Extraction method for total microcystins in cyanobacteria-laden sludge

Cyanobacteria in water treatment sludge pose a health risk as they continue to be viable, multiply, and produce potentially harmful secondary metabolites. To date, little research has focused on accurately determining cell bound microcystin (MC) concentrations of cyanobacterial cells in water treatment sludge. Three extraction methods (freeze–thaw, lyophilisation, direct methanolic extraction) with three different pre-treatments (homogenisation, (ultra)sonication, combination of both, and controls) were investigated for their MC extraction recovery. It was found that lyophilisation with prior sonication achieved the highest toxin recovery across the two MC analogues (MC-LR, MC-LA) tested. The method was able to extract 69 and 56% of MC-LR and MC-LA, respectively with good reproducibility. Comparable results were also obtained with direct methanolic extraction, with poor reproducibility. The least efficient method was freeze-thawing which achieved poor recoveries and was less reproducible. This study highlights a rapid, efficient, low-cost extraction method for determining total microcystins in cyanobacterial-laden sludge.