Disentangling the role of light and nutrient limitation on bacterivory by mixotrophic nanoflagellates.

Many phytoplankton taxa function on multiple trophic levels by combining photosynthesis and ingestion of bacteria, termed mixotrophy. Despite the recognition of mixotrophy as a universal functional trait, we have yet to fully resolve how environmental conditions influence community grazing rates in situ. A microcosm study was used to assess bacterivory by mixotrophic nanoflagellates following nutrient enrichment and light attenuation in a temperate lake. We found contrasting results based on assessment of mixotroph abundance or bacterivory. Despite an interactive effect of nutrient enrichment and light attenuation on mixotroph abundance, significant differences within light treatments were observed only after enrichment with P or N + P. The greatest abundance of mixotrophs across treatments occurred under co-nutrient enrichment with full exposure to irradiance. However, bacterivory by mixotrophic nanoflagellates was greatest under shaded conditions after either N or P enrichment. We suggest that PAR availability dampened the stimulatory effect of nutrient limitation, and bacterivory supplemented a suboptimal photosynthetic environment. In a saturating light regime, the mixotrophic community was less driven to ingest bacteria because photosynthesis was able to satisfy energetic demands. These findings quantify community bacterivory in response to environmental drivers that may characterize future ecosystem conditions and highlight the importance of considering grazing rates in conjunction with abundance of mixotrophic protists.

Production of Cyanotoxins by Microcystis aeruginosa Mediates Interactions with the Mixotrophic Flagellate Cryptomonas.

Eutrophication of inland waters is expected to increase the frequency and severity of harmful algal blooms (HABs). Toxin-production associated with HABs has negative effects on human health and aquatic ecosystem functioning. Despite evidence that flagellates can ingest toxin-producing cyanobacteria, interactions between members of the microbial loop are underestimated in our understanding of the food web and algal bloom dynamics. Physical and allelopathic interactions between a mixotrophic flagellate (Cryptomonas sp.) and two strains of a cyanobacteria (Microcystis aeruginosa) were investigated in a full-factorial experiment in culture. The maximum population growth rate of the mixotroph (0.25 day-1) occurred during incubation with filtrate from toxic M. aeruginosa. Cryptomonas was able to ingest toxic and non-toxic M. aeruginosa at maximal rates of 0.5 and 0.3 cells day-1, respectively. The results establish that although Cryptomonas does not derive benefits from co-incubation with M. aeruginosa, it may obtain nutritional supplement from filtrate. We also provide evidence of a reduction in cyanotoxin concentration (microcystin-LR) when toxic M. aeruginosa is incubated with the mixotroph. Our work has implications for "trophic upgrading" within the microbial food web, where cyanobacterivory by nanoflagellates may improve food quality for higher trophic levels and detoxify secondary compounds.

Temperature-dependent phagotrophy and phototrophy in a mixotrophic chrysophyte.

The roles of temperature and light on grazing and photosynthesis were examined for Dinobryon sociale, a common freshwater mixotrophic alga. Photosynthetic rate was determined for D. sociale adapted to temperatures of 8, 12, 16, and 20°C under photosynthetically active radiation light irradiances of 25, 66, and 130 µmol photons · m(-2)  · s(-1) , with concurrent measurement of bacterial ingestion at all temperatures under medium and high light (66 and 130 µmol photons · m(-2)  · s(-1) ). Rates of ingestion and photosynthesis increased with temperature to a maximum at 16°C under the two higher light regimes, and declined at 20°C. Although both light and temperature had a marked effect on photosynthesis, there was no significant difference in bacterivory at medium and high irradiances at any given temperature. At the lowest light condition (25 µmol photons · m(-2)  · s(-1) ), photosynthesis remained low and relatively stable at all temperatures. D. sociale acquired the majority of carbon from photosynthesis, although the low photosynthetic rate without a concurrent decline in feeding rate at 8°C suggested 20%-30% of the carbon budget could be attributed to bacterivory at low temperatures. Grazing experiments in nutrient-modified media revealed that this mixotroph had increased ingestion rates when either dissolved nitrogen or phosphorus was decreased. This work increases our understanding of environmental effects on mixotrophic nutrition. Although the influence of abiotic factors on phagotrophy and phototrophy in pure heterotrophs and phototrophs has been well studied, much less is known for mixotrophic organisms.