Psychrophilic diatoms in ice-covered Lake Erie

Winter surveys across Lake Erie between 2007 to 2011 documented diatom assemblages associated with extensive ice cover. This study aims to characterize these winter assemblages in terms of their community structure, their interactions, adaptations to a psychrophylic lifestyle and their role in the biogeochemistry of the lake. The winter assemblage was dominated by diatoms, specifically by Aulacoseira islandica (O. Müller) Simonsen (≈80%) and Stephanodiscus binderanus (Kützing) Krieger (≈19%), with other taxa like Fragilaria spp., Cyclotella spp., Asterionella spp., and Tabellaria spp. making up less than 1% of the assemblage. The detailed morphology of the two dominant taxa observed are described in this study, and the presence of resting cells and auxospores in filaments of A. islandica, but not in the other taxa are documented. The study compares different sample processing techniques for scanning electron microscopy (SEM), and describes a rapid and effective technique, substituting critical point drying (CPD) with hexa-methyl-di-silazane (HMDS) treatment, that allows viewing of fine details including the epiphytic colonization of the diatoms by flagellated rod-shaped bacteria embedded in an extracellular matrix. The study also describes ice nucleating activity associated with assemblages of filamentous diatoms sampled during winter and early spring across the Laurentian Great Lakes. The ability to promote ice formation offers a previously undescribed mechanism by which non-motile phytoplankton can attach to overlying ice and, thereby, maintain a favorable position in the photic zone. The ice nucleating activity is attributed to bacteria. Bacteria isolated from the phytoplankton showed high temperatures of crystallization (Tc) to -3°C. Ice-nucleating active (INA) isolates were identified as belonging to the genus Pseudomonas. Whereas INA bacteria have been isolated from lakes and streams, their presence in these environments is attributed primarily to runoff and atmospheric deposition as rain or snow consistent with their proposed role as biological ice nuclei in clouds. Far from a passive existence in the aquatic milieu, the INA microbes associated with winter diatoms in the Great Lakes may possess a role in promoting the formation of ice during winter and in so doing, promote the growth of their diatom hosts under ice. The novel mechanism presented may be relevant to temperate and polar ecosystems beyond the Great Lakes including coastal oceans. Finally, photosynthetic O2 evolution rates measured in winter support characterization of the assemblage as a photosynthetically robust population, with rates of primary production in winter higher than those measured in spring and comparable to those reported in summer. Results further suggest that the winter diatom assemblage may play an important role in the export of carbon to the benthos, thereby potentially driving the hypolimnetic oxygen deficits observed months later in summer