Cyanotoxin Occurrence and Diversity in 98 Cyanobacterial Blooms from Swedish Lakes and the Baltic Sea

Abstract

The Drinking Water Directive (EU) 2020/2184 includes the parameter microcystin LR, a cyanotoxin, which drinking water producers need to analyze if the water source has potential for cyanobacterial blooms. In light of the increasing occurrences of cyanobacterial blooms worldwide and given that more than 50 percent of the drinking water in Sweden is produced from surface water, both fresh and brackish, the need for improved knowledge about cyanotoxin occurrence and cyanobacterial diversity has increased. In this study, a total of 98 cyanobacterial blooms were sampled in 2016-2017 and identified based on their toxin production and taxonomical compositions. The surface water samples from freshwater lakes throughout Sweden including brackish water from eight east coast locations along the Baltic Sea were analyzed for their toxin content with LC-MS/MS and taxonomic composition with 16S rRNA amplicon sequencing. Both the extracellular and the total toxin content were analyzed. Microcystin's prevalence was highest with presence in 82% of blooms, of which as a free toxin in 39% of blooms. Saxitoxins were found in 36% of blooms in which the congener decarbamoylsaxitoxin (dcSTX) was detected for the first time in Swedish surface waters at four sampling sites. Anatoxins were most rarely detected, followed by cylindrospermopsin, which were found in 6% and 10% of samples, respectively. As expected, nodularin was detected in samples collected from the Baltic Sea only. The cyanobacterial operational taxonomic units (OTUs) with the highest abundance and prevalence could be annotated to Aphanizomenon NIES-81 and the second most profuse cyanobacterial taxon to Microcystis PCC 7914. In addition, two correlations were found, one between Aphanizomenon NIES-81 and saxitoxins and another between Microcystis PCC 7914 and microcystins. This study is of value to drinking water management and scientists involved in recognizing and controlling toxic cyanobacteria blooms.

Keywords: LC-MS/MS; analysis; blooms; cyanobacteria; cyanotoxins; survey.

Figure 1

Map of sampling sites where the blooms were observed, 2016 (red dots) and 2017 (blue triangles). The most northern sampling site was at 65.9150371 N, 22.273922 E and the most southern sampling site at 55.4675428 N, 13.4682083 E (distance: 1255 km).

Figure 2

Flowchart of the sampling procedure (three replicate samplings), specifically developed for this study, used at each sampling site.

Figure 3

The heatmap shows the correlation between the occurrence of cyanobacteria (indicated as operational taxonomic unit, OTU) and the cyanotoxin. The numbers indicate the proportion of all samples where both the OTU and the toxin could be detected. Red color indicates that the correlation between OTU and toxin is positive and blue color if the correlation is negative. The darker the color, the lower the p-value (stronger association between OTU and toxin). The p-value was calculated according to Spearman’s correlation test. The dendrogram was generated using complete linkage hierarchical clustering based on a correlation distance.

Figure 4

Cyanobloom sampling kit: (1) green transport container with 7–8 freezer bags and three brown filtrate bottles labeled “Toxins 1–3”; (2) a pair of sampling gloves; (3) a permanent marker; (4) a syringe; (5) freezer bag, marked “Steribag/For Sterivex” containing 3 sterile bags marked “Toxin cells 1–3” and 3 sterile bags marked “PCR 1–3”; (6) freezer bag, labeled “Sterivex” containing 6 Sterivex filters; (7) green transport container with “Sample collecting bottle” with 1 mL of Lugol’s solution and bubble wrap for transport; (8) large sampling bottle, amber; (9) sheet with the information to place the kit in the freezer the day before sampling; (10) algal bloom sampling protocol; and (11) padded return envelope with the accessories for closure.