Microbial community structure in contrasting Hawaiian coastal sediments

Microbe-mineral interactions play a fundamental role in marine sediments and global biogeochemical cycles. Here, we investigated the sediment microbial communities in two contrasting field sites on Big Island, Hawaii (USA), that differ in their bay morphology and sediment grain size distributions: Papakōlea Beach (exposed, finer sediment) and Richardson Ocean Park (sheltered, coarser sediment). We selected three stations within each bay and characterized the mineral and chemical composition of the sediment and porewater, and used 16S rRNA amplicon sequencing of the V4V5 hypervariable region to investigate the naturally occurring microbial communities. Microbial community structure differed significantly between the two bays, rather than within each bay, whereby microbial diversity was markedly lower at Papakōlea compared to Richardson. We correlated environmental variables to microbial community structure in order to identify the key drivers of community differences between and within the two bays. Our study suggests that differing physico-chemical properties of the sediment and porewater, resulting from the contrasting bay morphologies and geophysical drivers, are the main factors influencing microbial community structure in these two bays. Papakōlea Beach is a naturally occurring ‘green sand’ beach, due to its high olivine content. This site was selected in the broader context of a field campaign investigating olivine as a source mineral for ocean alkalinity enhancement (OAE), a carbon dioxide removal technology. Our results highlight the complexity of marine sediment environments, with implications for the monitoring, reporting and verification of future field trials involving olivine addition for ocean alkalinity enhancement.