Major loss of coralline algal diversity in response to ocean acidification.

Calcified coralline algae are ecologically important in rocky habitats in the marine photic zone worldwide and there is growing concern that ocean acidification will severely impact them. Laboratory studies of these algae in simulated ocean acidification conditions have revealed wide variability in growth, photosynthesis and calcification responses, making it difficult to assess their future biodiversity, abundance and contribution to ecosystem function. Here, we apply molecular systematic tools to assess the impact of natural gradients in seawater carbonate chemistry on the biodiversity of coralline algae in the Mediterranean and the NW Pacific, link this to their evolutionary history and evaluate their potential future biodiversity and abundance. We found a decrease in the taxonomic diversity of coralline algae with increasing acidification with more than half of the species lost in high pCO2 conditions. Sporolithales is the oldest order (Lower Cretaceous) and diversified when ocean chemistry favoured low Mg calcite deposition; it is less diverse today and was the most sensitive to ocean acidification. Corallinales were also reduced in cover and diversity but several species survived at high pCO2 ; it is the most recent order of coralline algae and originated when ocean chemistry favoured aragonite and high Mg calcite deposition. The sharp decline in cover and thickness of coralline algal carbonate deposits at high pCO2 highlighted their lower fitness in response to ocean acidification. Reductions in CO2 emissions are needed to limit the risk of losing coralline algal diversity.

Beta-1,3-glucanase inhibitors in Brazilian brown seaweed.

Beta-1,3-glucanases are enzymes that hydrolyze beta-1,3-glucans, and they are essential for the metabolism of seaweed, plants and fungi. These enzymes also participate in the digestion of herbivore and fungivore animals. Because of the importance of these enzymes in insects, beta-1,3-glucanase inhibitors may be used for the development of new control strategies against agricultural pests and disease vectors. Beta-1,3-glucanase inhibitors have been described in the brown seaweed Laminaria cichorioides, but were never recorded in Brazilian seaweed species. We evaluated the presence of beta-1,3-glucanase inhibitors in samples of Padina gymnospora, Dictyota sp., Colpomenia sinuosa, and Lobophora sp., collected in Arraial d'Ajuda (Bahia). Ethanolic or buffer extracts were used in inhibition tests against the beta-1,3-glucanase of Trichoderma sp. Extracts in buffer showed no inhibition, but ethanolic extracts from all species showed different extents of inhibition. Samples from Dictyota sp. and P. gymnospora showed inhibitions above 75% (absolute ethanol) or 50% (ethanol 50%). In summary, extraction with absolute ethanol resulted in better inhibitions, and P. gymnospora showed the higher inhibitions. Brazilian seaweed may be good sources of beta-1,3-glucanase inhibitors for biochemical and physiological studies of these enzymes. Besides that, these molecules show potential for the development of new biotechnological tools for insect control.

Decadal losses of canopy-forming algae along the warm temperate coastline of Brazil.

The loss of canopy-forming seaweeds from urbanized coasts has intensified in response to warming seas and non-climatic pressures such as population growth and declining water quality. Surprisingly, there has been little information on the extent of historical losses in the South-western Atlantic, which limits our ability to place this large marine ecosystem in a global context. Here, we use meta-analysis to examine long-term (1969-2017) changes to the cover and biomass of Sargassum spp. and structurally simple algal turfs along more than 1,000 kilometres of Brazil's warm temperate coastline. Analysis revealed major declines in canopy cover that were independent of season (i.e., displaying similar trends for both summer and winter) but varied with coastal environmental setting, whereby sheltered bays experienced greater losses than coastal locations. On average, covers of Sargassum spp. declined by 2.6% per year, to show overall losses of 52% since records began (ranging from 20% to 89%). This contrasted with increases in the cover of filamentous turfs (24% over the last 27 years) which are known to proliferate along human-impacted coasts. To test the relative influence of climatic versus non-climatic factors as drivers of this apparent canopy-to-turf shift, we examined how well regional warming trends (decadal changes to sea surface temperature) and local proxies of coastal urbanization (population density, thermal pollution, turbidity and nutrient inputs) were able to predict the changes in seaweed communities. Our results revealed that the most pronounced canopy losses over the past 50 years were at sites exhibiting the greatest degree of coastal warming, the highest population growth and those located in semi-enclosed sheltered bays. These findings contribute knowledge on the drivers of canopy loss in the South-western Atlantic and join with global efforts to understand and mitigate declines of marine keystone species.

