Chemische Ökologie
Final Report Abstract
The life cycle of most sessile marine invertebrates involves a motile larval stage. During this short stage, larvae choose quite selectively where to settle (i.e. permanently attach themselves to a solid substrate) and metamorphose into juveniles. The sources of indicators that “translate” the suitability of a substratum to larvae are generally well understood. For example, marine coralline algae have long been identified as a common source of settlement cues for corals, sea urchins, oysters and abalone but the specific nature of these cues has not been identified. This study aimed to describe the settlement cues of coralline algae and elucidate chemical signals that trigger larval metamorphosis. Four different coralline algae have been investigated in this study to elucidate the nature of settlement cues for the larvae of sea urchins. Different experimental treatments of algae, all of which aimed to modify or eradicate the bacterial biofilms on host algae, were analysed in larval settlement assays. These treatments revealed a clear effect of the manipulation of algal biofilms on the magnitude of larval settlement, indicating that algae-associated bacteria were indeed causing the effect on larvae. When algae that had received antibiotic treatments were re-introduced to the field for a period of 2 weeks, a faster and more potent recovery of inductive biofilms was observed on previously treated algae than on the inanimate control biofilms, suggesting that algal host attributes (surface characteristics and/or surface chemistry) drove the re-establishment of algal biofilms. The bacterial community on the host alga seemed to converge over 14 days to a community that was very similar to the original composition, i.e. prior to the manipulation by antibiotics/enzymes/oxidants. These results gave clear evidence of the bacterial nature of larval settlement cues present on coralline algae and supported the role of the host alga as an active player in shaping the community profile of its surface biofilms. Out of ca. 800 bacterial isolates only two bacteria showed potent induction of larval settlement in laboratory bioassays. We have isolated a compound from one of these bacteria that triggers larval settlement equally to the bacterium. In a related study focussing on larval settlement of broadcast spawning corals, we have isolated another compound from a bacterial isolate from a tropical calcareous coralline alga that triggers metamorphosis in the scleractinian coral Acropora millepora. The compound belongs to a compound family that is also produced by the strain that triggers sea urchin settlement. Given the similarity in metabolite chemistry of these inductive strains, we are currently cross-testing these cues between corals and sea urchins to identify common chemical signals that trigger invertebrate larval settlement. Any knowledge of such cues represents a breakthrough in marine prokaryote/eukaryote interactions in genreal and may provide biotechnologically interesting leads for mariculture and conservation of marine biodiversity.
