Coral elementomes diverge for colonies persisting in vegetative lagoons versus reef environments

Climate change, in tandem with localised stressors, continues to drive global declines in coral cover worldwide. Identifying where and how corals survive in present day extreme environments, characterised by suboptimum abiotic conditions, has become a key tool to better resolve coral stress tolerance and in turn future reef trajectories. Whilst several reef forming coral species routinely extend their ecological niche into extreme environments, whether corals have a distinct biogeochemical niche reflected by unique elementomes (the stoichiometry and quantity of elements) remains unknown. Here, through quantitative assessment and elemental mapping, we demonstrate that two functionally important Great Barrier Reef coral species, Acropora millepora and Porites lutea and their algal symbionts (Symbiodiniaceae) exhibit unique elementomes, that reflect a unique biogeochemical niche of species in the extreme mangrove lagoon compared to a neighbouring reef. Coral elementomes were distinct over multiple years, as were the elementomes of the seawater of each habitat. Furthermore, particulate organic matter was elevated in the mangrove lagoon which could support enhanced rates of heterotrophy. Collectively these findings reveal that vegetative mangrove island waters provide a unique biogeochemical environment for resident corals and that resident corals undergo bioelemental reorganisation, particularly via elevated micronutrient content, when living in extreme vegetative mangrove island lagoons. Results here reaffirm the importance of vegetative island systems in the survival of coral reefs and thus the critical need to ensure conservation efforts consider cross ecosystem protection measures.