Physiological and biochemical responses driven by different UV-visible radiation in Osmundea pinnatifida (Hudson) Stackhouse (Rhodophyta).

Light, or visible radiation, serves as a source of energy for photosynthesis of plants and most algae. In addition, light and ultraviolet radiation (UV-A and UV-B) act as a biological signal, triggering several cellular processes that are mediated by photoreceptors. The aim of this study was to evaluate the physiological and biochemical responses of Osmundea pinnatifida driven by different radiations through putative photoreceptors. For this, O. pinnatifida was grown under different radiation treatments composed by high intensity of light emitted by a low pressure sodium lamp (SOX), aiming to saturate photosynthesis, which was supplemented by low intensities of visible (red, green and blue) and ultraviolet radiation (UV-A and UV-B), in order to activate photoreceptors. Growth rates, photosynthesis, antioxidant activity, polyphenols, soluble proteins, phycobiliproteins, mycosporine-like amino acids (MAAs) and carotenoids were evaluated during the experiment. Complementary UV-A radiation positively influenced growth rates after 15 days of experiment, although the presence of a peak of blue light in this treatment can also have contributed. UV-B radiation increased the concentration of zeaxanthin and chlorophyll a. The blue light caused the accumulation of chlorophyll a, violaxanthin, phycoerythrin and polyphenols on different days of the experiment. Phycoerythrin also increased under green and red light conditions. Our results showed that some compounds can be modulated by different radiation, and the involvement of photoreceptors is suggested. In red algae, photoreceptors sensitive to red, green and blue light have been identified, however little is known about UV photoreceptors. The presence of photoreceptors sensitive to UV radiation in O. pinnatifida is discussed.

Reef-building coralline algae from the Southwest Atlantic: filling gaps with the recognition of Harveylithon (Corallinaceae, Rhodophyta) on the Brazilian coast.

The Southwest Atlantic is notable for having extensive reef areas cemented by nongeniculate coralline red algae. Based on an analysis of four genetic markers and morpho-anatomical features, we clarify the species of Harveylithon in the tropical and warm temperate Southwest Atlantic. Species delimitation methods (mBGD, ABGD, SPN, and PTP), using three markers (psbA, rbcL, and COI), support the recognition of three new species: H. catarinense sp. nov., H. maris-bahiensis sp. nov., and H. riosmenum sp. nov., previously incorrectly called Hydrolithon samoënse. Our findings highlight the importance of using an approach with several lines of evidence to solve the taxonomic status of the cryptic species.

Phytoremediation potential of Ulva ohnoi (Chlorophyta): Influence of temperature and salinity on the uptake efficiency and toxicity of cadmium.

Ulva ohnoi is a green macroalga with fast growth and high rates of nitrogen and phosphorus absorption. Recently, this species has been recorded in several places with record green tide formation in some of them. Using molecular tools, we herein report the first occurrence of this species in Brazil and demonstrate its potential for phytoremediation in typical environmental concentrations of Cd (0.625-15 µg L-1). Similarly, the effects of physicochemical parameters (salinity and temperature) on the toxicity and uptake efficiency of this species were evaluated. Molecular analysis of two sequences (1141 bp) obtained corroborates another 34 sequences for U. ohnoi obtained from GenBank. The addition of Cd in the medium affected photosynthetic parameters and reduced growth rate. U. ohnoi showed resistance to Cd when cultivated at 18 °C, S15 and 18-25 °C, S35, at concentrations between 0.625 and 2.5 µg. L-1 of Cd; yet, positive growth rate was maintained. Dose-dependent accumulation was observed in all combinations of factors used with a maximum value of 4.20 µg Cd per gram of dry seaweed at 15 µg. L-1 of Cd at 18 °C and S35. Maximum value of the concentration factor was 81.3 ±â€¯1.1% of Cd added at the concentration of 0.625 µg. L-1 to S15 and 18 °C. Our results demonstrate the potential of using U. ohnoi in the phytoremediation of Cd in saltwater or brackish water.

