Comparing the response of endolithic and sediment microorganisms at a dynamic cold seep on the Costa Rica Pacific Margin

Marine methane seeps are globally distributed, highly dynamic features that serve as islands of biological and geochemical diversity on the seafloor. Although methane seepage is known to vary on timescales ranging from days to centuries, little is known about the responses of the microbial community to these disruptions. We describe the results of a 17-month carbonate transplant experiment on ~50 rocks conducted on 13 DSV Alvin dives at Mound 12, a cold methane seep on the Costa Rica Pacific Margin. First order spatial analysis of the site was conducted by initially classifying each collection location as "active," "transition," or "inactive" based on observations of in situ megafauna or the presence of bacterial mat or shimmering water. These classifications were later confirmed and refined by 16S rRNA sequence and geochemical analysis of 30 sediment cores, as well as with AUV Sentry maps. Active cores were characterized by high sulfide concentrations, ¹³C-depleted dissolved inorganic carbon, and high relative abundance of anaerobic methane-oxidizing aggregates (ANME). Transition and inactive sites, surprisingly, contained similar sediment microbial communities. From each activity zone, the endolithic community of one carbonate was analyzed and five carbonates were transplanted to a zone of different activity until their recovery 17 months later. Carbonate and sediment core community comparisons revealed that carbonates do indeed host a markedly different microbial community than the surrounding sediments, with the ANME-1 subgroup always more abundant within carbonates than in the surrounding sediments. In contrast to sediment samples, carbonates from the active sites contained communities more similar to rocks from the inactive site than the transition zone. This phenomenon was also visible in transplanted carbonate samples. Active to inactive transplants continued to group with active carbonates, while active to transition transplants were more likely to group with other transition rocks. The data suggest that a reduction in methane concentration selects for a subset of the seep community, but that the community goes dormant in response to the absence of dissolved methane. These results indicate that the endolithic community may provide insight into the dynamic history of a methane seep.