The functional role of planktonic mixotrophs in altering seston stoichiometry

Abstract

Mixotrophic protists are widespread and relevant primary producers and consumers in planktonic food webs. Given their dual mode of nutrition, mixotrophs face different constraints in allocating resources to cellular structures compared to strict photoautotrophs. However, little is known about their stoichiometric requirements and their flexibility in nutrient content and thus food quality, or how this affects consumer performance and nutrient recycling. In the present study, we tested for systematic differences in elemental composition between photoautotrophic and mixotrophic protists. We compiled intracellular nutrient ratios of mixotrophic and phototrophic species from culture experiments and from 2 lake surveys. Overall, both laboratory and field data indicated that mixotrophy has a stabilizing effect on the nutrient stoichiometry of primary producers under changing nutrient supply. In laboratory experiments, mixotrophs showed a lower variability in intracellular N:P ratios compared to strict phototrophs and were more stable in their elemental composition in response to a gradient of dissolved N:P availability. With increasing contributions of mixotrophic phytoplankton taxa to total lake phytoplankton, both the mean and variance in seston C:P ratios decreased, i.e. communities with higher proportion of mixotrophs overall exhibited more constrained seston stoichiometry. Our results show that mixotrophy may have direct implications for nutrient cycling and secondary production through regulation of seston stoichiometry, buffering stoichiometric constraints for herbivores and enabling a more stable secondary production compared to systems dominated by phototrophic specialists.

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity, ecosystem productivity, food security, and human health

“…Although most studies of urban and crop wastes and leachate loads to rivers and estuaries (83.3%) have found increasing N:P ratios associated with increasing N:P ratios from human inputs, other studies (13.7%) tended to find decreasing ratios in areas with high livestock densities (Arbuckle & Downing, 2001;Johnson, Heck, & Fourqurean, 2006; Figure 6; Table S1). Changes in N and/or P availability and associated shifts in N:P ratios drive changes in species competition and dominance in communities of terrestrial plants (Sardans, Rodà, & Penuelas, 2004;Zhang, Liu, et al, 2019), animals (Jochum et al, 2017), microbes Fanin, Fromin, Biatois, & Hättenschwiler, 2013;Ren et al, 2017;Shao et al, 2017;Zechmeister-Bolstenstren et al, 2015), and plankton (Elser, Andersen, et al, 2009;Grosse, Burson, Stomp, Huisman, & Boschker, 2017;He, Li, Wei, & Tan, 2013;Moorthi et al, 2017;Plum, Husener, & Hillebrand, 2015). Changes in media (water or soil) N:P ratios affect the structure of terrestrial (Fanin et al, 2013;Scharler et al, 2015;Zechmeister-Bolstenstren et al, 2015) and aquatic (Sitters, Atkinson, Guelzow, Kelly, & Sullivan, 2015) food webs, but associated impacts on community diversity are unclear.…”

“Unifying” the Concept of Resource Use Efficiency in Ecology

“…This has been described as a mechanism to enhance nutrient supply under nutrient-limited conditions for phytoplankton (Rothhaupt, 1996;Klausmeier et al, 2004a,b;Hillebrand et al, 2013) as well as terrestrial plants (Wakefield et al, 2005;Farnsworth and Ellison, 2008). Recent evidence suggests that mixotrophs (i.e., organisms that are able to use different sources of energy and carbon) may buffer stoichiometric constraints for herbivores and thus stabilize secondary production compared to systems dominated by phototrophs (Moorthi et al, 2017).…”

Trait-based paradise – or only feeding the computer with biology?

“…For example, mixotrophic algae affects microbial trophic dynamics and the biological pump, which group is potentially favoured in a changing climate (Mitra et al, 2014). However, we have still limited information about traits characterizing mixotrophy; such as their stoichiometric requirements, or food quality of mixotrophic species for higher trophic levels (Moorthi et al, 2017).…”