Do spatial effects appear at low dilution rate in chemostat?

Abstract

The chemostat theory on two species competition has shown that the dilution rate where transition of dominance occurs-transision-dilution rate-is independent of limiting-nutrient concentration. However, we obtained the experimental data indicating that the transition-dilution rate changed with variations in limiting-ammonium concentrations, using the chemostat mixed-culture of the cyanobacterium Microcystis novacekii and the green alga Scenedesmus quadricauda. The transition-dilution rate was dependent on the concentration of limiting ammonium in the influx culture medium. We tried to simulate the experimental results. Though the dilution rate has been considered independent of nutrient concentration, we introduce the effective dilution rate that depends on nutrient concentration (ammonium concentration in this study). A hyperbolic Monodtype function is used to represent the effective dilution rate for each species. The maximum dilution rate of the function is set to be the mechanical dilution rate (nominal dilution rate) of the chemostat culture. The calculation shows that the nominal transitiondilution rate where transition of dominance occur decreases with increased concentration. This simulation is well consistent with our experimental data. These results may suggest that the species-specificity of limiting nutrients, here nitrogen. Or they may imply that the depreciation of nitrogen becomes critical when both dilution rate and concentration are very low, especially for the green algae. In the latter case, spatial effects are induced internally in the ecosystem.

The paradox of enrichment in phytoplankton by induced competitive interactions

“…According to Gause's law of competitive exclusion, two species competing for the same resources cannot coexist if all ecological factors remained constant202627. This proposition is supported by several experimental studies using chemostat162829. Many mathematical studies also demonstrate that the coexistence of many species is extremely unlikely when niches are not separated72531.…”

“…The birth rate of a species at low nutrient levels should be small with almost no variability and increase with eutrophication, since algal growth in natural oligotrophic waters is slow1516. Since birth rates increase proportionally as nutrient level rises, they are set to be logistic functions of the nutrient level17 (Fig.…”

“…1). We also place tradeoffs among species such that those with higher maximum birth rates at high nutrient condition have lower birth rates when nutrient is poor (close to r = 0), since no species are almighty in nature151618192021.…”

“…Due to these experimental and theoretical results17162226272829, many studies proposed to include extrinsic factors such as climate change, immigration, dormancy and spatial heterogeneity of habitats, and chaotic dynamics303132333435. For example, high disturbance may promote low diversity, since many individual organisms may be located in unsuitable habitats by mixing up the current distribution of phytoplankton363738.…”

Mechanisms of Algal Species Dominance among Cyanobacteria, Diatoms and Green Algae as a Consequence of Phosphorus Reduction Caused by River Water Dilution in Eutrophic Lake

“…For the competition between M. aeruginosa and S. quadricauda, dilution rate, D, would be one of the influential factors based on several reports (Takeya et al, 2004;Miyazaki et al, 2009). Takeya et al (2004) ) led to the predominance of M. novacekii.…”