# Delayed Shift in Microbiota Composition in a Marine Microcosm Pollution Experiment

## TL;DR

This study shows that pollution with nitrate and sulfate causes delayed changes in marine microbial communities, which could be missed in natural ecosystems.

## Contribution

The study reveals delayed abrupt shifts in microbiota composition following pollution, resembling tipping points in marine ecosystems.

## Key findings

- Nitrate addition caused a threefold reduction in species richness and dominance of Pseudarchobacter.
- Sulphate addition led to a weaker but delayed shift towards spore-forming Firmicutes.
- DNRA increased in oxic conditions, suggesting a shift in nitrogen metabolism.

## Abstract

Benthic habitats are the largest habitats on Earth, being essential for marine ecosystem functioning. Benthic habitats are particularly vulnerable towards pollution and anthropogenetic influence due to general oligotrophic nature. We, therefore, simulated pollution events involving nitrate and sulphate, in combination with organic carbon. We then observed the microbiota composition the following month. Surprisingly, upon nitrate addition, an abrupt response was observed between two and three weeks after the pollution event. We observed a threefold reduction in species richness, with a dominance of the genus Pseudarchobacter within the Campylobacteriota phylum, concurring with a decrease in nitrification potential and an increase in Dissimilatory Nitrate Reduction to Ammonium (DNRA) and a regain in denitrification. Likewise, addition of sulphate contributed to a delayed response with reduction in species richness albeit weaker than for nitrate, leading to a shift towards potential spore-forming Firmicutes. There was also an increase in DNRA, but only for the oxic conditions, concurring with a regain in sulphate reductio and denitrification. For the nitrate addition experiments, the delay in response could potentially be attributed to the genus Pseudarchobacter which rely on sulphides for denitrification, while for the sulphate addition experiments, the delayed response might be explained by the germination of spores. The late increase of DNRA may indicate a shift towards a different metabolic regime for nitrogen. In conclusion, our microcosm experiments revealed delayed abrupt microbiota shifts resembling tipping points that can potentially be overlooked in natural ecosystems.

The online version contains supplementary material available at 10.1007/s00284-024-03869-5.