A tale of two mixotrophic chrysophytes: Insights into the metabolisms of two Ochromonas species (Chrysophyceae) through a comparison of gene expression

Abstract

Ochromonas spp. strains CCMP1393 and BG-1 are phagotrophic phytoflagellates with different nutritional strategies. Strain CCMP1393 is an obligate phototroph while strain BG-1 readily grows in continuous darkness in the presence of bacterial prey. Growth and gene expression of strain CCMP1393 were investigated under conditions allowing phagotrophic, mixotrophic, or phototrophic nutrition. The availability of light and bacterial prey led to the differential expression of 42% or 45–59% of all genes, respectively. Data from strain CCMP1393 were compared to those from a study conducted previously on strain BG-1, and revealed notable differences in carbon and nitrogen metabolism between the 2 congeners under similar environmental conditions. Strain BG-1 utilized bacterial carbon and amino acids through glycolysis and the tricarboxylic acid cycle, while downregulating light harvesting and carbon fixation in the Calvin cycle when both light and bacteria were available. In contrast, the upregulation of genes related to photosynthesis, light harvesting, chlorophyll synthesis, and carbon fixation in the presence of light and prey for strain CCMP1393 implied that this species is more phototrophic than strain BG-1, and that phagotrophy may have enhanced phototrophy. Cellular chlorophyll a content was also significantly higher in strain CCMP1393 supplied with bacteria compared to those without prey. Our results thus point to very different physiological strategies for mixotrophic nutrition in these closely related chrysophyte species.

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

“…In contrast, in our study, the mixotroph may competitively exclude the phytoplankter even when, in monoculture, its I Ã out is greater, because it is also able to feed on the phytoplankter (Appendix S1: Fig. 3), other studies have shown that prey can enhance mixotroph photosynthetic capacity, perhaps by providing nutrients inaccessible in the culture media (Sanders et al 2001, Lie et al 2018. Because competition for light is mediated by standing stock biomass rather than production of new biomass, the competitive interactions between mixotrophs and phytoplankton mimic interference competition.…”

“…3). These results highlight the complexities of real mixotroph physiology, wherein different taxa may derive different forms and degrees of benefit from ingesting prey (Mitra et al 2016, Lie et al 2018. S5).…”

“…Second, carrying capacity increased with increasing light availability (Fig. However, transcriptomic data suggest that Ochromonas strain CCMP 1393 may not be capable of using nitrate (Lie et al 2018), which was the most abundant source of inorganic nitrogen in our culture medium (Appendix S1: Table S1). S6).…”

“…For Ochromonas, we included data in which Micromonas prey were initially available but were ultimately eliminated by grazing, because some Ochromonas strains have been shown to have grazing-enhanced phototrophy (Lie et al 2018). As part of our main experiment (details below) we measured carrying capacity as the maximum population size achieved by each taxon at each light level over a period of 20 (100 lmol quanta Á m À2 Á s À1 ), or 40 d (20 lmol quanta Á m À2 Á s À1 ).…”

“…As part of our main experiment (details below) we measured carrying capacity as the maximum population size achieved by each taxon at each light level over a period of 20 (100 lmol quanta Á m À2 Á s À1 ), or 40 d (20 lmol quanta Á m À2 Á s À1 ). For Ochromonas, we included data in which Micromonas prey were initially available but were ultimately eliminated by grazing, because some Ochromonas strains have been shown to have grazing-enhanced phototrophy (Lie et al 2018). Indeed, we observed that two of our Ochromonas strains (CCMP 1393 and CCMP 2951) achieved higher population sizes when prey were available (Appendix S1: Fig.…”