The frequency of cyanobacterial proliferations in fresh waters is increasing worldwide and the presence of associated cyanotoxins represent a threat for ecosystems and human health. While the occurrence of microcystin (MC), the most widespread cyanotoxin, is well documented in freshwaters, only few studies have examined its occurrence in estuarine waters. In this study we evaluated the transfer of cyanobacteria and cyanotoxins along a river continuum from a freshwater reservoir through an interconnecting estuary to the coastal area in Brittany, France. We sampled regularly over 2 years at 5 stations along the river continuum and analysed for phytoplankton and cyanotoxins, together with physico-chemical parameters. Results show that cyanobacteria dominated the phytoplanktonic community with high densities (up to 2x10 6 cells mL-1) at the freshwater sites during the summer and autumn periods of both years, with a cell transfer to estuarine (up to 10 5 cells mL-1) and marine (2x10 3 cells mL-1) sites. While the temporal variation in cyanobacterial densities was mainly associated with temperature, spatial variation was due to salinity while nutrients were non-limiting for cyanobacterial growth. Cyanobacterial biomass was dominated by several species of Microcystis that survived intermediate salinities. Intracellular MCs were detected in all the freshwater samples with concentrations up to 60 µg L-1 , and more intermittently with concentrations up to 1.15 µg L-1 , at the most upstream estuarine site. Intracellular MC was only sporadically detected and in low concentration at the most downstream estuarine site and at the marine outlet (respectively < 0.14 µg L-1 and < 0.03 µg L-1). Different MC variants were detected with dominance of MC-LR, RR and YR and that dominance was conserved along the salinity gradient. Extracellular MC contribution to total MC was higher at the downstream sites in accordance with the lysing of the cells at elevated salinities. No nodularin (NOD) was detected in the particulate samples or in the filtrates.

While transfer of freshwater cyanobacteria to estuaries has been observed worldwide, the associated transfer of cyanotoxins is less often reported, in particular the sediment contribution. During fall 2018, we monitored the co-occurrence of cyanobacteria and microcystin (MC) in both the water column and in surface sediments at 5 stations along a river continuum, from a freshwater reservoir to the coastal area in Brittany, France. Cyanobacteria dominated the phytoplankton community in the water column with high densities at the freshwater sites. Microcystis cells and intracellular MC transfer to estuarine and marine sites were observed with decreasing concentrations in accordance with flow dilution. Extracellular MC showed the opposite trend and increased from upstream to downstream in accordance with the lysing of the cells at elevated salinities. Surface sediment samples contained high densities of colonial Microcystis in freshwater and with decreasing concentrations along the salinity gradient, similarly to cells concentrations in the water column. Intracellular MC was detected in sediment at all sites except at the marine outlet suggesting the survival of intact cells. Extracellular MC concentrations in sediment were up to 5 times higher than intracellular

Ciguatera poisoning is caused by the ingestion of fish or shellfish contaminated with ciguatoxins produced by dinoflagellate species belonging to the genera Gambierdiscus and Fukuyoa. Unlike in the Pacific region, the species producing ciguatoxins in the Atlantic Ocean have yet to be definitely identified, though some ciguatoxins responsible for ciguatera have been reported from fish. Previous studies investigating the ciguatoxin-like toxicity of Atlantic Gambierdiscus species using Neuro2a cell-based assay identified G. excentricus as a potential toxin producer. To more rigorously characterize the toxin profile produced by this species, a purified extract from 124 million cells was prepared and partial characterization by high-resolution mass spectrometry was performed. The analysis revealed two new analogs of the polyether gambierone: sulfo-gambierone and dihydro-sulfo-gambierone. Algal ciguatoxins were not identified. The very low ciguatoxin-like toxicity of the two new analogs obtained by the Neuro2a cell-based assay suggests they are not responsible for the relatively high toxicity previously observed when using fractionated G. excentricus extracts, and are unlikely the cause of ciguatera in the region. These compounds, however, can be useful as biomarkers of the presence of G. excentricus due to their sensitive detection by mass spectrometry.

The transfer of Microcystis aeruginosa from freshwater to estuaries has been described worldwide and salinity is reported as the main factor controlling the expansion of M. aeruginosa to coastal environments. Analyzing the expression levels of targeted genes and employing both targeted and non-targeted metabolomic approaches, this study investigated the effect of a sudden salt increase on the physiological and metabolic responses of two toxic M. aeruginosa strains separately isolated from fresh and brackish waters, respectively, PCC 7820 and 7806. Supported by differences in gene expressions and metabolic profiles, salt tolerance was found to be strain specific. An increase in salinity decreased the growth of M. aeruginosa with a lesser impact on the brackish strain. The production of intracellular microcystin variants in response to salt stress correlated well to the growth rate for both strains. Furthermore, the release of microcystins into the surrounding medium only occurred at the highest salinity treatment when cell lysis occurred. This study suggests that the physiological responses of M. aeruginosa involve the accumulation of common metabolites but that the intraspecific salt tolerance is based on the accumulation of specific metabolites. While one of these was determined to be sucrose, many others remain to be identified. Taken together, these results provide evidence that M. aeruginosa is relatively salt tolerant in the mesohaline zone and microcystin (MC) release only occurs when the capacity of the cells to deal with salt increase is exceeded.

Cyanobacteria are a potential threat to aquatic ecosystems and human health because of their ability to produce cyanotoxins, such as microcystins (MCs). MCs are regularly monitored in fresh waters, but rarely in estuarine and marine waters despite the possibility of their downstream export. Over a period of two years, we monthly analysed intracellular (in phytoplankton) and extracellular (dissolved in water) MCs at five stations along a river continuum from a freshwater reservoir with ongoing cyanobacterial blooms to the coast of Brittany, France. MCs were quantified using two integrative samplers placed at each site: solid phase adsorption toxin tracking (SPATT) samplers for collecting extracellular MCs and caged mussels (Anodonta anatina and Mytilus edulis) filter-feeding on MC-producing cyanobacteria.The MC transfer was demonstrated each year during five months at estuarine sites and sporadically at the marine outlet. SPATT samplers integrated extracellular MCs, notably at low environmental concentrations (0.2 µg/L) and with the same variant profile as in water. The mussel A. anatina highlighted the presence of MCs including at intracellular concentrations below 1 µg/L. M. edulis more efficiently revealed the MC transfer at estuarine sites than punctual water samplings. Bivalves showed the same MC variant profile as phytoplankton samples, but with differential accumulation capacities between the variants and the two species.Using SPATT or bivalves can give a more accurate assessment of the contamination level of a freshwater-marine continuum, in which the MC transfer can be episodic. MC content in M. edulis represents a potent threat to human health if considering updated French guideline values, and particularly the total (free and protein-bound) MC content, highlighting the necessity to include cyanotoxins in the monitoring of seafood originating from estuarine areas.