Polar regions impose harsh conditions, including low temperatures, and prolonged periods of darkness on resident microbial communities. Despite these challenges, the conditions in these environments can also create opportunities for organisms utilizing combined trophic strategies (Mixotrophy). Only a limited number of studies have identified mixotrophic behavior in polar microbial eukaryotes, and even fewer studies have quantified the response of mixotrophs to likely environmental drivers of trophic behavior (light and nutrients). The goal of this work is to provide an identification of mixotrophic behavior and elucidate of some of the factors that influence algae isolated from polar environments. First, a study of the Arctic prasinophyte, Micromonas pusilla is presented in the first species-specific identification of mixotrophy in a eukaryotic phytoflagellate of this size class. M. pusilla grazed on bacteria under all experimental conditions, responding to nutrient limitation with increased rates of bacterivory. M. pusilla also showed evidence of prey selection. In contrast to the phagotrophic response, photosynthetic production was decreased under low-nutrient conditions. In an additional study of microbial eukaryotes from the Antarctic environment, identification of phagotrophy in photosynthetic nanoflagellates representing multiple evolutionary lineages: Cryptophyceae (Geminigera cryophila) and Prasinophyceae (Pyramimonas tychotreta and Mantoniella antarctica), showed that mixotrophy is more widespread in the Southern ocean that previously thought. G. cryophila and M. antarctica increased ingestions in dark treatments, but did not respond to difference in nutrient concentrations. In contrast, no significant grazing activity was observed in P. tychotreta under high nutrient conditions. When nutrients were reduced, ingestion of bacteria by P. tychotreta was observed and grazing increased in dark as compared to illuminated treatments. Finally, through a series of experimental assays, the competitive advantages of mixotrophic flagellates as opposed to monotrophic specialists were evaluated, using organisms isolated from the Southern Ocean. In these experiments, G. cryophila is emerged as a dominant competitor against two solely autotrophic diatoms (Fragilaria sp. and Fragilariopsis sp.). In contrast, P. tychotreta was outcompeted by the solely heterotrophic chrysophyte Paraphysomonas antarctica. These results show that mixotrophic ability can confer advantages to organisms in some cases, while in other interactions the cost associated with maintenance of multiple trophic strategies results in competitive exclusion by a specialist. These results present novel identification as well as rigorous investigation of mixotrophic behaviors in phototrophic flagellates from both polar (Arctic and Antarctic) environments representing two evolutionary lineages. This work provides a significant contribution to our understanding of the versatile nature of the physiology and trophic ecology of microbial eukaryotic organisms occupying polar marine ecosystems. / Biology

Marine microbes are an essential component of global biogeochemical cycles. In oligotrophic marine surface waters, the phytoplankton, phototrophic, single-celled (on occasion, colonial) organisms, is often dominated by the picoplankton (cells <2 micrometers in size), which constitute the base of the marine food chain. The picophytoplankton is composed of three main groups of organisms: two genera of cyanobacteria, Prochlorococcus and Synechococcus, and a third group, the picoeukaryotes. Even though numerically less abundant than cyanobacteria, picoeukaryotes can contribute significantly to biomass and primary production in this size fraction. Furthermore, picoeukaryotes are a diverse group but this diversity is still underexplored and their ecological roles and physiology is poorly understood. Here uncultured protists are investigated using 18S rRNA gene clone libraries, phylogenetic analyses, specific fluorescence in situ hybridization (FISH) probes and other methods in tropical and subtropical waters. Gene sequences comprising a unique eukaryotic lineage, biliphytes, were identified in most samples, whether from high (30 degrees Celsius) or low (5 degrees Celsius) temperature waters. Sequences within this uncultured group have previously been retrieved from mid and high latitudes. Phycobilin-like fluorescence associated with biliphyte-specific FISH probed cells indicated they may be photosynthetic. Furthermore, the data indicated biliphytes are nanoplanktonic in size, averaging between 3.0 and 4.1 micrometers. Using the 18S rRNA gene, sequences belonging to a broadly distributed but uncultivated pico-prymnesiophytes were retrieved. We investigated the ecological importance of these natural pico-prymnesiophyte populations and field experiments showed that they could grow rapidly and contributed measurably to primary production. They also appear to form a large portion of global picophytoplankton biomass, with differing contributions in five biogeographical provinces, from tropical to high latitudes. Finally, the physiology of the picoeukaryote Micromonas was studied under a shift from medium to high light and UV radiation. Results showed that the growth of these photosynthetic cells was synchronized with the light: dark period. Forward angle side scatter and red autofluorescence from chlorophyll increased throughout the light period and decreased during the dark period. This is consistent with cell division occurring at the beginning of the dark period. Additionally, genes proposed to have roles in photoprotection were up-regulated under high light and UV, but not in controls.