Photophysical acclimation of tropical free-living coralline algae to over-turning

Sammanfattning

Coral reefs have received global attention as a biodiverse ecosystem with valuable ecosystem services. However, a less-studied organism contributing to the reef structure accretion is the red calcifying algae, coralline algae. Photophysiology is the foundation of coralline algae growth and with their plasticity and ability to acclimate to different light environments, they can potentially have a resilience to climate change. However, the mechanisms behind and the extent and speed of coralline algae acclimation is not fully understood. With the risk of being over-turned thus facing contrasting light regimes, the effect morphological complexity and intra-thallus heterogeneity have on the photo- acclimation is also not yet fully recognized. Therefore, this study has investigated how experimentally over-turning of tropical free-living coralline algae (commonly termed maerl or rhodolith) by flipping them over affects their photophysiology by investigating maerl in situ on a Maldivian coral reef. The over-turned maerl showed signs of photoinhibition and stress, with reduced quantum efficiency (Fv/Fm) and vitality index (RFd), while maintaining functional photosynthetic capacity as the maximum electron capacity (ETRmax) and maximum saturation intensity (Ek) did not have a significant difference between the flipped and original sides. Temporal patterns suggested that natural environmental variability, rather than over-turning alone, was a major driver of photosynthetic responses. Therefore, longer observations might lead to more reliable results that could confirm acclimation of different parts of the thallus to a new light environment. Additionally, stronger and delayed responses in original as well as flipped bottom, i.e. the bottom side which has been a bottom since before the experiment and the bottom side which has been over-turned to a bottom during the experiment, supported the presence of intra-thallus heterogeneity to adapt. The results indicate that maerl exhibit some degree of acclimatory plasticity, but the non-uniform response between top and bottom parts of the thallus to over-turning with consistent signs of physiological stress anticipates, together with predicted climate chance, negative feedback to coral reef systems and associated ecosystem services. This study thus confirms the difficult trade-off between maintaining photosynthetic efficiency under decreased light exposure while deploying photoprotective mechanisms in a high irradiance system when over-turned.