Hypothesized life cycle of the snow algae Chlainomonas sp. (Chlamydomonadales, Chlorophyta) from the Cascade Mountains, USA.

Chlainomonas (Chlamydomonadales, Chlorophyta) is one of the four genera of snow algae known to produce annual pink or red blooms in alpine snow. No Chlainomonas species have been successfully cultured in the laboratory, but diverse cell types have been observed from many field-collected samples, from multiple species. The diversity of morphologies suggests these algae have complex life cycles with changes in ploidy. Over 7 years (2017-2023), we observed seasonal blooms dominated by a Chlainomonas species from late spring through the summer months on a snow-on-lake habitat in an alpine basin in the North Cascade Mountains of Washington, USA. The Bagley Lake Chlainomonas is distinct from previously reported species based on morphology and sequence data. We observed a similar collection of cell types observed in other Chlainomonas species, with the addition of swarming biflagellate cells that emerged from sporangia. We present a life cycle hypothesis for this species that links cell morphologies observed in the field to seasonally available habitat. The progression of cell types suggests cells are undergoing both meiosis and fertilization in the life cycle. Since the life cycle is the most fundamental biological feature of an organism, with direct consequences for evolutionary processes, it is critical to understand how snow algal life cycles will influence their responses to changes in their habitat driven by climate warming. For microbial taxa that live in extreme environments and are difficult to culture, temporal field studies, such as we report here, may be key to creating testable hypotheses for life cycles.

Patchy and Pink: Dynamics of a Chlainomonas sp. (Chlamydomonadales, chlorophyta) algal bloom on Bagley Lake, North Cascades, WA.

Snow algal blooms frequently occur throughout alpine and polar environments during spring and summer months; however, our understanding of bloom dynamics is limited. We tracked a recurrent bloom of Chlainomonas sp. on Upper Bagley Lake in the North Cascade Mountains, USA, to assess the spatiotemporal dynamics in bloom color intensity, community photophysiology, and community composition over eight weeks. We found that the algae biomass had a dynamic patchy distribution over space and time, which was decoupled from changes in community composition and life-cycle progress averaged across the bloom. The proportional representation of Chlainomonas sp. remained consistent throughout the study while the overall community composition shows a progression through the bloom. We found that community photophysiology, measured by the maximum quantum yield of PSII (Fv/Fm), decreased on average throughout the bloom. These findings suggest that the Chlainomonas sp. community on Bagley Lake is not simply an algal bloom with rapid increase in biomass followed by a population crash, as is often seen in aquatic systems, though there is a physiological trajectory and sensitivity to environmental stress. These results contribute to our understanding of the biology of Chlainomonas sp. and its response to environmental stress, specifically an extreme warming event.