Abstract
Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.
Highlights
An increasing number of recent studies point out the prevalence of mixotrophic phytoplankton in natural communities[5,6]
The occurrence of mixotrophs under these conditions is taken to indicate that they have an advantage over photoautotrophic phytoplankton by obtaining limiting nutrients through phagotrophy, much in the way carnivorous plants and stony corals thrive in oligotrophic environments due to the combination of photosynthesis with heterotrophic ingestion[2]
Heterotrophic bacteria are characterized by low C:N and C:P ratios[15], making them a attractive prey for mixotrophs which may cover their energy demand by photosynthesis but their nutrient requirements by phagotrophy
Summary
An increasing number of recent studies point out the prevalence of mixotrophic phytoplankton in natural communities[5,6]. A large compilation of freshwater phytoplankton traits derived from observational data suggests that the growth rates of strictly photoautotrophic representatives (chlorophyceae: Scenedesmus, Chlamydomonas; bacillariophyceae: Fragilaria, Cyclotella) clearly exceed those of typical mixotrophs (chrysophyceae: Dinobryon, Ochromonas; haptophyta: Chrysochromulina; dinophyta: Peridinium, Gymnodinium; Table S1 in[20]). From that it appears that the niche of mixotrophs is mostly defined by maintaining positive growth at simultaneously low prey and nutrient concentrations. We expect the ratio mixotrophs: heterotrophs among protist consumers to increase with light
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