Light as a possible regulator of MIB-producing Planktothrix in source water reservoir, mechanism and in-situ verification

Abstract

The typical musty/earthy odor-causing compound, 2-methylisoborneol (MIB), is usually associated with the occurrence and proliferation of benthic/subsurface-living cyanobacteria in source water. Control of MIB-producing cyanobacteria in source water may greatly reduce the processing burden for drinking water treatment plants. We explored the mechanism and feasibility of restricting the growth of subsurface-living Planktothrix sp. by reducing underwater light availability. The effects of light intensity (5, 17, 36, 85, and 250 μmol photons m^-2 s^-1) on the growth and MIB production of Planktothrix sp. were first determined using batch culture, followed by an in-situ experiment deployed at different depths (0.5, 1.5, 3.5, and 5.0 m) in a drinking source water reservoir (Miyun Reservoir, China) to verify the laboratory results. The optimum conditions for growth (7.5 × 10⁸ cells L^-1) and MIB production ((1300 ± 29) μg L^-1) of Planktothrix sp. were achieved at 85 μmol photons m^-2 s^-1 in the laboratory and at 1.5 m (the corresponding average light intensity of 66 μmol photons m^-2 s^-1) in the field. The minimum light requirement for the growth of Planktothrix sp. (4.4 μmol photons m^-2 s^-1) was determined according to the laboratory data. While the in-situ experiment further indicated that Planktothrix sp. could not successfully grow at depths of 5 m where light intensity was below the minimum light requirement. In addition, the history data also verified the negative relationships between underwater light availability and MIB concentration.

Keywords: 2-Methylisoborneol; In-situ experiment; Light intensity; Planktothrix sp.; Subsurface cyanobacteria.