Life at the Boundaries: a Multi-Approach Study Discloses Archaeal Diversity in the Black Sea.

Marine Archaea are involved in the global C and N cycle but their diversity in suboxic and anoxic systems and their lipid composition is unknown, as most of them remain uncultured. Archaeal membrane lipids comprise archaeol and various glycerol dibiphytanyl glycerol tetraethers (GDGTs). An assortment of polar head groups attached to the GDGT lipids further enhances the diversity of the archaeal membranes. Because of their long term preservation potential, GDGTs are considered powerful biomarkers of the presence of archaea. Hexose phosphohexose (HPH)-crenarchaeol, a specific thaumarchaeotal membrane lipid, has been previously suggested to be especially labile and thus be indicative of living biomass.

The Black Sea is the largest permanently stratified anoxic basin in the world and it is an ideal model for ancient and modern anoxic environments. Here, by combining metagenomic and lipidomic approaches we determine archaeal abundance and diversity across the Black Sea water column that was sampled at high resolution. Archaeal diversity was evaluated by means of 16S rRNA amplicon pyrosequencing and by quantifying the abundance of targeted groups by quantitative PCR. The archaeal lipids were investigated by UHPLC-HR AM MS (High Resolution Accurate Mass MS). Our combined results indicate a predominance of Thaumarchaeota in the upper oxic waters followed by a sharp decrease at the oxic sub-oxic boundary, as confirmed by a decrease in the relative abundance of their unique lipid HPH-crenarchaeol. At the oxic-suboxic interface, the archaeal community was dominated by Marine euryarchaeota group II and the lipid composition changed dramatically with an increase of the abundance of HPH-GDGT-0, 1, 2 by three to four times. In deeper anoxic waters, an increase in the relative abundance of Miscellaneous Crenarchaeota Group coincides with an increase in HPH-GDGT-0. Further analyses will improve the assignment of potential biomarker lipids to specific archaeal groups.