Coralline algal metabolites induce settlement and mediate the inductive effect of epiphytic microbes on coral larvae

Abstract

Settlement of invertebrates is a key process affecting the structure of marine communities and underpins the ability of benthic ecosystems to recover from disturbance. While it is known that specific crustose coralline algae (CCA) are important for settlement of some coral species, the role of algal chemical compounds versus surface microbial biofilms has long been ambiguous. Using a model system - a CCA of a genus that has been shown to induce high levels of settlement of Acropora corals (Titanoderma cf. tessellatum) and an abundant coral species (Acropora millepora)- we show that chemical effects of CCA are stronger than those from CCA surface microbial biofilms as drivers of coral settlement. Biofilms contributed to some extent to larval settlement via synergistic effects, where microbial cues were dependent on the CCA primary metabolism (production of dissolved organic carbon). We propose that optimal coral settlement is caused by complex biochemical communications among CCA, their epiphytic microbial community and coral larvae.

Settlement of larvae from four families of corals in response to a crustose coralline alga and its biochemical morphogens

“…Thus, future research should increase taxonomic replication to obtain a broader sense of settlement preferences across Monitpora taxa. While Acropora larvae (and those of some other families) responded strongly to P. onkodes, the CCA fragments tested in this study were not treated with antibiotics, heat or pressure, so we cannot discount the possibility that CCA-associated bacterial communities may contribute to settlement induction 24,29,31,33,34 .…”

“…( Table 1), indicating that this response is likely to be ecologically important and potentially useful for restoration efforts with this genus. However, specific CCA surface chemistry, which is likely to differ among algal species, is expected to play an influential role in determining the settlement preferences of each coral species 21,27,34 .…”

“…Diploastrea heliopora , Goniastrea retiformis, and Dipsastraea pallida ), we also found that alcohol or water-soluble morphogens from this CCA may be unimportant for settlement of corals outside Acropora . For many coral species there may well be multiple cues acting in concert to induce settlement 34 , and these cues are potentially constructed from multi-domain microbial communities associated with inductive reef substrata 29 , 42 – 44 that may interact with physical factors such as surface rugosity 25 . The declines in settlement observed following the removal of the surface texture associated with live CCA fragments, as well as the decline observed with increasing refinement of the chemical cues tested in this study, support this hypothesis.…”

Early life history of the Caribbean coral Orbicella faveolata (Scleractinia: Merulinidae)

“…Recent research has shown that morphogens produced by Titanoderma spp. are hydrophobic, and larvae must establish direct contact with the CCA surface to sense the cues and be induced to settle (Gómez-Lemos et al, 2018). No settlement occurred on the surface of dead coral fragments, suggesting that benthic biofilms are not as important as CCA to prompt settlement of O. faveolata larvae.…”

“…In 2018, in order to know if larvae were competent to settle, i.e., attach and metamorphose (Heyward & Negri, 1999), competence tests were performed by placing 3 small individual fragments (1 cm 2 ) of the crustose coralline algae (CCA) species Titanoderma prototypum in multi-well plates (Nest®). 10 larvae were added in each well in a ratio of 1 larva per ml and allowed to settle for 24 h as defined by Gómez-Lemos, Doropoulos, Bayraktarov, and Díaz-Pulido (2018). Once larvae attached and underwent metamorphosis, they were considered competent to run the settlement assay.…”