Abstract
Bacterial biofilms provide cues for the settlement of marine invertebrates such as coral larvae, and are therefore important for the resilience and recovery of coral reefs. This study aimed to better understand how ocean acidification may affect the community composition and diversity of bacterial biofilms on surfaces under naturally reduced pH conditions. Settlement tiles were deployed at coral reefs in Papua New Guinea along pH gradients created by two CO2 seeps. Biofilms on upper and lower tiles surfaces were sampled 5 and 13 months after deployment. Automated Ribosomal Intergenic Spacer Analysis was used to characterize 240 separate bacterial communities, complemented by amplicon sequencing of the bacterial 16S rRNA gene of 16 samples. Bacterial biofilms consisted predominantly of Alpha-, Gamma-, and Delta-proteobacteria, as well as Cyanobacteria, Flavobacteriia, and Cytophagia, whereas taxa that induce settlement of invertebrate larvae only accounted for a small fraction of the community. Bacterial biofilm composition was heterogeneous, with on average only ∼25% of operational taxonomic units shared between samples. Among the observed environmental parameters, pH was only weakly related to community composition (R2 ∼ 1%), and was unrelated to community richness and evenness. In contrast, biofilms strongly differed between upper and lower tile surfaces (contrasting in light exposure and grazing intensity). There also appeared to be a strong interaction between bacterial biofilm composition and the macroscopic components of the tile community. Our results suggest that on mature settlement surfaces in situ, pH does not have a strong impact on the composition of bacterial biofilms. Other abiotic and biotic factors such as light exposure and interactions with other organisms may be more important in shaping bacterial biofilms on mature surfaces than changes in seawater pH.
Highlights
The colonization of bare substrata by reef organisms is a crucial process affecting the resilience and recovery of tropical coral reefs
automated Ribosomal Intergenic Spacer Analysis (ARISA) of the bacterial communities on the settlement tiles detected a total of 451 OTUARISA with an average of 143 OTUARISA per sample (25th percentile: 127, 75th percentile: 163)
We evaluated the interactions between abiotic and biotic factors influencing the composition of the bacterial biofilms by constructing directional path models to assess the relationships between water pH, bacterial community composition (ARISA data), and the macroscopic community on the settlement tiles
Summary
The colonization of bare substrata by reef organisms is a crucial process affecting the resilience and recovery of tropical coral reefs. Crustose coralline algae (CCA) and their adherent bacterial biofilms play a major role in the mediation of coral larval settlement (Negri et al, 2001; Harrington et al, 2004). Bacterial biofilms can enhance settlement rates in the absence of CCA (Webster et al, 2004). E.g. Pseudoalteromonas or VC International Council for the Exploration of the Sea 2017.
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