Why Do Corals Bleach? Conflict and Conflict Mediation in a Host/Symbiont Community

Abstract

Coral bleaching has attracted considerable study, yet one central question remains unanswered: given that corals and their Symbiodinium symbionts have co-evolved for millions of years, why does this clearly maladaptive process occur? Bleaching may result from evolutionary conflict between the host corals and their symbionts. Selection at the level of the individual symbiont favors using the products of photosynthesis for selfish replication, while selection at the higher level favors using these products for growth of the entire host/symbiont community. To hold the selfish lower-level units in check, mechanisms of conflict mediation must evolve. Fundamental features of photosynthesis have been co-opted into conflict mediation so that symbionts that fail to export these products produce high levels of reactive oxygen species and undergo programmed cell death. These mechanisms function very well under most environmental conditions, but under conditions particularly detrimental to photosynthesis, it is these mechanisms of conflict mediation that trigger bleaching.

Limitations of Using Cultured Algae to Study Cnidarian‐Algal Symbioses and Suggestions for Future Studies

“…To maintain symbiosis, the symbiont must also evade destruction and/or rejection by host innate immunity pathways (Weis 2019). However, with heat stress, the delicate balance of symbiosis is disrupted by metabolic dysregulation and the breakdown of photosynthetic machinery (Blackstone and Golladay 2018). This results in dysbiosis and the removal of algal symbionts by the host, leading to coral bleaching and ultimately coral reef decline (Blackstone and Golladay 2018).…”

Can natural selection and druggable targets synergize? Of nutrient scarcity, cancer, and the evolution of cooperation

“…Typically, the coral host limits the provisioning of the symbionts with inorganic nutrients; thus, holding the replication of symbionts in check [ 26,27 ] and forcing the symbiont to export photosynthetically‐produced reduced carbon to the coral. [ 28 ] In the presence of excess nutrients, a breakdown of this mutualistic relationship, known as coral bleaching, is more likely to occur. [ 29,30 ] During bleaching, the chemiosmotic process of photosynthesis is usually strongly downregulated within the coral because of the loss or movement of symbionts, and ROS dramatically increase.…”

Opportunistic bacteria use quorum sensing to disturb coral symbiotic communities and mediate the occurrence of coral bleaching

“…; Suggett and Smith, ), and led to mass bleaching events at regional and global scales (Hughes et al ., ). Blackstone and Golladay () and Matthews et al . () further pointed out that understanding the interactions between coral hosts and the multitude of symbiotic microorganisms is of critical importance for predicting the ability of corals to adapt and evolve in response to environmental change.…”

“…This type of dynamic symbiotic system is highly sensitive and responsive to its biotic and abiotic parameters, and maintenance of symbiotic homeostasis is critical to host health (Bourne et al ., ; Suggett and Smith, ). However, under global climate change, some environmental stressors – extreme fluctuations in temperature, ocean acidification, and increased pollution – have made the coral undergo unprecedented degradation in recent decades (Blackstone and Golladay, ; Comte and Pendleton, ). In particular, the phenomenon of coral bleaching has become increasingly serious, which can be defined as the loss of symbiodinium or reduction in photosynthetic pigments of symbiotic algae (Rosenberg et al ., ; Pernice et al ., ).…”