3. Ecosystem health

Symbiosis is a close ecological relationship between 2 or more different species (collectively referred to as a holobiont) and is a particularly common feature on tropical coral reefs. Symbiotic associations are often mutually beneficial to both partners (mutualism)⁵⁷¹ although some symbioses may benefit one partner and be harmless to the other (commensalism) or benefit one to the detriment of the other (for example, parasitism).⁸⁴⁷

Symbiosis facilitates various ecological processes, such as nutrient exchange between symbiotic partners and promotion of biodiversity, whereby host species can support the existence of multiple symbionts, including many thousands of microbes.⁸⁴⁸ Well-known examples of symbioses within the Region include the coral–dinoflagellate and sea anemone–clownfish relationships, but many thousands of other associations exist.⁶⁷² Coral crabs (genus Trapezia) clean and defend their Pocillopora coral hosts,³²³ while gobies protect the shelters they share with burrowing shrimps.⁸⁴⁹ Giant clams have obligate symbiotic associations with photosynthetic dinoflagellates,²⁸⁵ and they also shelter a range of commensal and parasitic animals in their mantle cavities.⁸⁵⁰

Climate change is severely affecting many symbioses, as increasing sea temperatures and marine heatwaves ⁶⁴⁷ lead to various host species losing algal symbionts from their tissues in a process called bleaching.^851,852 The underlying mechanisms of coral bleaching are a subject of active research interest.⁸⁵³ Recent studies have found that heat stress can cause nutrient exchange between symbiotic partners to become unbalanced, contributing to the breakdown of the symbiosis.⁸⁵⁴ In sponge–microbe associations, heat stress can also lead to the breakdown of symbiosis and negative consequences for host health.^855,856 Compounding stressors, such as changes in light conditions ^683,684 and limited oxygen,⁸⁵⁷ have been shown to lower thermal thresholds for bleaching. Elevated nutrient conditions and localised anthropogenic disturbances may also influence bleaching susceptibility in corals ^532,571,858 and other hosts.^859,860

Coral survival post-bleaching, particularly when sea temperatures remain high, may be influenced by host genetics,³¹ the heat sensitivity of algal symbionts ^858,861 and the availability of key nutrients such as iron.^720,721 Shifts in symbiont partners to more heat‑tolerant variants were detected in natural populations in response to marine heatwaves, including the 2016 mass coral bleaching event,⁸⁵⁸ and have also been induced experimentally under controlled laboratory conditions.⁸⁶² Symbiont changes appear to be a species-specific response.²⁸⁸ In some cases, restructured symbiont communities may be inherited by offspring.⁸⁶³

Symbiotic microbial communities, or microbiomes, which encompass bacterial and archaeal populations, play a critical and diverse role within the holobiont unit.⁸⁶⁴ The full diversity and complexity of reef microbiomes in supporting symbiosis and other host functions is still being explored.^{777,848,865,866} However, members of the microbiome could have an important role in providing a mechanism for adapting to environmental change.^288,867,868

Due to the inherent interdependencies between symbiotic partners, symbiont abundance and condition and the presence of key threats are used as a proxy for the condition and trend of symbiosis. 

Despite some recovery of hard coral cover in offshore reefs over the past 5 years, rates of bleaching and increasing sea temperatures continue to be a major concern. Some evidence exists of shifts in symbiont diversity following the 2016 mass coral bleaching, but responses appear to be species-specific.