Light limitation increases multidimensional trait evenness in phytoplankton populations

Abstract

Individual-level variation arising from responses to environmental gradients influences population and community dynamics. How such responses empirically relate to the mechanisms that govern species coexistence is, however, poorly understood. Previous results from lake phytoplankton communities suggested that the evenness of organismal traits in multiple dimensions increases with resource limitation, possibly due to resource partitioning at the individual level. Here we experimentally tested the emergence of this pattern by growing two phytoplankton species (Pseudokirchneriella subcapitata and Microcystis aeruginosa) under a gradient of light intensity, in monoculture and jointly. Under low light (resource) conditions, the populations diversified into a wide range of phenotypes, which were evenly distributed in multidimensional trait space (defined by four pigment-related trait dimensions), consistent with the observed field pattern. Our interpretation is that under conditions of light limitation, individual phytoplankton cells alter photosynthetic traits to reduce overlap in light acquisition, acquiring unexploited resources and thereby likely maximizing individual success. Our results provide prime experimental evidence that resource limitation increases the evenness of conspecific and heterospecific microbial phenotypes along trait axes, advancing our understanding of trait-based coexistence.

Intraspecific niche partitioning in macrodetritivores enhances mixed leaf litter decomposition

“…However, the absence of non‐additive effects in their study might be due to the use of one single leaf litter species, which decreases the opportunities for intraspecific niche partitioning. Our results built upon this previous knowledge (the differences in consumption rates detected so far among conspecific individuals do not automatically imply complementary resource use) and allowed us to experimentally demonstrate for the first time that intraspecific trait variation matters for small‐scale niche partitioning in soil fauna, as it was already shown for primary producers in different systems (Fontana, Thomas, Reyes, & Pomati, ; He et al, ), and has consequences for fundamental ecosystem processes.…”

A Universal Phase Transition in Plankton Trait Dynamics

“…We use chlorophyll-a measurements in phytoplankton monocultures as experimental support for our theoretical findings (Section 3). Our chlorophyll measurements were derived from experimental data described in detail in Fontana et al (2019), which we summarise briefly here. Kirchneriella subcapitata (a eukaryotic green alga) and Microcystis aeruginosa (a prokaryotic cyanobacterium) were grown in monocultures under several different, but constant, light intensities and a constant temperature of 20 • C. All cultures were subjected to the same light-dark cycle, with 14 hours light and 10 hours darkness, and measurements were taken during the light period.…”

“…Additionally, for each replicate (species and light intensity), there was an additional monoculture of the other species in a separate container, that was shaded by the first (no mixing). The mixed cultures described in Fontana et al (2019) are not considered here. The data were collected using a scanning flow cytometer as described in Fontana et al (2019), from which we only analyse live cells that were identified through clustering and a trained random forest classification algorithm (Thomas et al, 2018).…”

“…The mixed cultures described in Fontana et al (2019) are not considered here. The data were collected using a scanning flow cytometer as described in Fontana et al (2019), from which we only analyse live cells that were identified through clustering and a trained random forest classification algorithm (Thomas et al, 2018). We look here at the total fluorescence responses (that is, the integral over the fluorescence pulse) of the cells in the red spectrum (677-700 nm).…”

“…Using flow cytometry data from Fontana et al (2019) (cf. Section 2.2), we analysed singlecell absolute chlorophyll content in phytoplankton monocultures exposed to different light intensities.…”