The influence of environmental features in the content of mycosporine-like amino acids in red marine algae along the Brazilian coast.

Mycosporine-like amino acids (MAA) are ultraviolet screen substances synthesized by marine algae. The physiological function of these substances is related to cellular protection against UV radiation and as a protective mechanism against oxidative stress. These substances can be found mainly in the ocean, among red seaweeds. Its concentration in organisms has been related to ultraviolet radiation and availability of inorganic nitrogen in the environment. We start our study of MAA content in different species to understand if environmental conditions influence the concentration of MAAs in red seaweeds. The Brazilian coast presents abiotic factors that interact to create different physical-chemical features in the environment. We collected 441 samples from 39 species of red seaweed easily found in the intertidal zone, in low tide, during the summer of 2015. The sampling encompassed a latitudinal gradient (3° S to 28°5' S) at 23 points along the coast. We quantified and identified the content of MAAs in species through the method of high performance liquid chromatography. We detected for the first time the occurrence of MAAs in certain species of red algae that have not been reported to contain MAAs before. We confirmed that some environmental factors influenced the content of MAAs. Enhanced MAA contents, for example, were found in environments with a basic pH, a high ultraviolet index, and high concentrations of phosphate and nitrate. Salinity, dissolved oxygen and variations of sea surface temperature also influenced, in a secondary way, MAA content in algae in their natural environments.

The Western South Atlantic Ocean in a High-CO2 World: Current Measurement Capabilities and Perspectives.

An international multi-disciplinary group of 24 researchers met to discuss ocean acidification (OA) during the Brazilian OA Network/Surface Ocean-Lower Atmosphere Study (BrOA/SOLAS) Workshop. Fifteen members of the BrOA Network (www.broa.furg.br) authored this review. The group concluded that identifying and evaluating the regional effects of OA is impossible without understanding the natural variability of seawater carbonate systems in marine ecosystems through a series of long-term observations. Here, we show that the western South Atlantic Ocean (WSAO) lacks appropriate observations for determining regional OA effects, including the effects of OA on key sensitive Brazilian ecosystems in this area. The impacts of OA likely affect marine life in coastal and oceanic ecosystems, with further social and economic consequences for Brazil and neighboring countries. Thus, we present (i) the diversity of coastal and open ocean ecosystems in the WSAO and emphasize their roles in the marine carbon cycle and biodiversity and their vulnerabilities to OA effects; (ii) ongoing observational, experimental, and modeling efforts that investigate OA in the WSAO; and (iii) highlights of the knowledge gaps, infrastructure deficiencies, and OA-related issues in the WSAO. Finally, this review outlines long-term actions that should be taken to manage marine ecosystems in this vast and unexplored ocean region.

Effects of eutrophic seawater and temperature on the physiology and morphology of Hypnea musciformis J. V. Lamouroux (Gigartinales, Rhodophyta).

As both food and source of a kappa-carrageenan, Hypnea musciformis represents a species of great economic interest. It also synthesizes substances with antiviral, anti-helminthic and anti-inflammatory potential and shows promise for use as a bioindicator of cadmium. In this study, we investigated the combined effects of seawater from three urbanized areas (area 1: natural runoff, NRA; area 2: urbanized runoff and sewage with treatment, RTA; area 3: urbanized runoff and untreated sewage, RUS) and three different temperatures (15, 25 and 30 °C) on the growth rate, photosynthetic efficiency, photosynthetic pigments and cell morphology of H. musciformis. After 4 days (96 h) of culture, the biomass of H. musciformis showed differences that fluctuated among the areas and temperature treatments. Specifically, the specimens cultivated in 35 °C had low values of ETRmax, α(ETR), ß(ETR), and Fv/Fm photosynthetic parameters, as well as changes in cell morphology, with reduction in photosynthetic pigments and drastic reduction in growth rates. When combined with the extreme temperatures, high concentrations of ammonium ion in seawater effluent caused an inhibition of photosynthetic activity, as well as significant variation in chlorophyll a and carotenoid contents. As observed by light microscopy, the synergism between different temperatures and pollutants found in eutrophic waters caused changes in cellular morphology with increased cell wall thickening and decreased floridean starch grains. H. musciformis also showed important changes in physiological response to each factor independently, as well as changes resulting from the synergistic interaction of these factors combined. Therefore, we can conclude that extreme temperature combined with the effect of eutrophic waters, especially RUS, caused distinct morphological and physiological changes in the red alga H. musciformis.

