Nitric oxide production and tolerance differ among Symbiodinium types exposed to heat stress.

Nitric oxide (NO) is a ubiquitous molecule and its involvement in metazoan-microbe symbiosis is well known. Evidence suggests that it plays a role in the temperature-induced breakdown ('bleaching') of the ecologically important cnidarian-dinoflagellate association, and this can often lead to widespread mortality of affected hosts. This study confirms that dinoflagellates of the genus Symbiodinium can produce NO and that production of the compound is differentially regulated in different types when exposed to elevated temperature. Temperature-sensitive type B1 cells under heat stress (8°C above ambient) exhibited significant increases in NO synthesis, which occurred alongside pronounced photoinhibition and cell mortality. Tolerant type A1 cells also displayed increases in NO production, yet maintained photosynthetic yields at levels similar to those of untreated cells and displayed less dramatic increases in cell death. Type C1 cells displayed a down-regulation of NO synthesis at high temperature, and no significant mortality increases were observed in this type. Temperature-induced mortality in types A1 and B1 was affected by the prevailing level of NO and, furthermore, photosynthetic yields of these temperature-tolerant and -sensitive types appeared differentially susceptible to NO donated by pharmacological agents. Taken together, these differences in NO synthesis and tolerance could potentially influence the varying bleaching responses seen among hosts harboring different Symbiodinium types.