DNA- and RNA-based bacterial communities and geochemical zonation under changing sediment porewater dynamics on the Aldabra Atoll

Abstract

The remote Aldabra Atoll, Seychelles, provides the rare opportunity to study bacterial communities in pristine carbonate sediments across an entire biome. The four sampled sites cover sand with high porewater exchange, bioturbated silt and mud with intermediate exchange, as well as a seasonally and episodically desiccated landlocked pool. As sediments harbour dead cells and environmental DNA alongside live cells, we used bacterial 16S rRNA gene and transcript analysis to distinguish between past and present inhabitants. Previously described laminated sediments mirroring past conditions in the Cerin, France could not be retrieved. Thus, the aim was adjusted to determine whether bacterial community composition and diversity follow typical geochemical zonation patterns at different locations of the atoll. Our data confirm previous observations that diversity decreases with depth. In the lagoon, the bacterial community composition changed from Pseudomonas dominating in the sand to diverse mixed surface and sulphate reduction zones in the anaerobic mud with strongly negative Eh. The latter correlated with high total alkalinity, ammonia, and total sulphide, alongside a decrease in SO42−/Cl− and high relative abundances of sulphate reducing (Halo-) Desulfovibrio, sulphur oxidizing Arcobacteraceae, photo(hetero)troph Cyanobacteria, Alphaproteobacteria, and fermenting Propionigenium. In contrast to expectations, deeper mud and pool sediments harboured high abundances of Halomonas or Alphaproteobacteria alongside high C/N and increased salinity. We believe that this atypical community shift may be driven by a change in the complexity of available organic matter.

An insight into the prokaryotic diversity from a polymetallic nodule-rich region in the Central Indian Ocean Basin using next generation sequencing approach

“…Similar results were reported by Zinger et al (2011) , where Alphaproteobacteria was the dominant group in the pelagic communities, while Gammaproteobacteria was dominant in the benthic communities in the samples collected from the water column as well as the seafloor from the World’s oceans. However, it contrasts with the results obtained by Hoyningen-Huene et al (2022) , where a higher abundance of Alphaproteobacteria was detected from the sediment porewater from the Aldabra Atoll in Seychelles. Other abundant classes in the sediments include Bacilli, Actinobacteria and Acidimicrobiia.…”

“…Microbial diversity in the sediments was found to decrease with an increase in depth from the seafloor. Similar results were obtained from the sediment porewater samples from the Aldabra Atoll, Seychelles where a decrease in phylogenetic diversity along with a shift in the microbial community was observed (Hoyningen-Huene et al, 2022). A higher abundance of Nitrosopumilaceae group of organisms can be related to their ammonia oxidizing potential in deep-sea sediments under aerobic conditions.…”

Seabird nutrient subsidies enrich mangrove ecosystems and are exported to nearby coastal habitats

“…Foliar N:P ratio in our study was 11.7 ± 2.45 (Mean ± SD), suggesting mangroves in our study are N-limited, which is typical for fringe mangroves influenced by frequent tidal flushing 7,44 . N limitation at our study site is further indicated by the equally low N:P ratios documented in previous analyses on mangrove soil 45 and lagoonal sediment porewater on Aldabra 46 . At our seabird sites, uptake of seabird-derived nutrients alleviated mangrove nutrient limitations of both N and P, observed by reductions in foliar C:N and C:P ratios of R. mucronata .…”

Seabird nutrient subsidies enrich mangrove ecosystems and are exported to nearby coastal habitats

“… 7 , 47 N limitation at our study site is further indicated by the equally low N:P ratios documented in previous analyses on mangrove soil 48 and lagoonal sediment porewater on Aldabra. 49 At our seabird sites, uptake of seabird-derived nutrients alleviated mangrove nutrient limitations of both N and P, observed by reductions in foliar C:N and C:P ratios of R. mucronata . Nutrient enrichment in N-limited mangroves can generate multiple higher order effects.…”