tinto and comparing sequence data with that from other acidic sites such as Davis Mine, an abandoned sulfuric acid mine in Massachusetts, and hot acidic furnas of the Azorean Islands. The physiology component of this project seeks to understand how pH-extremophiles maintain internal pH and whether there are different mechanisms by which internal pH is maintained by acidophiles. We have been making internal pH measurements using pH-sensitive indicators such as BCECF on algal cultures from the Tinto River. Our physiological experiments employ fluorescence ratio imaging and ion-selective probe technology accomplished at the single-cell level. Initial experiments conducted on cultures of Chlamydomonas sp. and Euglena sp. growing at pH 3 revealed striking differences between the two species. Our preliminary results show that Euglena sp. may have an internal pH below 6.5. In contrast, the internal pH of Chlamydomonas sp. appears nearer neutrality. The putative acidic values of Euglena sp. will be confirmed with alternative pH indicators.