Physiological and metabolic responses to hypersalinity reveal interpopulation tolerance in the green macroalga Ulva compressa with different pollution histories

Transcriptomic profiles and diagnostic biomarkers in the Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa reveal mechanistic insights of adaptative strategies upon desalination brine stress

Seawater desalination by reverse osmosis is growing exponentially due to water scarcity. Byproducts of this process (e.g. brines), are generally discharged directly into the coastal ecosystem, causing detrimental effects, on benthic organisms. Understanding the cellular stress response of these organisms (biomarkers), could be crucial for establishing appropriate salinity thresholds for discharged brines. Early stress biomarkers can serve as valuable tools for monitoring the health status of brine-impacted organisms, enabling the prediction of long-term irreversible damage caused by the desalination industry. In this study, we conducted laboratory-controlled experiments to assess cellular and molecular biomarkers against brine exposure in two salinity-sensitive Mediterranean seagrasses: Posidonia oceanica and Cymodocea nodosa. Treatments involved exposure to 39, 41, and 43 psu, for 6 h and 7 days. Results indicated that photosynthetic performance remained unaffected across all treatments. However, under 43 psu, P. oceanica and C. nodosa exhibited lipid oxidative damage, which occurred earlier in P. oceanica. Additionally, P. oceanica displayed an antioxidant response at higher salinities by accumulating phenolic compounds within 6 h and ascorbate within 7 d; whereas for C. nodosa the predominant antioxidant mechanisms were phenolic compounds accumulation and total radical scavenging activity, which was evident after 7 d of brines exposure. Finally, transcriptomic analyses in P. oceanica exposed to 43 psu for 7 days revealed a poor up-regulation of genes associated with brassinosteroid response and abiotic stress response, while a high down-regulation of genes related to primary metabolism was detected. In C. nodosa, up-regulated genes were involved in DNA repair, cell cycle regulation, and reproduction, while down-regulated genes were mainly associated with photosynthesis and ribosome assembly. Overall, these findings suggest that 43 psu is a critical salinity-damage threshold for both seagrasses; and despite the moderate overexpression of several transcripts that could confer salt tolerance, genes involved in essential biological processes were severely downregulated.

A risk assessment on Zostera chilensis, the last relict of marine angiosperms in the South-East Pacific Ocean, due to the development of the desalination industry in Chile

Seagrasses, which are considered among the most ecologically valuable and endangered coastal ecosystems, have a narrowly limited distribution in the south-east Pacific, where Zostera chilensis is the only remaining relict. Due to water scarcity, desalination industry has grown in the last decades in the central-north coasts of Chile, which may be relevant to address in terms of potential impacts on benthic communities due to their associated high-salinity brine discharges to subtidal ecosystems. In this work, we assessed ecophysiological and cellular responses to desalination-extrapolable hypersalinity conditions on Z. chilensis. Mesocosms experiments were performed for 10 days, where plants were exposed to 3 different salinity treatments: 34 psu (control), 37 psu and 40 psu. Photosynthetic performance, H₂O₂ accumulation, and ascorbate content (reduced and oxidized) were measured, as well as relative gene expression of enzymes related to osmotic regulation and oxidative stress; these, at 1, 3, 6 and 10 days. Z. chilensis showed a decrease in photosynthetic parameters such as electron transport rate (ETR_max) and saturation irradiance (Ek_ETR) under hypersalinity treatments, while non-photochemical quenching (NPQ_max) presented an initial increment and a subsequent decline at 40 psu. H₂O₂ levels increased with hypersalinity, while ascorbate and dehydroascorbate only increased under 37 psu, although decreased along the experimental period. Increased salinities also triggered the expression of genes related to ion transport and osmolyte syntheses, but salinity-dependent up-regulated genes were mostly those related to the reactive oxygen species metabolism. The relict seagrass Z. chilensis has shown to withstand increased salinities that may be extrapolable to desalination effects in the short-term. As the latter is not fully clear in the long-term, and considering the restricted distribution and ecological importance, direct brine discharges to Z. chilensis meadows may not be recommended.

Benzopyrene induces oxidative stress and increases expression and activities of antioxidant enzymes, and CYP450 and GST metabolizing enzymes in Ulva lactuca (Chlorophyta)

Benzopyrene is rapidly incorporated and metabolized, and induces oxidative stress and activation of antioxidant enzymes, and CYP450 and GST metabolizing enzymes in Ulva lactuca. To analyze absorption and metabolism of benzo[a]pyrene (BaP) in Ulva lactuca, the alga was cultivated with 5 µM of BaP for 72 h. In the culture medium, BaP level rapidly decreased reaching a minimal level at 12 h and, in the alga, BaP level increased until 6 h, remained stable until 24 h, and decreased until 72 h indicating that BaP is being metabolized in U. lactuca. In addition, BaP induced an initial increase in hydrogen peroxide decreasing until 24 h, superoxide anions level that remained high until 72 h, and lipoperoxides that initially increased and decreased until 72 h, showing that BaP induced oxidative stress. Activities of antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (AP), glutathione reductase (GR) and glutathione peroxidase (GP) were increased, whereas dehydroascorbate reductase (DHAR) activity was unchanged. The level of transcripts encoding these antioxidant enzymes was increased, but transcripts encoding DHAR remained unchanged. Interestingly, the activity of glutathione-S-transferase (GST) was also increased, and inhibitors of cytochrome P450 (CYP450) and GST activities enhanced the level of BaP in algal tissue, suggesting that these enzymes participate in BaP metabolism.