Unveiling the novel role of several Flavobacteriaceae isolates: Cultivable bacteria associated with Porolithon onkodes induce Pocillopora damicornis larval settlement and metamorphosis.

Crustose coralline algae (CCA) are pivotal for coral larval settlement and metamorphosis, yet the functional roles of their associated bacteria in this critical process remain underexplored. Here, we systematically isolated 554 cultivable bacteria from the dominant CCA species Porolithon onkodes and assessed their ability to induce larval settlement and metamorphosis of Pocillopora damicornis. The isolates were dominated by Pseudomonadota (50.72%) and Bacteroidota (32.85%), with 57.40% exhibiting < 98.60% 16S rRNA gene similarity to their closest type-strain matches, suggesting that Por. onkodes may harbor substantial numbers of potentially undescribed or poorly characterized bacteria. Notably, 33 isolates significantly induced larval settlement and metamorphosis, with rates exceeding 40.00% after 96 h, mainly including 22 Flavobacteriaceae and 9 Bacillaceae isolates, substantially expanding the known taxonomic range of bacteria involved in this process. Further investigation on Flavobacteriaceae isolate B1405 (a highly active inducer) revealed that its inductive activity was tightly dependent on growth phase and biofilm age: logarithmic- or stationary-phase cells induced > 38.00% settlement, while 3-day-old biofilms significantly enhanced the settlement rate to > 60.00%. These findings fill critical gaps in CCA-bacteria-coral interactions and highlight specific cultivable isolates within Flavobacteriaceae and Bacillaceae as potential microbial candidates for further investigation in coral settlement and restoration applications.