Benthic harmful microalgae and their impacts in South America

Toxic plastisphere: How the characteristics of plastic particles can affect colonization of harmful microalgae and adsorption of phycotoxins

Microplastics (MP) are suitable substrates for the colonization of harmful microalgal cells and the adsorption of their lipophilic compounds including phycotoxins. Moreover, such interactions likely change as physical-chemical characteristics of the MP surface are gradually modified during plastic degradation in aquatic environments. Using a combination of innovative laboratory experiments, this study systematically investigated, for the first time, the influence of various MP characteristics (polymeric composition, shape, size, and/or surface roughness) on its capacity to carry both living harmful algal cells and dissolved phycotoxins. Cell colonization by the dinoflagellate Prorocentrum lima started early (within 24 h) on particles of all shapes tested. However, cell colonization was much more intense on polystyrene ∼800 µm microspheres (0.63-46.4 cells mm-²; mean=11.7) and 500 × 1000 μm cuboid fragments (0.64-28.3 cells mm-²; mean=7.0), compared to polypropylene 11,000 × 50 µm microfibers of equivalent surface area (0.01-0.64 cells mm-²; mean=0.28), which were probably too narrow and light to interact with these benthic cells. Similar to lipophilic pollutants, adsorption of the diarrhetic toxin okadaic acid (OA) was greater on smaller MP particles (50 µm), attaining up to 8.0 pg mm² after 168 h of exposure. Moreover, in the short term (24 h), OA adsorption was significantly higher on aged MP, whose surface was modified following common degradation processes (abrasion, UV-photodegradation or microbial biodegradation), relative to virgin particles. During benthic P. lima blooms, the presence of aged MP covered by toxic cells and/or their dissolved compounds are expected to make diarrhetic toxins available to a greater diversity of organisms.

The second skin of macroalgae: Unveiling the biodiversity of epiphytic microalgae across environmental gradients of the Magellan Subantarctic ecoregion

The Magellan Subantarctic ecoregion (MSE) in the Southern Hemisphere (47°-56°S; 71°-73°W) is a unique natural laboratory subject to persistent and accelerated glacial ice melt, generating a complex system of environmental gradients (e.g., salinity and temperature) that influence the ecological patterns of marine biodiversity. However, the factors influencing marine epiphytic microalgal assemblages are still poorly understood. In this context, we characterized the richness and structure of epiphytic assemblages in different benthic macroalgal hosts (Acrosiphonia arcta, Ectocarpus siliculosus, and Leptosiphonia brodiei) in sites with glaciers and estuarine characteristics (Yendegaia Bay and Fouquet Estuary) and sites without glaciers and oceanic characteristics (Batchelor River and Offing Island) of the MSE, revealing how sites, host, and environmental variables influence variation of epiphytic assemblages. In 36 samples, 67 genera of epiphytes were recorded. The dominant divisions were Bacillariophyta (50 genera), Dinophyta (7 genera) and Cyanophyta (6 genera). We observed significantly high diversity in epiphytic assemblages, with contrasting patterns of variation depending on site and/or host macroalgae. Host specificity was not evident for most epiphytes. The most factor influencing the variation of the epiphythic assemblage was the marked environmental gradient (changes in temperature, salinity, nutrients, among others) between sites with and without glacial influence. Additionally, our research identified potentially toxic and/or harmful epiphytic microalgae belonging to the divisions Dinophyta (dinoflagellates) and Cyanophyta (cyanobacteria). The data on ecological patterns of epiphyte assemblages provides valuable insights into the current state of a poorly understood microscopic biodiversity, shaped by diverse environmental factors at different sites. Under current and future climate change scenarios in the MSE, environmental gradients may become more pronounced, with important positive and/or negative consequences on epiphyte assemblages. In light of these findings, we present a baseline for future research to further develop our understanding and facilitate the monitoring and conservation of epiphytic microalgae in the MSE.

In vitro effects of the harmful benthic dinoflagellates Prorocentrum hoffmannianum and Ostreopsis cf. ovata on immune responses of the farmed oyster Crassostrea gasar

