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The present study examines for the first time the effects of increased salinity on water relations and osmolyte (carbohydrates and amino acids) concentrations in two Mediterranean seagrass species, Posidonia oceanica and Cymodocea nodosa, which are adapted to growth in environments with contrasting salinity and have a known differential sensitivity to alterations in ambient salinity. The specific aim was to obtain insights into their respective capacities to cope with natural or anthropogenically induced (e.g. desalination plants) hypersaline stress and its ecological implications. To this end, large plant fragments of both seagrass species were maintained for 47 days in a laboratory mesocosm system under ambient salinity (37 psu; control) and three chronic hypersaline conditions (39, 41 and 43 psu). Analyses of leaf-tissue osmolality indicated that both species followed a dehydration avoidance strategy, decreasing their leaf water potential (W w ) as the external salinity increased, but using different physiological mechanisms: whereas P. oceanica leaves exhibited a reduction in osmotic potential (W p ), C. nodosa leaves maintained osmotic stability through a decrease in turgor pressure (W p ) probably mediated through cell-hardening processes. Accordingly, the concentrations of soluble sugars and some amino acids (mainly Pro and Gly) suggested the activation of osmoregulatory processes in P. oceanica leaves, but not in C. nodosa leaves. Osmotic adjustments probably interfered with leaf growth and shoot survival of P. oceanica under hypersaline stress, whereas C. nodosa showed a more efficient physiological capacity to maintain plant performance under the same experimental conditions. These results are consistent with the more euryhaline ecological behaviour of C. nodosa and contribute to understanding the high vulnerability shown by P. oceanica to even mild increments in seawater salinity.

Applying proteomics, we tested the physiological responses of the euryhaline seagrass Cymodocea nodosa to deliberate manipulation of salinity in a mesocosm system. Plants were subjected to a chronic hypersaline condition (43 psu) to compare protein expression and plant photochemistry responses after 15 and 30 days of exposure with those of plants cultured under normal/ambient saline conditions (37 psu). Results showed a general decline in the expression level of leaf proteins in hypersaline stressed plants, with more intense reductions after long-lasting exposure. Specifically, the carbon-fixing enzyme RuBisCo displayed a lower accumulation level in stressed plants relative to controls. In contrast, the key enzymes involved in the regulation of glycolysis, cytosolic glyceraldehyde-3-phosphate dehydrogenase, enolase 2 and triose-phosphate isomerase, showed significantly higher accumulation levels. These responses suggested a shift in carbon metabolism in stressed plants. Hypersaline stress also induced a significant alteration of the photosynthetic physiology of C. nodosa by means of a down-regulation in structural proteins and enzymes of both PSII and PSI. However we found an over-expression of the cytochrome b559 alpha subunit of the PSII initial complex, which is a receptor for the PSII core proteins involved in biogenesis or repair processes and therefore potentially involved in the absence of effects at the photochemical level of stressed plants. As expected hypersalinity also affects vacuolar metabolism by increasing the leaf cell turgor pressure and enhancing the up-take of Na+ by over-accumulating the tonoplast specific intrinsic protein pyrophosphate-energized inorganic pyrophosphatase (H(+)-PPase) coupled to the Na+/H+-antiporter. The modulation of carbon metabolism and the enhancement of vacuole capacity in Na+ sequestration and osmolarity changes are discussed in relation to salt tolerance of C. nodosa.

The dominant Mediterranean seagrass Posidonia oceanica inhabits sublittoral environments with very stable salinity regimes; the species is considered highly sensitive to even moderate increases in salinity caused by hypersaline effluents (brine) from desalination plants. We analysed the effect of salinity increase on seagrass vitality and survival by means of an in situ mesocosm experiment. To this end, we used the brine (70–75 psu) produced by a pilot desalination plant, which was diluted with seawater to obtain brine solutions of 1 psu (high salinity increase, HS) and 2.5 psu (very high salinity increase, VS) over the mean natural salinity (37.5±0.16 psu) and then interspersed a set of experimental units in a nearby P. oceanica meadow. At the end of the experimental period (3 months), these treatments had produced differential effects on all seagrass descriptors, i.e., intense and significant in the VS experimental units and more modest (or even negligible) in the HS ones. Seagrass meadow declined through a significant decrease in shoot density in both HS (12.4±3.4%, mean±SE) and VS (18.5±3.05%, mean±SE) in comparison to control experimental units (<5%). Surviving shoots had reduced size and lower leaf growth rate than control plants, but more generally a reduction in the photosynthetic leaf surface caused by the increase in the proportion of necrotic leaf area (up to 60% in VS treatments). Non-structural carbohydrate (soluble and reserve) concentrations decreased in rhizomes, probably in relation to physiological osmoregulation processes. All these responses support the hypothesis that the threshold salinity tolerance of P. oceanica is very close to the upper limit of its natural salinity range (ca. 38 psu).

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The frequency of marine heatwaves (MHWs) is increasing due to climate change. Although seaweeds are resilient to environmental changes, an increasing body of evidence shows that rising sea surface temperatures have deleterious effects on temperate kelp species. However, information on the vulnerability of juvenile kelp to these stressors and their population stability is limited. This study summarizes findings on the ability of juvenile sporophytes of Macrocystis pyrifera to survive and recover from simulated MHW conditions (22°C, 5 d) in combination with nitrate limitation (<1 µM) by evaluating photosynthetic capacity, nitrate uptake, tissue composition, bio‐optical properties, and oxidative stress of single‐blade juvenile sporophytes (<20 cm). Temperature, nitrate availability, and their interaction had significant effects on the physiological status of juvenile sporophytes after the exposure and recovery periods. Overall, as expected, the photosynthetic capacity of juvenile sporophytes decreased with increased temperature and lower nitrate availability. Short‐term exposure to simulated MHWs resulted in oxidative damage and reduced growth. The termination of the experimental warming allowed partial recovery to control values, indicating high physiological resilience. However, the interaction of both high temperature and nitrate scarcity induced irreversible damage to their photosynthetic capacity, with an increase in compensation irradiance, highlighting potential limitations in the carbon balance of juvenile sporophytes.