Evidence of the cost of the production of microcystins by Microcystis aeruginosa under differing light and nitrate environmental conditions

Abstract

The cyanobacterium Microcystis aeruginosa is known to proliferate in freshwater ecosystems and to produce microcystins. It is now well established that much of the variability of bloom toxicity is due to differences in the relative proportions of microcystin-producing and non-microcystin-producing cells in cyanobacterial populations. In an attempt to elucidate changes in their relative proportions during cyanobacterial blooms, we compared the fitness of the microcystin-producing M. aeruginosa PCC 7806 strain (WT) to that of its non-microcystin-producing mutant (MT). We investigated the effects of two light intensities and of limiting and non-limiting nitrate concentrations on the growth of these strains in monoculture and co-culture experiments. We also monitored various physiological parameters, and microcystin production by the WT strain. In monoculture experiments, no significant difference was found between the growth rates or physiological characteristics of the two strains during the exponential growth phase. In contrast, the MT strain was found to dominate the WT strain in co-culture experiments under favorable growth conditions. Moreover, we also found an increase in the growth rate of the MT strain and in the cellular MC content of the WT strain. Our findings suggest that differences in the fitness of these two strains under optimum growth conditions were attributable to the cost to microcystin-producing cells of producing microcystins, and to the putative existence of cooperation processes involving direct interactions between these strains.

Figure 1. Growth curves of the WT (black circles) and MT (white circles) strains in monoculture experiments under different environmental conditions.

A OLHN (optimal light and high nitrogen concentrations), B OLLN (optimal light and low nitrogen concentrations), C LLHN (low light and high nitrogen concentrations), and D LLLN (low light and low nitrogen concentrations). Error bars represent the standard deviation (N = 3). NO₃ was added (dashed line) on day 11 under OLLN conditions, and on day 26 under LLLN conditions.

Figure 2. Relationship between cell growth rates and MC production rates of the WT strain.

Each point represents the mean value of triplicate determinations. Rates were calculated between successive sampling points during the exponential growth phase in monoculture experiments (crosses and continuous line), and in co-culture experiments (triangles and dashed line).

Figure 3. Time-course of the intracellular MC content in monoculture (histograms with black points) and in co-culture (hatched histograms) experiments under different culture conditions.

A OLHN (optimal light and high nitrogen), B OLLN (optimal light and low nitrogen), C LLHN (low light and high nitrogen), and D LLLN (low light and low nitrogen). Error bars represent the standard deviation (N = 3). NO₃ was added (dashed line) on day 11 under OLLN conditions, and on day 26 under LLLN conditions.

Figure 4. Time-course of the relative proportions of the WT (black circles) and MT (white circles) strains in co-culture experiments under different culture conditions.

A OLHN (optimal light and high nitrogen), B OLLN (optimal light and low nitrogen), C LLHN (low light and high nitrogen), and D LLLN (low light and low nitrogen). Error bars represent the standard deviation (N = 3). NO₃ was added (dashed line) on day 11 under OLLN conditions, and on day 26 under LLLN conditions.