Effect of copper on cell division, nitrogen metabolism, morphology, and sexual reproduction in the life cycle of Closterium moniliferum (Chlorophyceae)

Abstract

Additions of copper were shown to affect cell morphology, growth rates and nutrient uptake in Closterium moniliferum. These parameters are interrelated in the total life cycle of the organism. It was found that the timing of events in the life cycle, including sexual reproduction, could be changed when copper was added. When increasing concentrations of copper were added to the growth medium, Closterium moniliferurn exhibited a stimulatory, inhibitory, or toxic dose-response typical of organisms to trace metals. The stimulatory effect, occurring at pCu* 14.4, was demonstrated by an early increase in cell number, increased nitrate uptake, and early onset of sexual reproduction. The inhibitory effects of greater concentrations of copper (pCu* 12.1) were a longer lag phase, decreased nitrate uptake, and later onset of sexual reproduction. Light micrographs and scanning electron micrographs of normal and aberrant cells demonstrated the effect of copper on morphology. It was demonstrated that the sexual phase in some srains of Closteriurn moniliferurn could be triggered by changes in the amount of nitrogen in the medium. Although it was expected that each cell would have a minimal cell quota (Q nitrogen/cell) before sexual reproduction occurred, work here demonstrated that Q was not the critical factor in the initiation of the process. Sexual reproduction occurred at Q = .05 micromoles nitrogen/cell up to .21 micromoles nitrogen/cell. Neither was cell density in batch cultures the critical factor. Sexual reproduction occurred in the range of 400 cells/ml up to 6100 cells/ml. Different nitrogen sources gave different responses in the onset of sexual reproduction. Nitrate depletion. not ammonia, appears to be the critical component in induction of sexual reproduction. The actual effect of metals like copper on the population dynamics of Closterium moniliferum in natural environments is yet to be determined. The stimulatory effect on nitrate uptake by copper, and its accompanying earlier formation of zygotes, may have survival value for the organism since the zygotes become thick-walled and are not as subject to environmental perturbations as vegetative cells.