Watermelon Snow: A Microscopic Serengeti [2019]
Figure 1 – Watermelon snow below the toe of the Lava Glacier, Mt. Garibaldi, BC. Photo: Casey Engstrom
Each summer, alpine snowfields across Canada undergo a startling transformation from white to pink, orange, or a startling red, known as watermelon snow. These blooms of microscopic algae are vast enough to show up on satellite imagery—in some regions covering more than a third of the snow surface (Fig 1). In recent years, snow algae have attracted attention for their possible impact on global climate and local water cycles: red snow absorbs more solar energy, driving faster snowmelt—indeed, mountain travelers can observe that the red snow often melts trenches substantially deeper than the surrounding white snow.
Microscopic examination reveals a hidden ecosystem that is stunningly beautiful and surprisingly complex. We see algae, fungi, bacteria, ciliates and rotifers, and from molecular work we know archaea and viruses are also present (Fig 2,3). Although specifics are not yet known, the snow algae microbiome undoubtedly comprises a web of grazers, parasites and symbionts. A favourite micrograph from our collecting trips in the mountains of southwestern B.C. shows a tardigrade with a belly full of red algae. Another image shows a row of chytrid fungi attached to an algal cell appearing to sip the energy-rich fatty red pigment through hyphae protruding into the alga. The algae themselves are brilliantly coloured, from ruby-red to yellow or orange, often encased in translucent shells with flanges, spikes or turrets. If snow algae are anything like their well-studied temperate cousins, some species may look quite different at various stages of the life cycle: tiny biflagellate green swimmers may grow into ruby-red armored cysts, or into brown hulking cells we have dubbed “motherships”—each one harbouring dozens of small green progeny.
Figure 2 – Microscopic image of two turreted Chlamydomonas cf. nivalis cells with three massive Chlainomonas sp. and the fungus Chionaster sp. in center. Scale bar is 25 microns. Photo: Casey Engstrom.
It remains a mystery how cells colonize the fresh snow surface each spring. By late summer this ephemeral ecosystem has melted, leaving behind red pools of water and small putty-like clumps of cells on the rocks. We’ve observed that these clumps contain what appears to be a microcosm of the core snow algae microbiome—algae, fungi, bacteria— possibly a “seed” to inoculate the pristine spring snow. If so, the organisms will need to survive the heat and desiccation of summer as they lie exposed on rocks, followed by a freezing dark winter under a new field of snow. Some species may colonize the spring snow by swimming to the surface (an epic migration for an organism 1/100th of a millimetre in diameter), while other species may be deposited on the snow surface by the wind. No one has yet identified the reservoirs from which snow surfaces are colonized each year.
Snow algae thrive on the snow, growing best at near-freezing temperatures. The algae flourish when both liquid water and nutrients are released from melting snow. We don’t yet know how the bulk of the bloom avoids being carried away with the meltwater. Biofilms may play a role, as may an intriguing family of molecules known as ice-binding proteins. When secreted, these molecules could anchor the cells to the rounded snow granules. Inside the cell they may offer protection by preventing the growth of damaging ice crystals.
A significant challenge to photosynthetic life on the snow is the bombardment with more solar energy than can be handled by the biochemical reactions of carbon fixation, which are slowed by the cold temperatures. The consequence is a surfeit of free radicals with the potential to damage the cell. It is thought that the red pigment (astaxanthin) does double duty as a light shade and antioxidant. Given the fatty nature of the astaxanthin molecule, we wonder whether it might also serve as a store of energy to launch development in the spring, possibly from the darker reaches of deep snow.
Figure 3 – A. Tardigrade with stomach full of snow algae. B. Chytrid fungi attached to the outside of a snow algae cell. C. Hatching “mothership” of unknown species. Photo: Casey Engstrom.
Might snow algae be abundant enough to significantly alter global warming and the global carbon cycle? One study estimated that one fifth of the seasonal melt on an Alaskan snowfield was due to the presence of algae. Given the patchiness of blooms and their transience, gauging global impact will be difficult. Similarly, their contribution to the global carbon cycle is difficult to calculate. We hypothesize from our field observations that the bulk of the snow algae bloom is eventually deposited into alpine soil and lakes, potentially acting as a carbon sink. Whatever roles snow algae may have played in maintaining the ecological balances of the past 10,000 years, it is inevitable that this microbiome will be an early casualty of global warming as alpine snowfields diminish on a warming Earth.
Casey Engstrom is a graduate student in the lab of Dr. Lynne Quarmby in the Department of Molecular Biology and Biochemistry at Simon Fraser University. If you would like to learn more about snow algae, please visit www.quarmby.ca.
REFERENCES
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3. Lutz, S. et al. The biogeogr aphy of red snow microbiomes and their role in melting ar ctic glaciers. Nat. Commun. 7, 11968 (2016).