Response of bacterial communities to nutrient enrichment differs between surface and subsurface layers of temperate mangrove sediments

Microbial communities in mangrove sediments are critical for carbon and nutrient cycling; yet despite their importance, responses to eutrophication, a pervasive global threat to coastal ecosystems, remain poorly understood. We assessed bacterial community responses to an in-situ fertilisation experiment in a temperate mangrove (Avicennia marina subsp. australasica) forest in Aotearoa New Zealand. Using 16S rRNA gene sequencing, we compared surface (0-2 cm) and subsurface (5-7 cm) community diversity and function, 4.5 months after enrichment. Differences in porewater nutrient concentrations between ambient and enriched plots were weak at the end of the experiment, however, bacterial communities exhibited significant shifts relative to controls. Community shifts in subsurface samples were stronger compared to the surface, with enrichment reducing alpha diversity and altering community composition to Proteobacteria and Bacteroidota favoured communities under fertilisation. Conversely, taxa from Desulfobacterota, Chloroflexi, and Nitrospirota were more abundant in controls. Similarly, functional changes in the surface remained limited, while they showed strong, selective shifts towards suppression of sulphur-cycling and hydrogen oxidising taxa and enrichment of heterotrophic guilds in the subsurface. Community assembly processes were more deterministic in subsurface sediments compared to surface sediments and became more homogenised under nutrient addition. Our findings reveal that short-term eutrophication can leave lasting imprints on subsurface bacterial communities in mangrove sediments, potentially altering coupled biogeochemical cycles beyond the enrichment period. These results underscore the need to better understand and integrate microbial responses at relevant spatial scales into predictions of ecosystem resilience under increasing anthropogenic pressures.