Negative Cues drive settlement of corals. Biofilms_neg_cues

Coral settlement is a poorly understood nonrandom process where planktonic larvae attach to a surface and undergo metamorphosis. Successful settlement is critical for the persistence of coral reefs, but can be disrupted by changing water quality. During two laboratory experiments, 8-week old biofilms from two locations were exposed to moderately turbid waters (50 mg/L sediment) and a doubling of particulate organic matter, generating distinct biofilm communities (determined through 16S rRNA gene amplicon sequencing). Acropora millepora and Pocillopora acuta coral larvae demonstrated varying settlement responses to biofilms from the different treatments. Using network analyses, we conclude there are many more negative associations between biofilm community taxa and coral settlement than positive associations. Gammaproteobacteria affiliated sequences, a diverse group with many known pathogens, and the family Rhodospirillaceae were associated with reduced larval settlement for A. millepora, whereas many Rhodobacterales affiliated sequences reduced settlement of P. acuta. Positive associations between bacterial taxa and A. millepora included dominant sequences recovered from all samples (Rhodobacterales; genera Phaeobacter and Rhodovulum). These findings indicate that the spatial distribution of early A. millepora and P. acuta recruitment may be driven by negative more than positive microbial cues. We suggest that corals must select sites with a greater probability of survival due to the absence of competition, predation, or disease, even when positive cues are present. We must therefore ask what makes corals not settle in order to improve our ability to predict successful coral settlement, and to help determine the resilience of a given reef.