Mixotrophy in Chlorophytes and Haptophytes-Effect of Irradiance, Macronutrient, Micronutrient and Vitamin Limitation

Abstract

Chlorophytes and haptophytes are key contributors to global phytoplankton biomass and productivity. Mixotrophic bacterivory has been detected for both groups, but a shortage of studies with cultured representatives hinders a consistent picture of the ecological relevance and regulation of this trophic strategy. Here, the growth, primary production, fraction of feeding cells (acidotropic probes) and bacterivory rates (surrogate prey) are tested for two species of the chlorophyte genus Nephroselmis and the haptophyte Isochrysis galbana under contrasting regimes of light (high vs. low) and nutrients (non-limited and macronutrient-, micronutrient- and vitamin-limited), at low bacterial concentrations (<10⁷ bacteria mL^-1). All three species were obligate phototrophs, unable to compensate for low light conditions through feeding. Under nutrient limitation, N. rotunda and I. galbana fed, but growth ceased or was significantly lower than in the control. Thus, mixotrophic bacterivory could be a survival rather than a growth strategy for certain species. In contrast, nutrient-limited N. pyriformis achieved growth rates equivalent to the control through feeding. This strikingly differs with the classical view of chlorophytes as primarily non-feeders and indicates mixotrophic bacterivory can be a significant trophic strategy for green algae, even at the low bacterial concentrations found in oligotrophic open oceans.

Keywords: bacterivory; chlorophyte; green algae; haptophyte; mixotrophic growth; mixotrophy; phytoflagellate.

Figure 1

Changes in phytoflagellate abundance, percentage of feeding cells and growth rate for N. rotunda Exp. 1 and 2; and the N. pyriformis and I. galbana experiments. In the plots for percentage of feeding cells and growth rate all time points for Ctl are plotted, while for the treatments only the time points with a significant difference to Ctl are plotted (t-test; P < 0.05). a, at this time point only duplicate values were available for the control and statistics were not possible. Treatments – control (Ctl), and media without the addition of macronutrients (Nma), micronutrients (NMi) or vitamins (NV).

Figure 2

Follow up experiments ascertaining the regulatory role of the “limiting” substrate on algae growth and feeding. These were conducted with the N. pyriformis and I. galbana treatments NMa, NMi and NV. Ad 1, addition 1 - addition of the “limiting” solution (e.g., vitmains to the NV treatment); Ad 2, addition 2 - addition of the other solutions (e.g., macro- and micronutrients to the NV treatment); No ad, unamended treatment; nt, not tested. ^*Indicates significant differences to t0 (t-test; P < 0.05).

Figure 3

Change in phytoflagellate abundance and percentage of feeding cells over time for a starved N. pyriformis culture after being re-inoculated into nutrient replete media.

Figure 4

Comparison of measured bacterivory rates and percentages of feeding cells. Data has been pooled for all three algal strains. The dashed line indicates the observed threshold for bacterivory rate detection.