Work Package 6 (Schupp, Gerlach, Wilkes)

"Role of bacteria and holobiont signaling molecules in coral settlement"

Broader background of the proposed research project

The proposed project will address the function of DOM in the settlement process of marine organisms. Specifically, it addresses the production and function of signaling compounds, which are released from macroorganisms and their associated microbial communities. Crustose coralline algae (CCA) and its associated bacterial community will serve as a model holobionts (macro- and associated microorganisms). Numerous studies have shown the importance of CCA and their bacterial biofilms in producing signaling molecules for the settlement process (Kitamura et al. 2007, 2009, Tebben et al. 2011, 2015). The proposed experiments investigating the production of settlement compounds for scleractinian coral larvae from holobiont biofilms will serve as an example for DOM function among aquatic organisms. Changes in biofilm composition and settlement compound production will be assessed under different environmental parameters (e.g. effects of climate change like increasing sea surface temperature and decreasing seawater pH), adding to our understanding of DOM function in future climate change scenarios. The planned experiments on coral settlement cues will highlight the importance of DOM by showing its role as important signaling compounds among organisms, even between organisms belonging to different kingdoms (e.g. bacteria, plantae, animalia).

Outline for the proposed PhD research project

The importance of CCA in the settlement and metamorphosis of scleractinian corals has been demonstrated in several publications. While settlement and metamorphosis inducing compounds have been isolated from CCA for certain coral larvae (Kitamura et al. 2007, 2009), in many cases it remains unclear if the macroorganism or the associated microorganisms are the producers of the signaling compounds. In the proposed project the following hypothesis will be tested: Changes in environmental conditions related to climate change will change the CCA associated bacterial community resulting in changes in the production of coral settlement cues. We will isolate and characterize settlement inducing bacteria and their settlement inducing compounds from different CCA species. Isolated bacteria will be screened for production of settlement and metamorphosis inducing compounds, using larvae of the brooding coral Leptastrea purpurea. L. purpurea is grown at the aquarium facility at the ICBM Terramare in Wilhelmshaven and releases daily larvae for settlement experiments. CCAs will be exposed to different climate change scenarios and holobiont microbiomes monitored for shifts in the abundance, presence or absence of the bacterial producers, as well as the production of settlement cues. While several inducing bacteria have already been isolated from the CCA Hydrolithion by our group, factors triggering or causing changes in the production of settlement cues need to be examined (e.g. different environmental conditions). In this project we will closely collaborate with WP 7 where a different model system is investigated in a similar context. Molecular analysis of settlement inducing compounds will be done in collaboration with Wilkes (WPs 3) and Dittmar (WP 4).