Insights into Microbial Mats and Possible Stromatolite Formation from Little Hot Creek, California

Abstract:

A carbonate-rich, partially lithified microbial mat from Little Hot Creek (Long Valley Caldera, CA) was studied as a potential proxy for ancient stromatolites. This microbial mat was characterized through a combination of water chemistry, 16S rRNA gene sequencing, and incubation experiments. Four distinct layers were observed in three replicate microbial mat samples based on color, texture, and microbial composition. The bacterial populations changed significantly across the mat layers: layer A (top) was dominated by the phyla Cyanobacteria (55%), Chloroflexi (14%), and Bacteroidetes (11%); layer B was dominated by Cholorflexi (57%) and Bacteroidetes (17%); layer C was dominated by Nitrospirae (23%) and Chlorobi (11%); and layer D was dominated by Nitrospirae (35%). Microbial diversity increased with depth (from layer A to layer D). The fraction of inorganic carbon and its δ¹³C values as well as the δ¹³C_org, however, remained nearly constant throughout the mat at 0.93 ± 0.03, -1.22 ± 0.10‰, and -20.44 ± 0.27‰, respectively. SEM images of the carbonate revealed similar features and highly conserved structure between layers. Incubations with δ¹³C_bicarbonate showed bicarbonate uptake in all layers of the mat with the highest rate of uptake occurring with the top layer in the light. The growth experiments, the microbial taxonomy and diversity, and the SEM analysis suggest top-down mat growth, with each layer originating from the “A” (top) layer. Incubation experiments with nutrient additions to each mat layer showed enhanced growth in the presence of added Mg²⁺ and Mn²⁺, but growth was inhibited with the addition of Fe²⁺. This data suggests that Mg²⁺ and Mn²⁺ may play an important and overlooked role in the growth of microbial mats and the biological origin of stromatolites.