Geology and land use shape nitrogen and sulfur cycling groundwater microbial communities in Pacific Island aquifers

Resource-constrained island populations have thrived in Hawai’i for over a millennium, but now face aggressive new challenges to fundamental resources, including the security and sustainability of water resources. Characterizing the microbial community in groundwater ecosystems is a powerful approach to infer changes from human impacts due to land management in
hydrogeological complex aquifers. In this study, we investigate how geology and land management influence geochemistry,
microbial diversity and metabolic functions. We sampled a total of 19 wells over 2-years across the Hualālai watershed of Kona,
Hawai’i analyzing geochemistry, and microbial communities by 16S rRNA amplicon sequencing. Geochemical analysis revealed
significantly higher sulfate along the northwest volcanic rift zone, and high nitrogen (N) correlated with high on-site sewage
disposal systems (OSDS) density. A total of 12,973 Amplicon Sequence Variants (ASV) were identified in 220 samples, including 865
ASVs classified as putative N and sulfur (S) cyclers. The N and S cyclers were dominated by a putative S-oxidizer coupled to
complete denitrification (Acinetobacter), significantly enriched up to 4-times comparatively amongst samples grouped by
geochemistry. The significant presence of Acinetobacter infers the bioremediation potential of volcanic groundwater for microbialdriven coupled S-oxidation and denitrification providing an ecosystem service for island populations dependent upon groundwater
aquifers.