Bacterial communitiy compositions on natural and artificial substrates in the Spermonde Archipelago, Indonesia

Populations on small islands surrounded by coral reefs often heavily depend on these reefs. The health and recovery of reefs are strongly influenced by recruitment of coral larvae. Their settlement relies on cues such as those emitted from bacterial communities forming biofilms on reef surfaces. Environmental conditions can change these bacterial community compositions (BCC) and may in turn affect settlement of coral larvae. We investigated BCC and coral larvae settlement at three small inhabited islands with different distance from the mainland in the Spermonde Archipelago, Indonesia. BCC and recruitment were analyzed on artificial ceramic tiles after 2-8 weeks exposure time and on natural reef substrate. Environmental analysis showed a clear separation in water parameters between inshore and near-shore/mid-shelf sites, as well as distinct benthic communities at all of the three sites. No coral recruitment was observed at the inshore site with highest anthropogenic impact. At the other two sites coral recruitment occurred on natural surfaces (recruits per 100 cm2: 0.73 ± 1.75 near-shore, 0.90 ± 1.97 mid-shelf), but there was no significant difference between the two sites. On artificial substrates coral recruitment differed between these two sites, with tile orientation and with exposure time of the tiles in the reef. The most abundant bacteria on both substrates were Gammaproteobacteria, Alphaproteobacteria and Cyanobacteria. BCC was strongly correlated with water quality and significant differences in BCC between the inshore site and near-shore/mid-shelf were found. On artificial substrates there was a significant difference in BCC with exposure time in the reef. Furthermore, bacterial communities on artificial tiles showed changes in their composition after being subjected to higher water temperatures in an experimental setting. We demonstrate how changes in water quality and temperature can affect BCC in coral reefs, which in turn could affect coral recruitment. Small islands such as these investigated feature a high spatial variability, as seen e.g. in environmental gradients, differences in BCC or in coral recruitment. Our study highlights the value of taking both BCC and coral recruitment into account in addition to the environmental conditions, when considering the recovery potential of coral reefs.