METAGENOMIC INSIGHTS INTO CONE FORMATION IN HOT SPRING MICROBIAL MATS
Initial observations of cones in Little Hot Creek revealed surface-normal growth on slopes and under overhangs rather than vertically, suggesting that neither phototaxis nor bubble production are driving cone formation. Bubble production was only observed in flat regions of the mat and not in cones, indicating that there is not a strong association between bubbles and cones. This was further supported by the absence of known phototaxis-associated genes in metagenomic data from both the cones and the surrounding flat mat. Comparison of the metagenomes from the cones and the surrounding mat revealed shared predominant taxa and functional genes between both sample types, differing only in relative abundance and the presence of some low-abundance sequences. Both samples were primarily composed of the phyla Cyanobacteria and Chloroflexi, with the coccoidal cyanobacterium genus Synechococcus and the filamentous, photoheterotrophic chloroflexi genus Roseiflexus as dominant members of both samples. The filamentous cyanobacterium genus Phormidium was highly enriched in the cone samples, suggesting a potential role for abundant filaments in cone formation.
Our study suggests that cone formation can be independent of bubble formation or phototaxis. Alternative mechanisms could be involved, such as differential gene expression, diffusion limitation of nutrients, or the stochastic aggregation of filamentous microbes. These mechanisms could potentially be driven by processes other than oxygenic photosynthesis, and therefore might have implications for interpretation of cones in the rock record.