The effect of cadmium under different salinity conditions on the cellular architecture and metabolism in the red alga Pterocladiella capillacea (Rhodophyta, Gelidiales).

The in vitro effect of cadmium (Cd) on apical segments of Pterocladiella capillacea was examined. Over a period of 7 days, the segments were cultivated with the combination of different salinities (25, 35, and 45 practical salinity units) and Cd concentrations, ranging from 0.17 to 0.70 ppm. The effects of Cd on growth rates and content of photosynthetic pigments were analyzed. In addition, metabolic profiling was performed, and samples were processed for microscopy. Serious damage to physiological performance and ultrastructure was observed under different combinations of Cd concentrations and salinity values. Elementary infrared spectroscopy revealed toxic effects registered on growth rate, photosynthetic pigments, chloroplast, and mitochondria organization, as well as changes in lipids and carbohydrates. These alterations in physiology and ultrastructure were, however, coupled to activation of such defense mechanisms as cell wall thickness, reduction of photosynthetic harvesting complex, and flavonoid. In conclusion, P. capillacea is especially sensitive to Cd stress when intermediate concentrations of this pollutant are associated with low salinity values. Such conditions resulted in metabolic compromise, reduction of primary productivity, i.e., photosynthesis, and carbohydrate accumulation in the form of starch granules. Taken together, these findings improve our understanding of the potential impact of this metal in the natural environment.

Modelling bioremediation of contaminated effluents by Ulva ohnoi. - A predictive perspective.

Ulva spp. are tolerant to salinity variations and exhibit easy acclimation, playing an essential role in the depollution of aquatic ecosystems precisely due to their high efficiency in absorbing and accumulating nutrients. For this reason, Ulva spp. becomes an attractive solution for recovering areas that suffer the impacts of problems such as the eutrophication of anthropogenic origin. In addition to being a promising alternative for the blue bioeconomy, it can contribute to the sustainability of economic activities in coastal areas. Therefore, the present study aimed to develop and elucidate the behavior of Ulva ohnoi using predictive surface response models. The algae were grown under different concentrations of nutrient and salinity levels, as predicted by the experimental design, and it was evaluated according to the potential of the biomass to absorb the nutrients, as well as its photosynthetic performance and biochemical parameters. Our study confirmed the high efficiency and preference of Ulva ohnoi in the absorption of nitrogen dissolved in the medium in the form of NH4+ and that salinity is an essential factor in the dynamics and speed of ammonium absorption. The absorption of orthophosphate by U. ohnoi is reverted to the culture medium when subjected to long-term cultivation. This process was more intense because of low salinity, even at conditions of availability of the compound. The 3D-models of response surfaces elucidate the behavior of Ulva ohnoi, attributing a correlation between nutrient availability and salinity and the biological behavior of the species. In view of what is exposed by these models, as well as the effects of saline distribution along the Lagoon, the following regions of the lagoon are suggested: Center-North, Center and South - as potential areas for the implementation of bioremediation projects with Ulva ohnoi.

Anti-infective potential of marine invertebrates and seaweeds from the Brazilian coast.

