Abstract
Benthic cyanobacterial proliferations in rivers are have been reported with increasing frequency worldwide. In the Eel and Russian rivers of California, more than a dozen dog deaths have been attributed to cyanotoxin toxicosis since 2000. Periphyton proliferations in these rivers comprise multiple cyanobacterial taxa capable of cyanotoxin production, hence there is uncertainty regarding which taxa are producing toxins. In this study, periphyton samples dominated by the cyanobacterial genera Anabaena spp. and Microcoleus spp. and the green alga Cladophora glomerata were collected from four sites in the Eel River catchment and one site in the Russian River. Samples were analysed for potential cyanotoxin producers using polymerase chain reaction (PCR) in concert with Sanger sequencing. Cyanotoxin concentrations were measured using liquid chromatography tandem-mass spectrometry, and anatoxin quota (the amount of cyanobacterial anatoxins per toxigenic cell) determined using droplet digital PCR. Sequencing indicated Microcoleus sp. and Nodularia sp. were the putative producers of cyanobacterial anatoxins and nodularins, respectively, regardless of the dominant taxa in the mat. Anatoxin concentrations in the mat samples varied from 0.1 to 18.6 μg g-1 and were significantly different among sites (p < 0.01, Wilcoxon test); however, anatoxin quotas were less variable (< 5-fold). Dihydroanatoxin-a was generally the most abundant variant in samples comprising 38% to 71% of the total anatoxins measured. Mats dominated by the green alga C. glomerata contained both anatoxins and nodularin-R at concentrations similar to those of cyanobacteria-dominated mats. This highlights that even when cyanobacteria are not the dominant taxa in periphyton, these mats may still pose a serious health risk and indicates that more widespread monitoring of all mats in a river are necessary.
Highlights
Reports of toxic benthic cyanobacterial proliferations have been described over the past 30 years (e.g. [1, 2, 3]) and are increasing in frequency globally [4,5,6,7,8,9]
Cyanotoxins are typically classified by their different toxicological properties into neurotoxins and saxitoxins (STXs [12]), hepatotoxins, cytotoxins and dermatotoxins [13, 14]
This study detected the anaC gene at sites where [18] did not find anatoxin biosynthesis gene clusters in their metagenomes; our results correspond with previous anatoxin detections in the upper reaches of the Eel watershed [16]
Summary
Reports of toxic benthic cyanobacterial proliferations have been described over the past 30 years (e.g. [1, 2, 3]) and are increasing in frequency globally [4,5,6,7,8,9]. [1, 2, 3]) and are increasing in frequency globally [4,5,6,7,8,9] Despite these reports, investigations into benthic cyanotoxin producers, and variability in toxin production are limited [7]. Many benthic cyanobacteria are known to produce cyanotoxins, for example, Anabaena, Phormidium, Lyngbya, Oscillatoria, Nostoc, Nodularia and Microcoleus. While these taxa are often dominant in proliferations [7], they are common components of periphyton mats where other algae are more abundant (e.g. Cladophora glomerata [10, 11]), and the risks these mats pose is relatively unknown. Anatoxins comprise four main structural congeners; anatoxin-a (ATX), dihydroanatoxin-a (dhATX), homoanatoxin-a (HTX) and dihydrohomoanatoxin-a (dhHTX), and their relative proportions vary in environmental samples [15]
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