Modelling mixotrophic functional diversity and implications for ecosystem function

Abstract

Mixotrophy is widespread among protist plankton displaying diverse functional forms within a wide range of sizes. However, little is known about the niches of different mixotrophs and how they affect nutrient cycling and trophodynamics in marine ecosystems. Here we built a plankton food web model incorporating mixotrophic functional diversity. A distinction was made between mixotrophs with innate capacity for photosynthesis (constitutive mixotrophs, CMs) and those which acquire phototrophy from their prey (non-constitutive mixotrophs, NCMs). We present simulations of ecosystems limited by different light and nutrient regimes. Our simulations show that strict autotrophic and heterotrophic competitors increased in relative importance in the transition from nutrient to light limitation, consistent with observed oceanic biomass ratios. Among CMs, cells < 20 µm dominate in nutrient poor conditions while larger cells dominate in light-limited environments. The specificity of the prey from which NCMs acquire their phototrophic potential affects their success, with forms able to exploit diverse prey dominating under nutrient limitation. Overall, mixotrophy decreases regeneration of inorganics and boosts the trophic transfer efficiency of carbon. Our results show that mixotrophic functional diversity has the potential to radically change our understanding of the ecosystem functioning in the lower trophic levels of food webs.

Contrasting Mixotrophic Lifestyles Reveal Different Ecological Niches in Two Closely Related Marine Protists

“…In modeling studies, constitutive mixotrophs are typically considered to be primarily autotrophic, supplementing their nutrition by ingestion of prey if either light or nutrients become limiting and thus have an advantage under nutrient limitation (Leles et al. ). This is a strategy, for example, found in prymnesiophytes and some dinoflagellates (Hansen and Hjorth , Hansen ).…”

“…Although based on different underlying physiology, kleptoplastic protists dominated predicted plankton biomass in a nutrient‐limited scenario (Leles et al. ), hinting at the success of obligate mixotrophs in oligotrophic waters. From our results, the narrow range of environmental conditions that permit growth of the obligate mixotroph might seem disadvantageous, but this strategy is likely efficient in stable environments with low‐resource availability.…”

“…, Leles et al. ), but underlying metabolic attributes will need to be resolved to understand the biogeochemical and ecological impact as well as evolutionary trajectories of photosynthetic eukaryotes.…”

Abiotic Changes Driving Microphytoplankton Functional Diversity in Admiralty Bay, King George Island (Antarctica)

“…Heterotrophy and mixotrophy confer advantages to dinoflagellates under conditions of low nutrient concentrations (Litchman and Klausmeier, 2008) and, associated with the motility of these organisms, represent unique strategies for competing in stratified environments (Bellinger and Sigee, 2011;Klais et al, 2016;Roselli et al, 2017). When compared to heterotrophic competitors, mixotrophy confers the ability to compete and overcome both strictly autotrophic and heterotrophic organisms by using nutrients more efficiently (Leles et al, 2018). In addition, the presence of vacuoles, associated mainly with mixotrophic dinoflagellates, may be indicative of a digestive function for these organelles and the consequent occurrence of phagotrophy by these organisms (Onuma and Horiguchi, 2013;Almada et al, 2017).…”

Spatial variation of abundant picoeukaryotes in the subtropical Kuroshio Current in winter

“…To cope with stressful conditions, that is, if the environment is unfavorable for photosynthesis or if the cell lacks certain nutrients, mixotrophic picoeukaryotes will temporarily convert their nutritional strategy from phototroph to bacterivore (Leles et al, 2018;McKie-Krisberg & Sanders, 2014;Stoecker, Hansen, Caron, & Mitra, 2017;Unrein, Gasol, Not, Forn, & Massana, 2014;Zubkov & Tarran, 2008). Therefore, special nutritional modes determine their niche in marine ecosystems, which in turn affect the process of the food chain and the carbon cycle (Leles et al, 2018;Stoecker et al, 2017;Worden et al, 2015). However, as of now, the associations between environmental factors and the community composition of picoeukaryotes are not yet fully understood.…”