Stable coexistence of equivalent nutrient competitors through niche differentiation in the light spectrum

Abstract

Niche-based theories and the neutral theory of biodiversity differ in their predictions of how the species composition of natural communities will respond to changes in nutrient availability. This is an issue of major environmental relevance, as many ecosystems have experienced changes in nitrogen (N) and phosphorus (P) due to anthropogenic manipulation of nutrient loading. To understand how changes in N and P limitation may impact community structure, we conducted laboratory competition experiments using a multispecies phytoplankton community sampled from the North Sea. Results showed that picocyanobacteria (Cyanobium sp.) won the competition under N limitation, while picocyanobacteria and nonmotile nanophytoplankton (Nannochloropsis sp.) coexisted at equal abundances under P limitation. Additional experiments using isolated monocultures confirmed that Cyanobium sp. depleted N to lower levels than Nannochloropsis sp., but that both species had nearly identical P requirements, suggesting a potential for neutral coexistence under P-limited conditions. Pairwise competition experiments with the two isolates seemed to support the consistency of these results, but P limitation resulted in stable species coexistence irrespective of the initial conditions rather than the random drift of species abundances predicted by neutral theory. Comparison of the light absorption spectra indicates that coexistence of the two species was stabilized through differential use of the underwater light spectrum. Our results provide an interesting experimental example of modern coexistence theory, where species were equal competitors in one niche dimension but their competitive traits differed in other niche dimensions, thus enabling stable species coexistence on a single limiting nutrient through niche differentiation in the light spectrum.

Why Do Phytoplankton Evolve Large Size in Response to Grazing?

“…Zooplankton was eliminated prior to their experiments. In all competition experiments, the two to four smallest phytoplankton taxa, ranging in size from 1.5 to 8.3 mm, competitively displaced the larger phytoplankton species (Burson et al 2018(Burson et al , 2019. Similarly, in the oceans small picocyanobacteria, such as Prochlorococcus and Synechococcus, dominate the oligotrophic subtropical gyres (Chisholm et al 1988;Agawin et al 2000;Li 2002).…”

“…This size shift is supported by many empirical studies that show that small size confers a major competitive advantage under nutrient-limited conditions when grazing pressure is low (Grover 1989;Litchman et al 2007;Edwards et al 2011;Marañón 2015). For example, Burson et al (2018Burson et al ( , 2019 performed multispecies competition experiments with natural phytoplankton communities encompassing a wide range of cell sizes. Zooplankton was eliminated prior to their experiments.…”

Changes in water color shift competition between phytoplankton species with contrasting light‐harvesting strategies

“…Therefore, the earlier competition models were extended by incorporation of the full spectrum of light (Stomp et al , ). These spectral models predict stable coexistence of species if they absorb different parts of the light spectrum, a theoretical prediction that has been verified by both laboratory competition experiments and field data (Stomp et al , , Burson et al ).…”

“…Why not? Previous competition studies showed that species with different photosynthetic pigments can coexist in white light (Stomp et al , , Burson et al ). One might therefore expect ample opportunities for species coexistence when cyanobacteria and green algae compete for different colors of light.…”

Temperature Stress Induces Shift From Co-Existence to Competition for Organic Carbon in Microalgae-Bacterial Photobioreactor Community – Enabling Continuous Production of Microalgal Biomass

“…In which stabilizing effects of increasing niche differentiation (the use of different resources), in combination with the equalizing effects of decreasing fitness (more evenly distributed abundances) describe a situation favoring coexistence, while the opposite conditions favor competition and competitive exclusion. Examples of niche differentiation, of either light or nutrient preferences, and coexistence of different microalgal groups have been suggested by previous studies both in laboratory experiments and nature ( Alexander et al, 2015 ; Burson et al, 2019 ). Studies of interactions among microbial communities often focus on niche overlaps/differentiations between similar organisms.…”