Defining Site-Specific Water Quality Guidelines for Marine Microbiomes under Contaminant Stress

Marine ecosystem resilience is closely linked to microbiome
health,
yet prokaryotes are typically overlooked in conservation management
due to challenges in quantitatively defining microbial sensitivity
thresholds. This study introduces a novel approach that combines 16S
rRNA gene amplicon sequencing with ecotoxicological concentration–response
modeling to assess marine microbial responses to contamination stress
at both community and taxon levels. By applying propidium monoazide
to differentiate intact from membrane-compromised cells combined with
normalization of relative to absolute abundances via flow cytometry,
no-significant-effect concentration values can be derived from individual
taxon-specific stress–response curves. These thresholds inform
prokaryotic sensitivity distributions, enabling the derivation of microbiome hazard concentrations that estimate the contaminant
concentrations affecting x% of the microbial communitya
metric fully compatible with existing eukaryotic datasets and regulatory
frameworks. Applied to seawater microbiomes exposed to copper along
a disturbance gradient, higher copper sensitivities were observed
in microbiomes from ecologically valuable sites, such as coral reefs,
compared to disturbed areas. The site-specific microbiome hazard concentrations for copper affecting 5% of community taxa
ranged from 0.10 to 0.94 μg L–1, highlighting
the need for stricter environmental protection of ecologically valuable
sites. This quantitative approach provides a robust pathway for direct
integration of site-specific microbiome sensitivity data into environmental
risk assessments.