Differential tolerance and resilience of Mediterranean seagrasses to short-term heat stress

The aim of the present study was to determine tolerance to ocean warming of the two main Mediterranean seagrass species Posidonia oceanica and Cymodocea nodosa. To this end, photophysiological and transcriptomic responses of shallow (5m) and deep (25m) plants were studied during a 5-d exposure to heat (32°C) and after a 5-d recovery to ambient temperature (24°C). Observed responses evidenced interspecific differences in heat tolerance in accordance with the biological attributes and ecological strategies of the species, but also intraspecific differences in relation to the depth origin of plants. The smaller and fast-growing species C. nodosa, was able to maintain unaltered leaf carbon budget along heat exposure, thanks to proportionally similar increments in photosynthesis and respiration. Along with this, the lack of alterations at the photochemical and photoprotective levels point to a high tolerance of the species to heat stress. Contrarily, the higher increment in respiration than in photosynthesis showed by the long-lived and slow-growing P. oceanica provoked leaf carbon imbalance after 24h of heat exposure, evidencing its higher sensitivity to heat. Interestingly, shallow plants displayed a higher acclimation capacity and after 5d of exposure their photosynthetic and respiratory rates converged to control conditions, whereas deep plants still evidenced symptoms of heat stress. The progressive photochemical efficiency reduction observed in deep plants, reinforced the idea that deeps plants are less able to tolerate heat. In any case, plants from both depths were able to recover their physiological status after a recovery period, indicating that they did not suffer lethal injury under the experimental conditions here tested. An analysis of gene expression (both RT-qPCR and RNA-seq) is ongoing, to dig into the molecular and physiological mechanisms involved in seagrass tolerance to heat stress. In conclusion, the study suggests that warming caused by human-induced climate change will likely have significant negative effects mainly on P. oceanica populations. The sensibility of the species to heat will finally depend on the local environmental conditions where plants grow (e.g. different depths or latitudes), a fact that must be taken in to account for the establishment of suitable management and conservation strategies.