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Climate models predict increases in frequency of summer heat waves. In Europe, such events have already caused declines in seagrass meadows, highlighting the importance of short-term responses of local communities to climate stress. Understanding the variability among populations along the European thermal gradient in response to heat waves is crucial for seagrass conservation and management. Using a mesocosm we compared effects of a simulated heat wave on the photophysiology of Zostera marina populations coming from low (43° N, Adriatic Sea) and high latitudes (56° N, North and Baltic Seas). Measurements before, during and up to 4 wk after the heat wave included photophysiological parameters derived from light response curves generated by PAM fluorometry and gene expression using qRT-PCR. In all 3 populations, initial exposures to thermal stress were characterized by increases in dark adapted effective quantum yield (Y0), maximum electron transfer rate of PSII (ETRmax) and slope of the light response curve (α), coinciding with upregulations of the gene superoxidase dismutase [Mn]. With continuation of the heat wave these initial effects disappeared, demonstrated by declines in Y0, ETRmax and α relative to controls. Z. marina from the Adriatic suffered from the simulated heat wave as much as its high-latitude counterparts. However, we also demonstrate slight photophysiological differences between the populations during the recovery phase, where performance of high-latitude populations continued declining even after water temperatures returned to control levels, while photochemical activity fully recovered in the Adriatic population. These results might draw the attention of future studies and seagrass conservation efforts.

Photoacclimation of the symbiotic hermatypic coral Stylophora pistillata was studied both for colonies growing at different depths (1 to 20 m) sampled within the same season ('spatial photoacclimation') and for colonies growing at the same depth (5 m) but sampled throughout the year ('temporal photoacclimation'). Specimens were darker and had increased chlorophyll densities (chlorophyll per coral surface area) under low-light conditions caused both by increased depth and, in shallow-growing specimens, by the winter season. However, while increased depth involved increased zooxanthella chlorophyll content, acclimation to low irradiances by shallow-growing corals during the winter mostly involved an increase in algal density. In shallow-growing corals, both zoox- anthella chlorophyll content and coral chlorophyll density fluctuated seasonally in correlation with light more than with nitrogen or temperature. This could be related to the fact that reefs in Eilat, Israel, are exposed to more solar radiation than many other reefs in the world. Histological analyses of high-light corals showed that zooxanthellae were located deeper inside the host tissue and were smaller in diameter compared to zooxanthellae from low-light corals. Additionally, while deep-water colonies hosted clade C zooxanthellae, colonies growing in shallow waters, both in high- and very low-light environments, hosted clade A. These clade differences were found to affect coral physiol- ogy, as examined in a short-term thermal stress experiment, with clade-A colonies having higher thermal resilience than clade-C colonies. Taken together, our results show evidence of different mechanisms that help adjust the light-capturing abilities of S. pistillata zooxanthellae to spatial and temporal changes in irradiance.