This manuscript describes the evaluation of anti-infective potential in vitro of organic extracts from nine sponges, one ascidian, two octocorals, one bryozoan, and 27 seaweed species collected along the Brazilian coast. Antimicrobial activity was tested against Staphylococcus aureus (ATCC 25923), Enterococcus faecalis (ATCC 29212), Pseudomonas aeruginosa (ATCC 27853), Escherichia coli (ATCC 25922) and Candida albicans (ATCC 10231) by the disk diffusion method. Antiprotozoal activity was evaluated against Leishmania braziliensis (MHOM/BR/96/LSC96-H3) promastigotes and Trypanosoma cruzi (MHOM/BR/00/Y) epimastigotes by MTT assay. Activity against intracellular amastigotes of T. cruzi and L. brasiliensis in murine macrophages was also evaluated. Antiviral activity was tested against Herpes Simplex Virus type 1 (HSV-1, KOS strain) by the plaque number reduction assay (IC50). Cytotoxicity on VERO cells was evaluated by the MTT assay (CC50). The results were expressed as SI = CC50/IC50. The most promising antimicrobial results were obtained against S. aureus and C. albicans with Dragmacidon reticulatum. Among the seaweeds, only Osmundaria obtusiloba showed moderate activity against P. aeruginosa. Concerning antiprotozoal activity, Bugula neritina, Carijoa riseii, Dragmaxia anomala and Haliclona (Halichoclona) sp. showed the most interesting results, mainly against extracellular promastigote forms of L. braziliensis (66, 35.9, 97.2, and 43.6% inhibition, respectively). Moreover, six species of seaweeds Anadyomene saldanhae, Caulerpa cupressoides, Canistrocarpus cervicornis, Dictyota sp., Ochtodes secundiramea, and Padina sp. showed promising results against L. braziliensis (87.9, 51.7, 85.9, 93.3, 99.7, and 80.9% inhibition, respectively), and only Dictyota sp. was effective against T. cruzi (60.4% inhibition). Finally, the antiherpes activity was also evaluated, with Haliclona (Halichoclona) sp. and Petromica citrina showing the best results (SI = 11.9 and SI > 5, respectively). All the active extracts deserve special attention in further studies to chemically characterize the bioactive compounds, and to perform more refined biological assays.

Effects of ocean warming and pollution on Sargassum forests.

The combined effects of climate change and ocean pollution have resulted in a noteworthy decline of canopy-forming species, impacting marine biodiversity and ecosystem functioning significantly. In this context, Sargassum cymosum, which is widely distributed along the southwestern Atlantic Ocean, serves as an excellent model among canopy-forming species to investigate these impacts on populations in different regions and environmental conditions. Here, we evaluate the ecophysiological responses of two populations of S. cymosum, from Florianopolis (warm-temperate province; WTP) and Fernando de Noronha (tropical province, TP), through of interaction of temperatures and nutrient concentrations, representing marine heatwaves and acute pollution levels. Our findings revealed a decrease in biomass in both populations, highlighting the significance of nutrient enrichment as an anthropogenic filter that might potentially inhibit the expansion of the populations from tropical regions and temperature for WTP ones. These stressors directly impacted the physiological performance of S. cymosum populations, including relative growth rates, photosynthesis, chlorophylls, carotenoids and phenolic compound levels. Although there was an increase in both parameters for the TP population, a significant loss of biomass was observed, with growth rates reaching -1.5% per day. Our results highlight the need for urgent actions to manage the eutrophication process due to its negative association with global warming, which can enhance the impacts and preclude the settlement and survival of Sargassum in warm-temperate areas considering the observed and predicted tropicalization process.

From ecological functions to ecosystem services: linking coastal lagoons biodiversity with human well-being.

In this review we highlight the relevance of biodiversity that inhabit coastal lagoons, emphasizing how species functions foster processes and services associated with this ecosystem. We identified 26 ecosystem services underpinned by ecological functions performed by bacteria and other microbial organisms, zooplankton, polychaetae worms, mollusks, macro-crustaceans, fishes, birds, and aquatic mammals. These groups present high functional redundancy but perform complementary functions that result in distinct ecosystem processes. Because coastal lagoons are located in the interface between freshwater, marine and terrestrial ecosystems, the ecosystem services provided by the biodiversity surpass the lagoon itself and benefit society in a wider spatial and historical context. The species loss in coastal lagoons due to multiple human-driven impacts affects the ecosystem functioning, influencing negatively the provision of all categories of services (i.e., supporting, regulating, provisioning and cultural). Because animals' assemblages have unequal spatial and temporal distribution in coastal lagoons, it is necessary to adopt ecosystem-level management plans to protect habitat heterogeneity and its biodiversity, ensuring the provision of services for human well-being to multi-actors in the coastal zone.

