Isopropylthiol emission by bloom-forming Microcystis: Biochemistry, ecophysiology and semiochemistry of a volatile organosulfur compound

Microcystis species not only produce toxic cyanobacterial blooms, but can be a significant source of taste and odour. Previous studies have associated foul-smelling volatile organic sulfur compounds (VOSCs) with Microcystis blooms, but have largely attributed these compounds to bacterial bloom decomposition. However, earlier reports of the production of isopropylthio compounds by several Microcystis strains suggests that these cyanobacteria may themselves be a source of these VOSCs. Sulphur compounds have been shown to play important semiochemical roles in algal cell protection and grazer interactions in marine systems, but little is known about the production and chemical ecology of freshwater cyanobacterial VOSCs. To address this knowledge gap, we undertook the first detailed investigation of the biochemistry, ecophysiology and semiochemistry of these compounds and their production by Microcystis, and tested the hypothesis that they act as multifunctional semiochemicals in processes related to cell protection and grazer defence. Using short-term incubations and an adapted headspace-GC-MS technique, we investigated VOSC production by axenic and non-axenic strains, and verified that isopropylthio compounds are in fact produced by these cyanobacteria, identifying 5 isopropyl moiety-containing VOSCs (isopropylthiol (ISH), isopropylmethyl sulfide, isopropyl methyl disulfide, diisopropyl disulfide (ISSI) and diisopropyl trisulfide) as well as methanethiol in three strains. Further studies with the axenic strain Microcystis PCC 7806 using different light regimes, metabolic inhibitors (sodium azide, DCMU), the antioxidant enzyme catalase and stable labelled precursors (hydrogencarbonate, acetates and sulfate) demonstrated that ISH is a true exo-metabolite, synthesized via the acetate pathway. It is actively produced and continuously excreted by the cyanobacteria during growth, with minimal internal storage or post-lysis catalytic generation. The molar ratios of the redox pair ISH/ISSI are not directly involved in the photosynthetic and respiratory electron transport chains, but dependant on the redox state of the cell - likely mediated by reactive oxygen species (ROS), as shown by a marked effect of catalase. These results, along with toxicological and behavioural assays using the two aquatic invertebrates Thamnocephalus platyurus and Daphnia magna indicate that ISH plays multiple important physiological and ecological roles. It acts as an effective antioxidant against high ROS levels, as often experienced in surface blooms, it elicits avoidance-related behavioural responses in grazer communities and at high levels, it can be toxic to some invertebrates.