Elevated ammonium delays the impairment of the coral-dinoflagellate symbiosis during labile carbon pollution

Labile dissolved organic carbon (DOC) is a major pollutant in coastal marine environments affected by anthropogenic impacts, and may significantly contribute to coral bleaching and subsequent mortality on coastal reefs. DOC can cause bleaching indirectly through the rapid proliferation of copiotrophic and pathogenic bacteria. Here we demonstrate that labile DOC compounds can also impair the coral-dinoflagellate symbiosis by directly affecting coral physiology on both the host and algal symbiont level. In a controlled aquarium experiment, we monitored over several weeks key physiological parameters of the tropical coral Stylophora pistillata exposed to ambient and elevated labile DOC levels (0.1 and 1.0 mM) in combination with low and high nitrogen (i.e. ammonium) conditions (0.2 and 4.0 μM). At the symbiont level, DOC exposure under low ammonium availability decreased the photosynthetic efficiency accompanied by ∼75 % Chl a and ∼50 % symbiont cell reduction. The photosynthetic functioning of the symbionts recovered once the DOC enrichment ceased indicating a reversible shift between autotrophic and heterotrophic metabolism. At the host level, the assimilation of exogenous DOC sustained the tissue carbon reserves, but induced a depletion of the nitrogen reserves, indicated by ∼35 % decreased protein levels. This suggests an imbalanced exogenous carbon to nitrogen supply with nitrogen potentially limiting host metabolism on the long-term. We also demonstrate that increased ammonium availability delayed DOC-induced bleaching likely by keeping symbionts in a photosynthetically competent state, which is crucial for symbiosis maintenance and coral survival. Overall, the present study provides further insights into how coastal pollution can de-stabilize the coral-algal symbiosis and cause coral bleaching. Therefore, reducing coastal pollution and sustaining ecological integrity are critical to strengthen the resilience of coral reefs facing climate change.