Effects of light limitation on the photophysiology of the coral Pachyseris speciosa from Singapore's turbid reefs

Light limitation caused by increased sedimentation and turbidity, mainly due to intense human activities, poses a particularly pronounced threat to coral reefs located near urban areas, such as those in Singapore. In this study, we investigated the photophysiological and bleaching responses of Pachyseris speciosa, a common reef building coral species found in Singapore's turbid reefs, under low-light conditions over a four-week period. The experimental treatments included intermittent light and complete darkness, simulating possible current and potential future light scenarios for Singapore's reefs under heightened turbidity due to coastal development and climate change. Our results showed that P. speciosa experienced declines in Symbiodiniaceae density, chlorophyll-a levels, and color scores under both treatment conditions, with severe bleaching occurring in complete darkness. However, corals exposed to intermittent light exhibited only tissue paling and significant photoacclimatory adjustments. Notably, these corals maintained the stability of the maximum quantum yield of photosystem II, increased their effective quantum yield, and decreased the saturating irradiance. This suggested a potential photoadaptation to intermittent light conditions and fluctuating light levels, allowing them to sustain elevated photosynthetic performance. Our findings highlighted and confirmed the resilience and ability of P. speciosa to tolerate various limited light conditions and enhanced our understanding of coral photoacclimation under extremely low-light scenarios.