Human-induced salinity changes impact marine organisms and ecosystems

Influence of salinity on the thermal tolerance of aquatic organisms

Aquatic organisms are challenged by changes in their external environment, such as temperature and salinity fluctuations. If these variables interacted with each other, the response of organisms to temperature changes would be modified by salinity and vice versa. We tested for potential interaction between temperature and salinity effects on freshwater, brackish, and marine organisms, including algae, macrophytes, heterotrophic protists, parasites, invertebrates, and fish. We performed a meta-analysis that compared the thermal tolerance (characterised by the temperature optimum, lower and upper temperature limits, and thermal breadth) at various salinities. The meta-analysis was based on 90 articles (algae: 15; heterotrophic protists: 1; invertebrates: 43; and fish: 31). Studies on macrophytes and parasites were lacking. We found that decreasing salinity significantly increased and decreased the lower and upper temperature limits, respectively, in all groups. Thus, a lowered salinity increased the thermal sensitivity of organisms. These findings mainly reflect the response of brackish and marine organisms to salinity changes, which dominated our database. The few studies on freshwater species showed that their lower thermal limits increased and the upper thermal limits decreased with increasing salinity, albeit statistically nonsignificant. Although non-significant, the response of thermal tolerance to salinity changes differed between various organism groups. It generally decreased in the order of: algae > invertebrates > fish. Overall, our findings indicate adverse effects of salinity changes on the temperature tolerance of aquatic organisms. For freshwater species, studies are comparatively scarce and further studies on their thermal performance at various salinity gradients are required to obtain more robust evidence for interactions between salinity and temperature tolerance. Considering test conditions such as acclimation temperature and potential infection with parasites in future studies may decrease the variability in the relationship between salinity and thermal tolerance.

Salinity influence on Mytilus galloprovincialis exposed to antineoplastic agents: a transcriptomic, biochemical, and histopathological approach

Nowadays, aquatic species face a variety of environmental risks associated with pharmaceutical consumption. More specifically, the increased number of cancer patients has been accompanied by an increased consumption of antineoplastic drugs, such as ifosfamide (IF) and cyclophosphamide (CP). These drugs have been found in aquatic ecosystems, raising concerns about their impact, especially on estuarine species, as marine waters are the final recipients of continental effluents. Simultaneously, predicted climatic changes, such as salinity shifts, may threaten organisms. Considering this, the present research aims to investigate the combined effects of IF and CP, and salinity shifts. For this, a transcriptomic, biochemical, and histopathological assessment was made using the bivalve species Mytilus galloprovincialis exposed for 28 days to IF and CP (500 ng/L), individually, at different salinity levels (20, 30, and 40). IF and CP up-regulated metabolism-related gene cyp3a1, with CP also affecting abcc gene, showing minimal salinity impact and highlighting the importance of these metabolic routes in mussels. Salinity shifts affected the transcription of genes related to apoptosis and cell cycle growth, such as p53, as well as the aerobic metabolism, the antioxidant and biotransformation mechanisms. These findings indicate mussels' high metabolic adaptability to osmotic stress. Under CP exposure and low salinity, mussels exhibited increased cellular damage and histopathological effects in digestive gland tubules, revealing detrimental effects towards M. galloprovincialis, and suggesting that a metabolic slowdown and activation of antioxidant mechanisms helped prevent oxidative damage at the control and high salinities. Overall, results reinforce the need for antineoplastics ecotoxicological risk assessment, especially under foreseen climate change scenarios.

Repeated Hyposalinity Pulses Immediately and Persistently Impair the Sea Urchin Adhesive System

