Differences in Responses to Darkness betweenMicrocystis aeruginosaandChlorella pyrenoidosa

The effect of darkness on the cyanobacterium Microcystis aeruginosa and the green alga Chlorella pyrenoidosa were observed. Cell growth, metabolic activity, chlorophyll a fluorescence, cell size, and maximum quantum yield (Fv: Fm) were determined. M aeruginosa and C. pyrenoidosa both maintained their initial cell abundances while in the dark, but they showed distinctly different physiological responses to darkness. During the dark period metabolic activity decreased slightly, cell size and Fv: Fm decreased significantly, and chlorophyll a increased markedly in M. aeruginosa cells. However, metabolic activity, cell size, and Fv: Fm decreased dramatically, and chlorophyll a decreased trivially in C. pyrenoidosa cells. After re-illumination, metabolic activity, cell size, and Fv: Fm of M. aeruginosa and metabolic activity, chlorophyll a, and Fv: Fm of C. pyrenoidosa recovered significantly. No recovery was found in either chlorophyll a of M. aeruginosa or cell size of C. pyrenoidosa. Compared with C. pyrenoidosa, M. aeruginosa is more capable of withstanding the dark circumstances, which are unfavorable to the growth of alga but usually exists in water and sediment of eutrophic lakes. Its greater tolerance of darkness may be one of the reasons that M. aeruginosa is commonly the dominant species in eutrophic lakes.