The multifaceted lifestyle of ocean plants

The ecology of mixotrophs, i.e. organisms that combine autotrophy and heterotrophy, is not adequately understood, which is why these organisms are often misrepresented in biogeochemical models. Wilken et al. (2019) provide evidence for variation in the mixotrophic lifestyle of marine chrysophytes (genus Ochromonas) from coastal and pelagic ecosystems. The two eukaryotes were cultured under varying light and prey (Vibrio fischeri) availability. Subsequently, growth rate, ingestion rate and carbon fixation were quantified. While prey-depletion reduced growth in both organisms, the coastal chrysophyte maintained similar photosynthetic rates in the absence of prey. In contrast, primary production of the oceanic strain halved. Ingestion rates emulated photosynthetic rates in the oceanic chrysophyte, while they displayed an opposite trend in the coastal type. 

This study highlights the stark contrasts in mixotrophic metabolism found among congeners. While the coastal chrysophyte has the ability to switch effortlessly between photoautotrophy and heterotrophy, both metabolic pathways are coupled in the pelagic strain. The authors thus define the former as a facultative and the latter as an obligate mixotroph. The observed functional diversity can be interpreted as an adaptation to a fluctuating and stable environment, respectively. Nevertheless, it remains unclear how widespread these energy-acquisition strategies are in their respective habitat.

Wilken, S., Choi, C. J., & Worden, A. Z. (2019). Contrasting mixotrophic lifestyles reveal different ecological niches in two closely related marine protists. Journal of Phycology, 0(0), 0-0.