Climate change will increase the frequency and intensity of extreme climatic events (e.g., storms) that result in repeated pulses of hyposalinity in nearshore ecosystems. Sea urchins inhabit these ecosystems and are stenohaline (restricted to salinity levels ∼32‰), thus are particularly susceptible to hyposalinity events. As key benthic omnivores, sea urchins use hydrostatic adhesive tube feet for numerous functions, including attachment to and locomotion on the substratum as they graze for food. Hyposalinity severely impacts sea urchin locomotor and adhesive performance but several ecologically relevant and climate change-related questions remain. First, do sea urchin locomotion and adhesion acclimate to repeated pulses of hyposalinity? Second, how do tube feet respond to tensile forces during single and repeated hyposalinity events? Third, do the negative effects of hyposalinity exposure persist following a return to normal salinity levels? To answer these questions, we repeatedly exposed green sea urchins (Strongylocentrotus droebachiensis) to pulses of three different salinities (control: 32‰, moderate hyposalinity: 22‰, severe hyposalinity: 16‰) over the course of two months and measured locomotor performance, adhesive performance, and tube foot tensile behavior. We also measured these parameters 20 h after sea urchins returned to normal salinity levels. We found no evidence that tube feet performance and properties acclimate to repeated pulses of hyposalinity, at least over the timescale examined in this study. In contrast, hyposalinity has severe consequences on locomotion, adhesion, and tube foot tensile behavior, and these impacts are not limited to the hyposalinity exposure. Our results suggest both moderate and severe hyposalinity events have the potential to increase sea urchin dislodgment and reduce movement, which may impact sea urchin distribution and their role in marine communities.

Does climate change increase the risk of marine toxins? Insights from changing seawater conditions

Marine toxins produced by marine organisms threaten human health and impose a heavy public health burden on coastal countries. Lately, there has been an emergence of marine toxins in regions that were previously unaffected, and it is believed that climate change may be a significant factor. This paper systematically summarizes the impact of climate change on the risk of marine toxins in terms of changes in seawater conditions. From our findings, climate change can cause ocean warming, acidification, stratification, and sea-level rise. These climatic events can alter the surface temperature, salinity, pH, and nutrient conditions of seawater, which may promote the growth of various algae and bacteria, facilitating the production of marine toxins. On the other hand, climate change may expand the living ranges of marine organisms (such as algae, bacteria, and fish), thereby exacerbating the production and spread of marine toxins. In addition, the sources, distribution, and toxicity of ciguatoxin, tetrodotoxin, cyclic imines, and microcystin were described to improve public awareness of these emerging marine toxins. Looking ahead, developing interdisciplinary cooperation, strengthening monitoring of emerging marine toxins, and exploring more novel approaches are essential to better address the risks of marine toxins posed by climate change. Altogether, the interrelationships between climate, marine ecology, and marine toxins were analyzed in this study, providing a theoretical basis for preventing and managing future health risks from marine toxins.

The impact of acute low salinity stress on Antarctic echinoderms

Climate change is causing increased coastal freshening in Antarctica, leading to reduced salinity. For Antarctica's endemic echinoderms, adapted to the stable polar environment, the impact of rapid reductions in coastal salinity on physiology and behaviour is currently unknown. Six common Antarctic echinoderms (the sea urchin Sterechinus neumayeri; the sea star Odontaster validus; the brittle star Ophionotus victoriae; and three sea cucumbers Cucumaria georgiana, Echinopsolus charcoti and Heterocucumis steineni), were directly transferred from ambient salinity (34.5‰) to a range of salinity dilutions (29-9‰) for 24 h. All species showed reduced activity and the establishment of a temporary osmotic gradient between coelomic fluid and external seawater. Most species exhibited a depression in oxygen consumption across tolerated salinities; however, at very low salinities that later resulted in mortality, oxygen consumption increased to levels comparable to those at ambient. Low salinity tolerance varied substantially between species, with O. victoriae being the least tolerant (24 h LC50 (lethal for 50% of animals) = 19.9‰) while E. charcoti and C. georgiana demonstrated the greatest tolerance (24 h LC50 = 11.5‰). These findings demonstrate the species-specific response of Antarctica's endemic echinoderms to short-term hypoosmotic salinity events, providing valuable insight into this phylum's ability to respond to an underreported impact of climate change.

