Abstract
Nutritional modes of unicellular eukaryotes range from pure photoautotrophy of some phytoplankton to pure heterotrophy of species typically called protozoa. Between these two extremes lies a functional continuum of nutrient and energy acquisition modes termed mixotrophy. Prymnesium parvum is an ecologically important mixotrophic haptophyte alga that can produce toxins and form ecosystem disruptive blooms that result in fish kills and changes in planktonic food web structure. We investigated carbon and nitrogen acquisition strategies of single cells of P. parvum using a combined experimental-imaging approach employing labeling of live cells with stable isotope tracers (13C and 15N) followed by measurement of cellular isotopic ratios using nanometer-scale secondary ion mass spectrometry (NanoSIMS). With this method, we were able to quantify the relative contributions of photosynthesis and heterotrophy to the nutrition of the alga. Our results suggest that P. parvum relies on predation primarily for nitrogen, while most carbon for cellular building blocks is obtained from inorganic sources. Our analysis further revealed that nitrogen assimilation can vary up to an order of magnitude among individual cells, a finding that would be difficult to determine using other methods. These results help to improve our understanding of mixotrophy across the enormous diversity of eukaryotes, one cell and one species at a time.
Highlights
It is often assumed that the vast diversity of the eukaryotic domain can be neatly divided into plant-like or animal-like categories, with the few taxa that combine both capabilities viewed as curiosities
We investigated the mixotrophic potential of P. parvum at the single cell level by comparing carbon and nitrogen uptake from prey and inorganic nutrients using stable isotope tracers and imaging of cells with NanoSIMS
Prior to this study of P. parvum, the only previous application of stable isotope tracers and NanoSIMS to investigate protistan mixotrophy was completed by Terrado et al (2017) on the chrysophyte alga Ochromonas sp. strain BG-1
Summary
It is often assumed that the vast diversity of the eukaryotic domain can be neatly divided into plant-like (exclusively photoautotrophic) or animal-like (exclusively heterotrophic) categories, with the few taxa that combine both capabilities (i.e., mixotrophy) viewed as curiosities. Mixotrophy is very common across the eukaryotic tree (Raven et al, 2009), is ubiquitous in aquatic ecosystems, and is of great importance in aquatic biogeochemical cycles where it influences carbon and energy flow in food webs (e.g., Sanders, 2011; Mitra et al, 2014; Ward and Follows, 2016). The term mixotrophy has been broadly applied to protists that combine photoautotrophic with heterotrophic nutrition (primarily phagotrophy) (Flynn et al, 2013; Mitra et al, 2016). The great abundances of mixotrophs in most aquatic systems suggest this nutritional mode should be considered the rule rather than the exception (Matantseva and Skarlato, 2013)
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