Intraguild predation enables coexistence of competing phytoplankton in a well‐mixed water column

Abstract

Citation: Moeller, H. V., M. G. Neubert, and M. D. Johnson. 2019. Intraguild predation enables coexistence of competing phytoplankton in a well-mixed water column. Ecology 100(12):Abstract. Resource competition theory predicts that when two species compete for a single, finite resource, the better competitor should exclude the other. However, in some cases, weaker competitors can persist through intraguild predation, that is, by eating their stronger competitor. Mixotrophs, species that meet their carbon demand by combining photosynthesis and phagotrophic heterotrophy, may function as intraguild predators when they consume the phototrophs with which they compete for light. Thus, theory predicts that mixotrophy may allow for coexistence of two species on a single limiting resource. We tested this prediction by developing a new mathematical model for a unicellular mixotroph and phytoplankter that compete for light, and comparing the model's predictions with a laboratory experimental system. We find that, like other intraguild predators, mixotrophs can persist when an ecosystem is sufficiently productive (i.e., the supply of the limiting resource, light, is relatively high), or when species interactions are strong (i.e., attack rates and conversion efficiencies are high). Both our mathematical and laboratory models show that, depending upon the environment and species traits, a variety of equilibrium outcomes, ranging from competitive exclusion to coexistence, are possible. W Varied December 2019 MIXOTROPHY IN PLANKTONIC COMMUNITIES Article e02874; page 3 FIG. 4. Dependence of mathematical model predicted dynamics on surface light I in and attack rate a. Legends and notation as in Fig. 1. As available light increases, the system transitions from competitive exclusion by prey, to alternate competitive exclusion states or coexistence, to bistability of coexistence and competitive exclusion by the mixotroph, and finally to competitive exclusion by the mixotroph. Nullclines are shown for increasing values of I in at a fixed value of a = 1 9 10 À7 (right column); Roman numerals are used to indicate specific parameter values and are chosen for consistency with Roman numerals in Fig. 1. Note that the basins of attraction for the bistable equilibria shift. For example, as I in increases, the basin of attraction for the coexistence equilibrium (purple square) shrinks (nullclines corresponding to locations IVa and IVb). Parameters are as listed in Table 1, with p m = 0.3, h m = 250, ' m = 0.1, k m = 5 9 10 À7 , and b = 0.005.

Modeling the metabolic evolution of mixotrophic phytoplankton in response to rising ocean surface temperatures

“…In particular, light is assumed to attenuate from a surface input irradiance of I in with depth due to absorption by photosynthetic cells which have a per-cell absorptivity k. Huisman and Weissing computed the average photosynthetic rate across depth, and obtained the per capita photosynthetic growth rate, defined here as P( θ , z, I in , T, M ): Mixotrophs also obtain resources by grazing on bacteria. Therefore, we included a grazing term (sensu Moeller et al 2019 [41]) assuming that, at relevant bacterial population densities, mixotroph grazing on bacteria at density B can be approximated using a Type I functional response [42], where α ( θ , T) represents the mixotroph attack rate and b represents the conversion efficiency of captured bacteria into mixotroph biomass: Metabolic rates are known to scale with temperature [3]. To simplify our model, we assume that photosynthetic and grazing rates are linear functions of temperature (Figure 1B): Here, θ is an investment parameter that controls the reliance of mixotrophs on phagotrophy and photosynthesis.…”

Marine Protists: A Hitchhiker’s Guide to their Role in the Marine Microbiome

“…However, some constitutive mixotrophs show a stronger reliance on heterotrophy, as is the case for several chrysophytes. Unfortunately, evolutionary relatedness does not align well with differences in physiological strategies of mixotrophs, and closely related strains can show divergent ecophysiologies differing in both resource requirements and responses to environmental conditions (Moeller et al 2019;Wilken et al 2020). This makes inferences of functional roles in nature difficult.…”

Selective Feeding of a Mixotrophic Dinoflagellate (Lepidodinium sp.) in Response to Experimental Warming and Inorganic Nutrient Imbalance

“…Phagotrophic algae, one common type of mixotrophs, are capable of acquiring carbon from photosynthesis as well as ingestion of prey ( Stoecker, 1998 ; Mitra et al, 2016 ). Many studies have found that phagotrophic algae can benefit from prey ingestion, which allows them to thrive in the waters where the resource availability (e.g., inorganic nutrients and light) is limited to strict autotrophs ( Wilken et al, 2014 ; Moeller et al, 2019 ). Although there is increasing recognition and understanding of the ecological significance of mixotrophic protists, it remains substantially understudied how phagotrophic algae take advantage of mixotrophic metabolisms in rapidly changing environments.…”