The coral microbiome in sickness, in health and in a changing world

Stony corals, the engines and engineers of reef ecosystems, face unprecedented threats from anthropogenic environmental change. Corals are holobionts that comprise the cnidarian animal host and a diverse community of bacteria, archaea, viruses and eukaryotic microorganisms. Recent research shows that the bacterial microbiome has a pivotal role in coral biology. A healthy bacterial assemblage contributes to nutrient cycling and stress resilience, but pollution, overfishing and climate change can break down these symbiotic relationships, which results in disease, bleaching and, ultimately, coral death. Although progress has been made in characterizing the spatial-temporal diversity of bacteria, we are only beginning to appreciate their functional contribution. In this Review, we summarize the ecological and metabolic interactions between bacteria and other holobiont members, highlight the biotic and abiotic factors influencing the structure of bacterial communities and discuss the impact of climate change on these communities and their coral hosts. We emphasize how microbiome-based interventions can help to decipher key mechanisms underpinning coral health and promote reef resilience. Finally, we explore how recent technological developments may be harnessed to address some of the most pressing challenges in coral microbiology, providing a road map for future research in this field.

Habitat- and lifestyle-dependent structural and functional characteristics of viruses in mangrove wetlands of different functional zonings

Mangrove wetlands, as one of the natural ecosystems with the most ecological services, have garnered widespread attention about their microbial driven biogeochemical cycling. Urbanization have led to different spatial patterns of environmental conditions and microbial communities in mangroves. However, viruses, as the pivotal drivers of biogeochemical cycling in mangroves, remain inadequately explored in terms of how their ecological potential and complex interactions with host respond to functional zonings. To address this knowledge gap, we conducted a comprehensive investigation on the structural and functional properties of temperate and lytic viruses in mangrove wetlands from different functional zonings by jointly using high-throughput sequencing, prokaryotic and viral metagenomics. Multiple environmental factors were found to significantly influence the taxonomic and functional composition, as well as lysogen-lysis decision-making of mangrove viruses. Furthermore, enriched auxiliary metabolic genes (AMGs) involved in methane, nitrogen and sulfur metabolism, and heavy metal resistance were unveiled in mangrove viruses, whose community composition was closely related to lifestyle and host. The virus-host pairs with different lifestyles were also discovered to react to environmental changes in different ways, which provided an empirical evidence for how virus and bacteria dynamics were specific to viral lifestyles in nature. This study expands our comprehension of the intricate interactions among virus, prokaryotic host and the environment in mangrove wetlands from multiple perspectives, including viral lifestyles, virus-host interactions, and habitat dependence. Importantly, it provides a new ecological perspective on how mangrove viruses are adapted to the stress posed by urbanization.

Coping with salinity extremes: Gill transcriptome profiling in the black-chinned tilapia (Sarotherodon melanotheron)

Steeper and sometimes extreme salinity gradients increasingly affect aquatic organisms because of climate change. Hypersalinity habitats demand powerful physiological adaptive strategies. Few teleost species have the capacity to spend their whole life cycle in salinities way over seawater levels. Focusing on the multifunctional gill, we unraveled the tilapia S. melanotheron key strategies to cope with different environmental conditions, ranging from freshwater up to hypersaline habitats. De novo transcriptome assembly based on RNAseq allowed for the analysis of 40,967 annotated transcripts among samples collected in three wild populations at 0, 40 and 80 ‰. A trend analysis of the expression patterns revealed responses across the salinity gradient with different gene pathways involved. Genes linked to ion transport, pH regulation and cell surface receptor signaling were mainly upregulated in the high salinity habitat. We identified tight junction proteins that were critical in high salinity habitats and that were different from the well-known tightening junctional proteins identified and expressed in fresh water. Expression profiles also suggest a change in the vascular tone that could be linked to an osmorespiratory compromise not only in fresh water, but also in high salinity environments. A striking downregulation of genes linked to the immune system and to the heat shock response was observed suggesting an energetic trade-off between immunity and acclimation/adaptation in the hypersaline habitat. The high expression of transcripts coding for immune and heat shock response in the freshwater habitat suggests the establishment of powerful mechanisms to protect gills from environmental threats and to maintain protein integrity. Non-directional expression trends were also detected with an upregulation of genes only in the hypersaline habitat (80 ‰) or only in the marine habitat (40 ‰). Unravel physiological strategies in S. melanotheron populations will help to better understand the molecular basis of fish euryhalinity in salinity-contrasted environments.

