Unstandardized UV-C dose used for killing harmful cyanobacteria may instead initiate accelerated growth in the target organisms

Abstract

Although UV-C radiation has been in use for killing unwanted cyanobacteria, experiments with lower doses of UV-C radiation instead showed induction of growth related parameters and enhanced biomass production in the cyanobacterium Nostoc muscorum Meg1. When the cyanobacterial cultures were exposed to UV-C radiation of varying doses (6, 12 and 18 mJ/cm2), concentrations of various photo-absorbing pigments, RuBisCO and D1 protein of PSII; activities of oxygen evolving complex, nitrogenase and glutamine synthetase were significantly increased upon 6 and 12 mJ/cm2 UV-C radiation exposures. Resulting higher photosynthetic performance was evident from the augmentation in carbohydrate content by ∼49% under single exposure to 6 mJ/cm2 UV-C by fifteenth day. The increased performances of both RuBisCO and D1 proteins were in part also due to induction at the genetic level as seen from the increase in their mRNA and protein levels under treatment. Similar increase was also observed in protein (16%) and in lipid contents (43%) that reflected an upsurge in the total biomass. Highest biomass (463 mg/L/d) was noted in culture exposed to 6 mJ/cm2 UV-C radiation, representing a ∼25% increase. Furthermore the possibility of this organism using part of the incident UV-C radiation as an additional source of energy was deduced from an experiment where the thylakoid membranes excited within UV (226–400 nm) range showed emission at longer wavelengths with an emission maximum at ∼640 nm. Thus this work provides evidence that lower UV-C doses can potentially augment cyanobacterial growth and use of unstandardized UV-C doses for restricting cyanobacterial growth may in fact produce contrary result.