Long range gene flow beyond predictions from oceanographic transport in a tropical marine foundation species.

The transport of passively dispersed organisms across tropical margins remains poorly understood. Hypotheses of oceanographic transportation potential lack testing with large scale empirical data. To address this gap, we used the seagrass species, Halodule wrightii, which is unique in spanning the entire tropical Atlantic. We tested the hypothesis that genetic differentiation estimated across its large-scale biogeographic range can be predicted by simulated oceanographic transport. The alternative hypothesis posits that dispersal is independent of ocean currents, such as transport by grazers. We compared empirical genetic estimates and modelled predictions of dispersal along the distribution of H. wrightii. We genotyped eight microsatellite loci on 19 populations distributed across Atlantic Africa, Gulf of Mexico, Caribbean, Brazil and developed a biophysical model with high-resolution ocean currents. Genetic data revealed low gene flow and highest differentiation between (1) the Gulf of Mexico and two other regions: (2) Caribbean-Brazil and (3) Atlantic Africa. These two were more genetically similar despite separation by an ocean. The biophysical model indicated low or no probability of passive dispersal among populations and did not match the empirical genetic data. The results support the alternative hypothesis of a role for active dispersal vectors like grazers.

Levelling-up rhodolith-bed science to address global-scale conservation challenges.

Global marine conservation remains fractured by an imbalance in research efforts and policy actions, limiting progression towards sustainability. Rhodolith beds represent a prime example, as they have ecological importance on a global scale, provide a wealth of ecosystem functions and services, including biodiversity provision and potential climate change mitigation, but remain disproportionately understudied, compared to other coastal ecosystems (tropical coral reefs, kelp forests, mangroves, seagrasses). Although rhodolith beds have gained some recognition, as important and sensitive habitats at national/regional levels during the last decade, there is still a notable lack of information and, consequently, specific conservation efforts. We argue that the lack of information about these habitats, and the significant ecosystem services they provide, is hindering the development of effective conservation measures and limiting wider marine conservation success. This is becoming a pressing issue, considering the multiple severe pressures and threats these habitats are exposed to (e.g., pollution, fishing activities, climate change), which may lead to an erosion of their ecological function and ecosystem services. By synthesizing the current knowledge, we provide arguments to highlight the importance and urgency of levelling-up research efforts focused on rhodolith beds, combating rhodolith bed degradation and avoiding the loss of associated biodiversity, thus ensuring the sustainability of future conservation programs.

Functional redundancy and stability in a subtidal macroalgal community in the Southwestern Atlantic coast.

Functional redundancy can stabilize ecological functions as asynchronous fluctuations among functionally similar species may buffer environmental changes. We investigated the temporal dynamics of a subtidal macroalgal community in the warm temperate Southwestern Atlantic coast (SWA) to evaluate whether functional redundancy stabilize ecosystems functions through compensatory dynamics under realistic environmental scenarios. Despite temporal variations in the community structure occurred, a high stability in macroalgal coverage was found at the community-level driven by taxa asynchronous fluctuations. No relationship between functional redundancy and stability occurred, suggesting that functional compensation cannot surpass the influence of environmental fluctuations on the performance of ecological functions. Declines in Sargassum species abundance, along with its low functional redundancy, indicate that this canopy-forming algae must be prioritized in conservation efforts in the SWA. Our study adds to the comprehension and generalization of biodiversity-stability findings in natural systems across distinct geographical areas, also contributing to their operationalization in marine ecosystems.

Physiology, niche characteristics and extreme events: Current and future habitat suitability of a rhodolith-forming species in the Southwestern Atlantic.

