Toxic cyanobacteria Microcystis blooms often occur in eutrophic lakes around the world. During bloom-occurring, Microcystis population aggregates at the surface of water and is often exposed to strong UV radiation of sunlight. To evaluate the ecological function of solar UV on Microcystis bloom, we studied effects of UV radiation on two M. aeruginosa strains, the unicellular strain PCC7806 and the colonial strain XW01, by using a transparent UV protection film filtering out the solar UV from sunlight (cut off more than 90% UV radiation, "without UV") in outdoor-culture, and using a 30 W UV lamp (1.25 μW/cm² at the surface of culture medium) adding artificial UV radiation in lab-culture. Results showed that the growth of XW01 in the whole solar light (the highest intensity of UV-297 and UV-254 was about 120 μW/cm² at noon, "with UV") was better than in the light without UV in the outdoor-culture condition; the cell density (OD650nm) of 6-day culture with UV was 35.8% higher than that without UV. In the lab condition, the low-intensity artificial UV at 18.75 J/m² daily radiation (repeat 5-second UV light with 20-second intervals, 30 cycles) improved the growth of XW01, the cell density of 6-day culture was 14.5% higher than the control (without UV). However, the same UV intensities killed the strain PCC7806 both of in the lab and outdoor experiments. These data show that a suitable UV radiation increase the growth of colonial XW01, and the colonial strain has a stronger resistance to UV than the unicellular strain.
To discover the mechanism of XW01 against UV, we investigated the changes of antioxidant protective enzymes superoxide dismutase (SOD) and catalase (CAT) in XW01 cultured with or without solar UV. The activities of SOD and CAT of XW01 cultured with solar UV were 86% and 34% above that without solar UV respectively. However, the same solar UV decreased the activities of SOD and CAT of PCC7806 by 42% and 39% respectively and caused the cell death. The data indicate that XW01 has stronger antioxidant ability than PCC7806. Solar UV could induced accumulation of UV-protection compounds mycosporine-like amino acids (MAAs) and scytonemin (Scy) in XW01, the relative contents of MAA and Scy of the cells with solar UV were 27.3% and 23.4% higher than that of the cells without UV. These physiological reaction could be helpful for XW01 protecting from the damages caused by the solar UV.
Solar UV also increased the production of extracellular polysaccharides. In the 4-day culture of XW01, extracellular soluble polysaccharides and extracellular wall polysaccharides increased 12.5% and 22% respectively. The UV obviously improved the formation of larger colonies. The percentage of smallest colonies decreased from 20% to less than 3%, and larger colonies increased from 35% to 47%, average colonial size increased about 68%. The colonial size increasing was consistent with the production of extracelullar polysaccharides. More extracellular polysaccharides should be benefit for bigger colony formation.
Above all, the results suggest that the solar UV should be helpful for colonial Microcystis growth. Microcystis bloom could effectively protect themselves from UV damages, and solar UV be helpful for Microcystis bloom forming in natural water body.