Robert W. Sanders

Phaeocystis antarctica , a dominant phytoplankton species found in the Southern Ocean, is known for colony formation and large seasonal blooms. Blooms play a key role in Southern Ocean biogeochemistry, contributing to carbon export and the production of dimethylsulfoniopropionate (DMSP) and its climatically active breakdown product, dimethyl sulfide (DMS). Here we provide evidence of phagotrophy, i.e., mixotrophic behavior in the alga P. antarctica. Mixotrophy is widespread among marine protists and was recently identified in the temperate sister species P. globosa. We examined potential triggers for bacterivory using a 4 × 4 factorial design manipulating irradiance (diurnal changes versus static) and nutrient availability and evaluated bacterivory using fluorescent microspheres as tracers. Our results show that bacterivory in P. antarctica was driven more strongly by variability in irradiance than by nutrient availability, with greater ingestion occurring in treatments with diurnal cycles. Because macronutrient limitations are generally negligible in the Southern Ocean, bacterial ingestion of P. antarctica may represent an adaptive response to seasonal changes in light availability.

Through its role as an interface between ocean and atmosphere, sea ice is important both physically and biologically. We propose here that the resident microbial community can influence the structure of sea ice, particularly near its ocean interface, by effectively lowering the local freezing point via an osmolytic mechanism. This lowered freezing point can enhance fluid flow, linking a bottom, convective ice layer with the underlying ocean, resulting in improved nutrient uptake and byproduct removal. A mathematical model based on a previously suggested abiotic one dimensional simplification of mushy ice fluid dynamics is used to illustrate, and supporting measurements of freezing point depression by lab grown sea ice-associated organisms are provided.

Rising temperature, variation of light levels, and changes in nutrient availability are among the top environmental factors known to have profound effects on growth, grazing, and photosynthesis in mixotrophic species. Polar species, in particular, are facing rapid changes in their environment. In the present study, phagotrophic and phototrophic responses of an Arctic chrysophyte, Dinobryon faculiferum, were investigated at different temperatures (2, 4, and 6°C), light levels (0-200 μmol photons m^-2 s^-1), and nutrient concentrations (12.5 and 50% f/2 + Si nutrient media in 32 PSU artificial seawater). Bacterivory by D. faculiferum, estimated by ingestion of fluorescent proxies, increased with rising temperature, while photosynthetic rates, determined by ¹⁴C uptake, were low under all experimental regimes; phagotrophy appeared to contribute more to this species’ carbon budget than phototrophy. Furthermore, D. faculiferum survived long periods of darkness, implying it may not be an obligate phototroph, as found for some other species in the genus. In experiments limiting nitrogen and/or phosphorus, there was no effect of limiting phosphorus on either phototrophy or phagotrophy, but both were reduced in treatments with limited nitrogen, presumably impeding metabolic processes when strongly limited. Our results demonstrate the influences of abiotic environmental factors on phagotrophic and phototrophic responses, providing further understanding of how mixotrophic protists in polar systems may respond to a changing climate.

Mixotrophic chrysophytes are known for significant contributions to primary productivity and grazing within microbial loops in aquatic systems. Understanding how projected changes in a warming environment might alter physiological responses in mixotrophs is crucial. In this study, we investigated mixotrophic responses of the freshwater chrysophyte, Chrysolepidomonas dendrolepidota, under an array of temperatures (14–20°C) and inorganic nutrient conditions (1%–50% DY-IV nutrient media). Photosynthetic rates of C. dendrolepidota were reduced in lower inorganic nutrient concentrations and increased with rising temperature. Bacterivory rates were not significantly different across inorganic nutrient regimes but decreased with rising temperature, and in some measure with decreasing bacterial abundance. The highest temperature tested promoted rapid initial growth in higher inorganic nutrient condition, with slower more continuous growth observed at lower temperatures. Our results indicate phototrophy and phagotrophy by C. dendrolepidota are strongly affected by temperature, shifting toward more phototrophic nutritional mode as temperature increased. While some mixotrophs have been shown to be more heterotrophic with increasing temperature, photosynthesis contributes more to C. dendrolepidota’s carbon budget with increasing temperature. These findings demonstrate environmental factors can induce different physiological responses along gradient of photo-osmo-phago-mixotrophic abilities, which should be taken into consideration in future works involving models of climate-change impacts on phagotrophic phytoplankton.

Through its role as a an interface between ocean and atmosphere, sea ice is important both physically and biologically. We propose here that the resident microbial community can influence the structure of sea ice, particularly near its ocean interface, by effectively lowering the local freezing point via an osmolytic mechanism. The lower freezing point can enhance fluid flow, linking a bottom, convective ice layer with the underlying ocean, resulting in improved nutrient uptake and byproduct removal. A mathematical model based on a previously suggested abiotic one dimensional simplification of mushy ice fluid dynamics is used to illustrate, and supporting measurements of freezing point depression by lab grown sea ice-associated organisms are provided.

Through its role as a an interface between ocean and atmosphere, sea ice is important both physically and biologically. We propose here that the resident microbial community can influence the structure of sea ice, particularly near its ocean interface, by effectively lowering the local freezing point via an osmolytic mechanism. The lower freezing point can enhance fluid flow, linking a bottom, convective ice layer with the underlying ocean, resulting in improved nutrient uptake and byproduct removal. A mathematical model based on a previously suggested abiotic one dimensional simplification of mushy ice fluid dynamics is used to illustrate, and supporting measurements of freezing point depression by lab grown sea ice-associated organisms are provided.

Through its role as a an interface between ocean and atmosphere, sea ice is important both physically and biologically. We propose here that the resident microbial community can influence the structure of sea ice, particularly near its ocean interface, by effectively lowering the local freezing point via an osmolytic mechanism. The lower freezing point can enhance fluid flow, linking a bottom, convective ice layer with the underlying ocean, resulting in improved nutrient uptake and byproduct removal. A mathematical model based on a previously suggested abiotic one dimensional simplification of mushy ice fluid dynamics is used to illustrate, and supporting measurements of freezing point depression by lab grown sea ice-associated organisms are provided.

Through its role as a an interface between ocean and atmosphere, sea ice is important both physically and biologically. We propose here that the resident microbial community can influence the structure of sea ice, particularly near its ocean interface, by effectively lowering the local freezing point via an osmolytic mechanism. The lower freezing point can enhance fluid flow, linking a bottom, convective ice layer with the underlying ocean, resulting in improved nutrient uptake and byproduct removal. A mathematical model based on a previously suggested abiotic one dimensional simplification of mushy ice fluid dynamics is used to illustrate, and supporting measurements of freezing point depression by lab grown sea ice-associated organisms are provided.

Through its role as a an interface between ocean and atmosphere, sea ice is important both physically and biologically. We propose here that the resident microbial community can influence the structure of sea ice, particularly near its ocean interface, by effectively lowering the local freezing point via an osmolytic mechanism. The lower freezing point can enhance fluid flow, linking a bottom, convective ice layer with the underlying ocean, resulting in improved nutrient uptake and byproduct removal. A mathematical model based on a previously suggested abiotic one dimensional simplification of mushy ice fluid dynamics is used to illustrate, and supporting measurements of freezing point depression by lab grown sea ice-associated organisms are provided.

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.