The sum is more than its parts: Key species in the functioning of cold-water coral reef communities

Abstract

Cold-water coral reefs are hotspots of biodiversity and biogeochemistry in the deep-sea, solely fuelled by external energy sources. Hence, food input, processing and recycling play a major role in the functioning of these ecosystems. In this thesis we aimed to study energy flow and metabolism of cold-water coral reef ecosystems by assessing the uptake and processing of different food sources by key species of the community (corals, worms and sponges) while including their (non-) trophic interactions and their impact on ecosystem functioning. Stable isotope as deliberately added tracers (13C and 15N) and bulk and compound specific isotope analysis were the major tools used throughout this study. Feeding experiments on the reef-building cold-water coral Lophelia pertusa, described in chapter 2, revealed that the coral can assimilate a broad range of organic food sources at comparable rates, indicating an opportunistic feeding strategy. Together with its microbial symbionts it is able to even access inorganic nutrients, such as bicarbonate and various nitrogen species, highlighting the previously unrecognized mixotrophic nature of the coral (chapter 3). Observations on the interaction between L. pertusa, the worm Eunice norvegica and the sponge Hymedesmia coriacea further showed that next to microbial symbionts associated macrospecies can also influence feeding and growth of the coral. Accordingly, results of the study observing the interaction between the worm E. norvegica and L. pertusa, described in chapter 4, demonstrated that the worm can enhance its food uptake in the presence of the coral while the coral kept assimilation constant by increasing the consumption of smaller algae particles less favored by the polychaete. Furthermore in the presence of the worm the coral increased skeletal formation (calcification). Given its effect on coral calcification the relationship might be of high importance for reef development and resilience. The interaction between the sponge H. coriacea and the coral, studied in chapter 5, however was characterized by a trophic transfer between both species via coral mucus, partly mediated by bacteria associated with the sponge. Subsequently, the sponge utilized the coral mucus to produce sponge-derived detritus, which can be feed upon by many species of the community, including the coral itself. The result might be a recycling of energy and nutrients which increases ecosystem functioning. In conclusion this thesis shows that the cosmopolitan reef building coral L. pertusa is an opportunistic feeder, exploiting a broad range of organic and inorganic resources. It further demonstrates that associates species can positively affect coral nutrition and growth and thereby can enhance cold-water coral reef ecosystem functioning.