Optimal phytoplankton cell size in an allometric model

Abstract

: The competitive ability of phytoplankton cells is simulated in a model chemostat in which cell size is considered to be an adaptive trait. Parameters describing nutrient uptake kinetics are constrained by empirically derived allometric rules. With a steady input of a single nutrient, the evolutionarily stable cell size is selected through competition among phytoplankton. We find that large cells may be favored when (1) phytoplankton growth is limited by the rate at which internally stored inorganic nutrients can be converted into biomass, and (2) maximum quotas increase with size faster than minimum quotas. Increased internal quotas then accelerate the rate of biomass production in large cells, despite their enhanced requirements for resources. The evolutionarily stable strategy is set by the allometric relationships for nutrient uptake kinetics and by metabolism. Images: Anne W. Thompson (left); Jim Ehrman, Zoe Finkel, Mount Allison Digital Microscopy Lab (right)

Isometric size-scaling of metabolic rate and the size abundance distribution of phytoplankton

“…However, in marine ecosystems near to steady-state, grazing pressure is higher on small phytoplankton and the losses of smaller cells may be compensated by the higher sedimentation rates experienced by larger cells [11,46,59], resulting in a broad cell size-independence of loss processes. Furthermore, it has been hypothesized that resource competition between phytoplankton species of different size should lead to the dominance of phytoplankton community by the single size that requires the lowest resource concentration to grow [47,48]. However, here we assume that, given that different phytoplankton species have different nutrient requirements and phytoplankton taxonomic composition changes with cell size [60,61], the nature of the limiting nutrient may vary along the size spectrum.…”

“…For instance, a number of structural and biophysical features of large phytoplankton may counterbalance their geometric constraints on resources acquisition, which otherwise favour small cells when resources are limiting [45,48]. These traits include the possession of intracellular vacuoles to increase nutrient storage capacity [47,49], changes in cell shape [50,51], the use of non-limiting substrates to increase cell size without increasing nutrient requirements [52], the ability to migrate vertically in the water column [53] and the establishment of associations with nitrogen fixers [54].…”

“…Microalgal species must maximize their resource acquisition and assimilation rates while minimizing loss rates in order to survive in aquatic pelagic ecosystems [46]. Thus, the size-scaling relationships in phytoplankton might have evolved, in part, as a consequence of adaptation processes that involved the acquisition of taxa-specific physiological strategies by species in certain size classes [47]. For instance, a number of structural and biophysical features of large phytoplankton may counterbalance their geometric constraints on resources acquisition, which otherwise favour small cells when resources are limiting [45,48].…”

Comparative analysis of the interaction between habitat and growth form in diatoms

“…It is unlikely, however, that these traits are selected solely by the requirements of the planktonic environment. Large cell size, for example, is a strategy for defense against predators (Thingstad et al, 2005;Verdy et al, 2009;Yokota and Sterner, 2010) and appears to be favored in marine environments in general (Litchman et al, 2009;Nakov et al, 2014).…”

Scaling of growth rate and mortality with size and its consequence on size spectra of natural microphytoplankton assemblages in the East China Sea

“…While other studies focused on the selective grazing behaviour of microzooplankton and inferred the potential effect on the phytoplankton size structure, they did not measure the phytoplankton size structure together with feeding experiments Teixeira et al, 2011). Moreover, while size-specific phytoplankton responses were examined in modelling research to explain the increase of large phytoplankton with nutrient concentration (Irwin et al, 2006;Verdy et al, 2009), empirical studies on size-specific growth rate and grazing mortality would help clarify the mechanisms affecting the phytoplankton size structure.…”