Department of Environmental Science, Stockholm University, Stockholm, Sweden.
School of Natural Sciences, Technology and Environmental Studies, Södertörn University, Huddinge, Sweden.
PLoS One. 2020 Mar 16;15(3):e0230310. doi: 10.1371/journal.pone.0230310. eCollection 2020.
Methylmercury (MeHg) is a potent neurotoxin that biomagnifies in marine food webs. Inorganic mercury (Hg) methylation is conducted by heterotrophic bacteria inhabiting sediment or settling detritus, but endogenous methylation by the gut microbiome of animals in the lower food webs is another possible source. We examined the occurrence of the bacterial gene (hgcA), required for Hg methylation, in the guts of dominant zooplankters in the Northern Baltic Sea. A qPCR assay targeting the hgcA sequence in three main clades (Deltaproteobacteria, Firmicutes and Archaea) was used in the field-collected specimens of copepods (Acartia bifilosa, Eurytemora affinis, Pseudocalanus acuspes and Limnocalanus macrurus) and cladocerans (Bosmina coregoni maritima and Cercopagis pengoi). All copepods were found to carry hgcA genes in their gut microbiome, whereas no amplification was recorded in the cladocerans. In the copepods, hgcA genes belonging to only Deltaproteobacteria and Firmicutes were detected. These findings suggest a possibility that endogenous Hg methylation occurs in zooplankton and may contribute to seasonal, spatial and vertical MeHg variability in the water column and food webs. Additional molecular and metagenomics studies are needed to identify bacteria carrying hgcA genes and improve their quantification in microbiota.
甲基汞（MeHg）是一种强效神经毒素，在海洋食物网中具有生物放大作用。无机汞（Hg）的甲基化由栖息在沉积物或沉降碎屑中的异养细菌进行，但动物肠道微生物组的内源性甲基化是另一个可能的来源。我们研究了北方波罗的海中占优势的浮游动物肠道中汞甲基化所需的细菌基因（hgcA）的发生情况。使用针对三个主要进化枝（Delta 变形菌门、Firmicutes 和古菌）的 hgcA 序列的 qPCR 检测方法，对在野外采集的桡足类（Acartia bifilosa、Eurytemora affinis、Pseudocalanus acuspes 和 Limnocalanus macrurus）和枝角类（Bosmina coregoni maritima 和 Cercopagis pengoi）的样本进行了检测。所有桡足类动物的肠道微生物组中都携带 hgcA 基因，而枝角类动物则没有扩增。在桡足类动物中，只检测到属于 Delta 变形菌门和 Firmicutes 的 hgcA 基因。这些发现表明，内源性 Hg 甲基化可能发生在浮游动物中，并可能导致水柱和食物网中季节性、空间性和垂直性 MeHg 变异性。需要进行额外的分子和宏基因组学研究，以鉴定携带 hgcA 基因的细菌，并提高其在微生物组中的定量分析。
