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 five 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, similar 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 five times higher than intracellular concentrations suggesting incomplete MC degradation. mcyB genes were present at all sites, while mcyA genes were absent at the marine outlet suggesting the presence of toxic strains along the estuary. The high densities of intact colonies of potentially toxic Microcystis in the estuarine sediment strongly suggest that sediments can act as an inoculum of cyanobacteria and cyanotoxins in estuaries.

中文翻译：

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河口水和沉积物中的蓝细菌和氰毒素

虽然全世界范围内都观察到了淡水蓝细菌向河口的转移，但鲜有相关的蓝藻毒素转移的报道，特别是沉积物的贡献。在2018年秋季，我们监测了从淡水水库到法国布列塔尼沿海地区的五个河流沿河连续体的水柱和表面沉积物中蓝细菌和微囊藻毒素（MC）的共存情况。蓝细菌在淡水场所以高密度控制了水柱中的浮游植物群落。微囊藻随着血流稀释，细胞浓度和细胞内MC转移至河口和海洋位置的浓度降低。细胞外MC显示相反的趋势，并且随着盐度升高细胞的裂解而从上游到下游增加。表面沉积物样品在淡水中含有高密度的结肠微囊藻，并且沿盐度梯度的浓度降低，类似于水柱中的细胞浓度。除海洋出口外，所有位置的沉积物中均检测到细胞内MC，表明完整细胞的存活。沉积物中的细胞外MC浓度比细胞内浓度高五倍，表明MC降解不完全。cyb基因在所有位点都存在，而海洋出口处没有mcyA基因，表明沿河口存在毒株。河口沉积物中潜在毒性微囊藻完整集落的高密度强烈表明，沉积物可以充当河口中蓝细菌和氰毒素的接种物。