Alternative splicing plays a nonredundant role in greater amberjack (Seriola dumerili) in acclimation to ambient salinity fluctuations

Alternative splicing (AS) is an important post-transcriptional mechanism for adaptation of fish to environmental stress. Here, we performed a genome-wide investigation to AS dynamics in greater amberjack (Seriola dumerili), an economical marine teleost, in response to hypo- (10 ppt) and hyper-salinity (40 ppt) stresses. Totally, 2267-2611 differentially spliced events were identified in gills and kidney upon the exposure to undesired salinity regimes. In gills, genes involved in energy metabolism, stimulus response and epithelial cell differentiation were differentially spliced in response to salinity variation, while sodium ion transport and cellular amide metabolism were enhanced in kidney to combat the adverse impacts of salinity changes. Most of these differentially spliced genes were not differentially expressed, and AS was found to regulate different biological processes from differential gene expression, indicative of the functionally nonredundant role of AS in modulating salinity acclimation in greater amberjack. Together, our study highlights the important contribution of post-transcriptional mechanisms to the adaptation of fish to ambient salinity fluctuations and provides theoretical guidance for the conservation of marine fishery resources against increasingly environmental challenges.

Long-term warming and human-induced plankton shifts at a coastal Eastern Mediterranean site

Plankton are key ecological indicators for assessing the impacts of human-induced pressures like climate change and waste-water discharge. Here, 26 years (1988-2015) of biweekly in-situ chlorophyll-a concentration, mesozooplankton biomass and remotely-sensed sea surface temperature (SST) data are utilized to investigate long-term changes of plankton biomass and timing of growth (phenology) in relation to warming, in a coastal region of the Saronikos Gulf (Aegean Sea). A Waste-Water Treatment Plant (WWTP) was established in 1995, leading to decreased nutrient concentrations circa 2004. Overall, the results indicate an interplay between warming and changes in ecological status. During higher nutrient input (1989-2004), a temporal mismatch between zooplankton and phytoplankton, and a positive zooplankton growth-SST association, are evident. Conversely, in the warmer, less mesotrophic period 2005-2015, an earlier timing of zooplankton growth (related to copepod abundance) synchronizes with phytoplankton growth, including a secondary autumn growth period. Concurrently, an abrupt negative interannual relationship between SST and mesozooplankton, and a summer biomass decrease (linked with cladoceran abundance) are observed. This work provides evidence that current warming could alter plankton abundance and phenology in nearshore Eastern Mediterranean ecosystems, suggesting shifts in plankton community composition that could trigger potential cascading effects on higher trophic levels.

Nitrogen enrichment ameliorates the stimulatory effects of reduced salinity on photosynthesis and growth of phytoplankton assemblages in the northern South China sea

Anthropogenic activities and climate change are exacerbating the occurrence of extreme rainfall that normally brings large amounts of nutrient-rich freshwater from the land to the sea, resulting in acute salinity decrease and nutrient increase. To evaluate the effects of such changes in salinity and nutrients, we tracked the changes in photosynthetic efficiency and growth of phytoplankton assemblages from the northern South China Sea at 5 salinity levels and at an intermediate salinity level with 3 or 4 nitrogen concentrations. The results showed that the reduction of salinity reduced the maximum photochemical quantum yield (FV/FM) of photosystem II of phytoplankton within a short-term cultivation (i.e. 24-72 h), followed by a stimulatory effect. The reducing effect of reduced salinity lasted longer in the nearshore area than in the offshore area, so the stimulatory effect occurred later in the former area. Nitrogen enrichment mitigated the negative effect of reduced salinity in short-term cultivation and showed a positive effect on FV/FM in long-term cultivation. Moreover, both reduced salinity and enriched nitrogen stimulated phytoplankton growth after an acclimation period. Our results suggest that the reduced salinity stresses phytoplankton in the short term, which is mitigated by nitrogen enrichment, but benefits them in the long term. This sheds light on how phytoplankton thrive and even flourish in coastal or estuarine environments where salinity and nutrients typically covary strongly after extreme rainfall.