Oyster culture is a sustainable solution to food production. However, this activity can be severely impacted by the presence and proliferation of harmful microalgae such as the benthic dinoflagellates Prorocentrum hoffmannianum and Ostreopsis cf. ovata. This study aimed to evaluate the in vitro effects of P. hoffmannianum and O. cf. ovata on immune system cells (hemocytes) of the native cultured oyster Crassostrea gasar. The direct toxicity of both dinoflagellates was first evaluated assessing hemocyte viability exposed to eight concentrations of each HAB species. No reduction in hemocyte viability was found with the exposure to cell culture or the crude extract of P. hoffmannianum, but O. cf. ovata culture induced hemocyte death in a concentration-dependent manner. Ostreopsis cf. ovata concentration that promoted half of maximal reduction in hemocyte viability (EC50) was 779 cells mL-1. Posteriorly, hemocytes were exposed to both dinoflagellate cells and crude extracts to investigate their effects on hemocyte functional parameters. Despite no direct toxicity of the dinoflagellate cells, P. hoffmannianum extract caused a threefold increase in ROS production and decreased the phagocytosis rate by less than half. Ostreopsis cf. ovata cells and crude extracts also triggered an increase in ROS production (two-fold), but the phagocytosis rate was reduced (by half) only in response to the two lower cell concentrations. These results indicate a harmful potential of both dinoflagellates through a direct toxicity (only for O. cf. ovata) and functional impairment of hemocytes (both species) which could expose C. gasar oyster to opportunistic infections.

Subcellular effects and lipid metabolism alterations in the gilthead seabream Sparus aurata fed on ovatoxins-contaminated mussels

The marine microalgae Ostreopsis cf. ovata are a well-known producer of palytoxin (PlTXs) analogues, i.e. ovatoxins (OVTXs) among others, which arouse concern for animal and human health. Both in field and laboratory studies, presence of OVTXs, detected in species directly feeding on O. cf. ovata, was frequently correlated with impairment on organisms' physiology, development and behaviour, while similar knowledge is still lacking for animals feeding on contaminated preys. In this study, transfer and toxicity of OVTXs were evaluated in an exposure experiment, in which gilthead seabream Sparus aurata was fed with bivalve mussel Mytilus galloprovincialis, contaminated by a toxic strain of O. cf. ovata. Mussels exposed to O. cf. ovata for 21 days accumulated meanly 188 ± 13 μg/kg OVTXs in the whole tissues. Seabreams fed with OVTX-contaminated mussels started to reject the food after 6 days of contaminated diet. Although no detectable levels of OVTXs were measured in muscle, liver, gills and gastro-intestinal tracts, the OVTX-enriched diet induced alterations of lipid metabolism in seabreams livers, displaying a decreased content of total lipid and fatty acid, together with overexpression of fatty acid biosynthetic genes, downregulation of β-oxidation genes and modulation of several genes related to lipid transport and regulation. Results from this study would suggest the hypothesis that OVTXs produced by O. cf. ovata may not be subject to bioaccumulation in fish fed on contaminated preys, being however responsible of significant biological effects, with important implications for human consumption of seafood products.

Occurrence of potentially toxic microalgae and diarrhetic shellfish toxins in the digestive tracts of green sea turtles (Chelonia mydas) from southern Brazil

Algal toxins are involved in the mortality and/or illness of marine organisms via consumption of contaminated prey, or upon direct exposure to toxic cells. In this study, the presence of potentially toxic microalgal cells was investigated within the digestive tract contents of a threatened species of green turtle (Chelonia mydas). Additionally, lipophilic toxins were determined by LC-MS/MS in tissue samples (liver, stomach and/or intestine) of selected animals (n = 39 individuals) found dead-stranded in southern Brazil, from winter/2015 to autumn/2016. Thirteen potentially toxic species of microalgae (both benthic and planktonic), including seven dinoflagellates, six cyanobacteria and one diatom, were found in the digestive tract contents of green turtles. Among them, dinoflagellates belonging to the Dinophysis acuminata species complex were the most frequent (36%) and abundant (maximum average abundance of 566 cells g-1 in spring/2015). Moreover, 23% of the examined sea turtles exhibited detectable levels of the diarrhetic shellfish toxin okadaic acid (OA) in washed digestive tissues. Seven individuals accumulated OA in their intestines (max. 24.1 ng g-1) and two in the stomachs (max. 7.4 ng g-1). Toxin levels in the tissues were directly and significantly (r = 0.70, p < 0.025) associated with the cell abundance of OA-producing D. acuminata and Prorocentrum lima species complexes within the digestive contents of green turtles. Although OA concentrations were relatively low, possible chronic exposure might deteriorate general health conditions of exposed sea turtles, increasing the risk for diseases. Okadaic acid has been regarded as a tumor-promoting compound and an environmental co-factor in the incidence of fibropapillomatosis, a frequent disease in juvenile green turtles inhabiting this geographic region. Even though, only one green turtle containing OA in the digestive tissues (out of six examined) also presented fibropapillomatosis in this study. Notwithstanding, sea turtles are sentinels of ocean health. Monitoring the accumulation of algal toxins and their negative effects on these organisms contributes to conserving biodiversity and marine habitats.