Symbiotic interactions

Symbiosis, where different species live together for prolonged periods, is ubiquitous and extremely important on coral reefs. The most important symbiosis is between corals and the microalgae (zooxanthellae) that live in their cells, without which coral reefs would not exist. This chapter focuses on the diversity of zooxanthellae, the linkage with coral calcification and the nutrition of the symbiosis, particularly the supply of photosynthetically fixed carbon to coral, and the conservation and recycling of essential nutrients (especially nitrogen and phosphorus) by this symbiosis. The acquisition and breakdown of the symbiosis, particularly under thermal stress (i.e. coral bleaching), is described. Other important coral–microbe symbioses involve cyanobacteria, heterotrophic bacteria, viruses, protozoans and endolithic algae and fungi that live in the coral skeleton. Symbioses between sponges and bacteria or algae are also important, as are the iconic associations between fish and various invertebrates (e.g. the sea anemone–anemonefish symbiosis) or other fish species.

Relative roles of endolithic algae and carbonate chemistry variability in the skeletal dissolution of crustose coralline algae

The susceptibility of crustose coralline algae (CCA) skeletons to dissolution is predicted to increase as oceans warm and acidify. Skeletal dissolution is caused by bioerosion from endolithic microorganisms and by chemical processes associated with undersaturation of carbonate minerals in seawater. Yet, the relative contribution of algal microborers and seawater carbonate chemistry to the dissolution of organisms that cement reefs under projected pCO2 and temperature (pCO2-T) scenarios have not been quantified. We exposed CCA skeletons (Porolithon onkodes) to four pCO2-T treatments (pre-industrial, present-day, SRES-B1 "reduced" pCO2, and SRES-A1FI "business-as-usual" pCO2 emission scenarios) under natural light cycles vs. constant dark conditions for 8 weeks. Dissolution rates of skeletons without photo-endoliths were dramatically higher (200%) than those colonized by endolithic algae across all pCO2-T scenarios. This suggests that daytime photosynthesis by microborers counteract dissolution by reduced saturation states resulting in lower net erosion rates over day–night cycles. Regardless of the presence or absence of phototrophic microborers, skeletal dissolution increased significantly under the spring A1FI "business-as-usual" scenario, confirming the CCA sensitivity to future oceans. Projected ocean acidity and temperature may significantly disturb the stability of reef frameworks cemented by CCA, but surficial substrates harbouring photosynthetic microborers will be less impacted than those without algal endoliths.

Relative roles of endolithic algae and carbonate chemistry variability in the skeletal dissolution of crustose coralline algae

The susceptibility of crustose coralline algae (CCA) skeletons to dissolution is predicted to increase as oceans warm and acidify. Skeletal dissolution is caused by bioerosion from endolithic microorganisms and by chemical processes associated with undersaturation of carbonate minerals in seawater. Yet, the relative contribution of algal microborers and seawater carbonate chemistry to the dissolution of organisms that cement reefs under projected CO2 and temperature (CO2-T) scenarios have not been quantified. We exposed CCA skeletons (Porolithon onkodes) to four CO2-T treatments (pre-industrial, present-day, SRES-B1 reduced CO2 emission scenario, SRES-A1FI business-as-usual CO2 emission scenario) under natural light cycles vs. constant dark conditions for 8 weeks. Dissolution rates of skeletons without photo-endoliths were dramatically higher (200%) than those colonized by endolithic algae across all CO2-T scenarios. This suggests that daytime photosynthesis by microborers counteract dissolution by reduced saturation states resulting in lower net erosion rates over day-night cycles. Regardless of the presence or absence of phototrophic microborers, skeletal dissolution increased significantly under the spring A1FI "business-as-usual" scenario, confirming the CCA sensitivity to future oceans. Projected ocean acidity and temperature may significantly disturb the stability of reef frameworks cemented by CCA, but surficial substrates harboring photosynthetic microborers will be less impacted than those without algal endoliths.