Given the ecological and biogeochemical importance of rhodolith beds, it is necessary to investigate how future environmental conditions will affect these organisms. We investigated the impacts of increased nutrient concentrations, acidification, and marine heatwaves on the performance of the rhodolith-forming species Lithothamnion crispatum in a short-term experiment, including the recovery of individuals after stressor removal. Furthermore, we developed an ecological niche model to establish which environmental conditions determine its current distribution along the Brazilian coast and to project responses to future climate scenarios. Although L. crispatum suffered a reduction in photosynthetic performance when exposed to stressors, they returned to pre-experiment values following the return of individuals to control conditions. The model showed that the most important variables in explaining the current distribution of L. crispatum on the Brazilian coast were maximum nitrate and temperature. In future ocean conditions, the model predicted a range expansion of habitat suitability for this species of approximately 58.5% under RCP 8.5. Physiological responses to experimental future environmental conditions corroborated model predictions of the expansion of this species' habitat suitability in the future. This study, therefore, demonstrates the benefits of applying combined approaches to examine potential species responses to climate-change drivers from multiple angles.

Calcification in free-living coralline algae is strongly influenced by morphology: Implications for susceptibility to ocean acidification.

Rhodolith beds built by free-living coralline algae are important ecosystems for marine biodiversity and carbonate production. Yet, our mechanistic understanding regarding rhodolith physiology and its drivers is still limited. Using three rhodolith species with different branching morphologies, we investigated the role of morphology in species' physiology and the implications for their susceptibility to ocean acidification (OA). For this, we determined the effects of thallus topography on diffusive boundary layer (DBL) thickness, the associated microscale oxygen and pH dynamics and their relationship with species' metabolic and light and dark calcification rates, as well as species' responses to short-term OA exposure. Our results show that rhodolith branching creates low-flow microenvironments that exhibit increasing DBL thickness with increasing branch length. This, together with species' metabolic rates, determined the light-dependent pH dynamics at the algal surface, which in turn dictated species' calcification rates. While these differences did not translate in species-specific responses to short-term OA exposure, the differences in the magnitude of diurnal pH fluctuations (~ 0.1-1.2 pH units) between species suggest potential differences in phenotypic plasticity to OA that may result in different susceptibilities to long-term OA exposure, supporting the general view that species' ecomechanical characteristics must be considered for predicting OA responses.

Saxitoxins from the freshwater cyanobacterium Raphidiopsis raciborskii can contaminate marine mussels.

Raphidiopsis raciborskii (formerly Cylindrospermopsis raciborskii) is a freshwater cyanobacterium potentially producing saxitoxins (STX) and cylindrospermopsin. Its ecophysiological versatility enables it to form blooms in the most diverse types of environments, from tropical to temperate, and from relatively pristine to polluted. In Peri Lake, located in the subtropical south of Brazil, growing populations of STX-producing R. raciborskii have been detected since 1994, posing risks to the use of its waters that supply a population of about 100,000 inhabitants. Despite the existence of a monitoring system for the presence and toxicity of cyanobacteria in Peri Lake water, no assessment has been made in the coastal region, downstream of outflowing lake water, thereby potentially making available a toxic biomass to natural and cultivated shellfish populations in the salt water ecosystem. To address this problem, the present study evaluated environmental variables and STX concentration by profiling the outflowing waters between Peri Lake and the adjacent coastal zone. Laboratory experiments were carried out with three strains of R. raciborskii in order to confirm the effect of salinity on STX production and verify if Perna Perna mussels fed with R. raciborskii cultures would absorb and accumulate STX. Results showed that environmental concentrations of STX reach high levels (up to 6.31 µg L-1 STX eq.), especially in the warmer months, reaching the coastal zone. In laboratory tests, it was found that the strains tolerate salinities between 4 and 6 and that salinity influences the production of STX. In addition, mussels fed with R. raciborskii effectively absorb and accumulate STX, even in typically marine salinities (22 to 30), suggesting that R. raciborskii biomass remains available and toxic despite salinity shock. These results draw attention to the ecological and health risk associated with R. raciborskii blooms, both in the lake environment and in the adjacent marine environment, calling attention to the need to improve the monitoring and management systems for water and shellfish toxicity in the region of interest, as well as other places where toxic cyanobacteria of limnic origin can reach the coastal zone.