Algal growth rate fluctuations observed under uniform ambient test conditions using static and semicontinuous assay techniques

Abstract

A simple, manually diluted, semicontinuous, 96-h algal growth assay technique was developed to measure changing rates of population growth following sublethal chemical exposure. Rates were estimated directly from changing cell counts in a fixed volume of cell suspension. Short-term rate fluctuations in exponential rate parameters were observed in Selenastrum capricornutum populations using this method and similar fluctuations were also documented by reanalyzing conventional static culture assay data. Replicate cultures tended to fluctuate in unison, and patterns of population increase were similar in static assays initiated on different dates. The latter suggested that nonuniform rates of S. capricornutum population increase were not due simply to environmental variation. All populations were preacclimated to test conditions, but growth lags were consistently observed for 12–24 h following inoculation. Subsequent rate fluctuations probably resulted from a high degree of cell-cycle synchronization. Treating systematic rate fluctuations as random error lowered measurement precision, especially with respect to estimates of rate changes over time. Systematic variance may be difficult to eliminate in practice, but repeated-measures regression methods can account for this effect and substantially reduce rate parameter confidence intervals. Findings are expected to apply to endpoints such as dry weight, total cell volume, chlorophyll, or DNA.