Mass adaptation to selenomethionine in populations of Chlorella vulgaris

Abstract

Two lines of evidence show that all cells of Chlorella vulgaris are capable of adapting to resist the growth uncoupling effect of selenomethionine. All cells plated in the presence of this methionine analogue eventually developed into colonies morphologically indistinguishable from normal colonies. Microscopic examination showed that the cells 1st grew into giant forms before they resumed divisions, and that their renewed ability to divide was not synchronized. Subculture of cells taken from several adapted colonies gave colonies that appeared much sooner on selenomethionine plates than did colonies from cells exposed to selenomethionine for the 1st time. During the course of the experiments approximately 6.5 to 7.0 x 107 cells were plated with selenomethionine, but never were colonies observed to develop in the same time as normal, untreated controls. Mutants resistant to selenomethionine, if such exist, are probably of very low frequency. In liquid cultures, selenomethionine initially prevents cell division, but about 50% of the inoculum cells grow into giant forms. At the time when a population increase was noted in the division-inhibited culture, autospores were observed within some of the giant cells. The proportion of these multiplying giants increased during the ensuing exponential phase of multiplication and then dropped to a constant low level during the stationary phase. The appearance of autospores was not synchronized in all giant cells, indicating a heterogeneous population of inoculum cells with varying abilities to adapt, as was observed in the plating experiments. The persistence of small numbers of giants during the stationary phase, as well as during subsequent passages with the analogue, is attributed to this heterogeneity. Two passages with selenomethionine sufficed to give a normal-sized population marked by the absence of a lengthy uncoupled phase in the presence of selenomethionine.