the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2026
Reviews and syntheses: Snow algae on the move – biased motility and snowpack interaction from a biophysics perspective
Abstract. Snow algae are psychrophilic and psychrotolerant photosynthetic microorganisms found on every continent, predominantly in polar and alpine environments. Along with contributing to terrestrial carbon cycling and food webs, colourful snow algal blooms formed on snow surfaces can substantially reduce albedo and accelerate snowmelt. Despite their ecological importance, the mechanisms governing snow algae motility and migration within snow remain poorly understood. This review synthesises current knowledge of snow algae migration, spanning microscopic cell-level motility to macroscopic population-level redistribution within snowpacks. We consider snow algae as biologically active particles within the framework of active matter physics, exploring their non-equilibrium dynamics and self-propelled motion in response to environmental stimuli. Particular attention is given to directional behaviours in response to light, temperature and chemical gradients, gravity and fluid flow. Where data gaps exist, we draw parallels from studies on a model motile microalga, Chlamydomonas reinhardtii. Finally, we identify key knowledge gaps and highlight future research directions, with implications for understanding cryosphere processes, microswimmer tactic behaviour, and the development of emerging biotechnologies.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2167', Anonymous Referee #1, 05 Jun 2026
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RC2: 'Comment on egusphere-2026-2167', Anonymous Referee #2, 08 Jun 2026
This review investigates the mechanisms underlying the motility in snow algae, a phenomena that is still not fully understood. There is an updated overview of the importance of snow algae in the global context, including some updates to the known diversity of this group of microalgae. The majority of the review then gives an in-depth overview of the different environmental parameters that can trigger this motility, and the mechanisms behind this.
This is a detailed and well-reasoned review that synthesises a wide range of relevant literature to provide new insights into the responses of mobile snow algae to different environmental parameters. There are many interesting arguments raised as to what may govern the movement of snow algae within snowpacks, alongside how these understandings could be scaled up to larger scales and possibly into the future. The explanations given for the biophysical mechanisms described are very accessible, alongside clear illustration. That said, there are a number of generalisations and potential oversights that this review feels needs to be addressed before publication.
Specific comments
Line 68: Include reference to Engstrom et al for the description of the new Rosetta genus.
Line 69: Please include Chlamydomonas on this list. As I understand there are still snow algal species assigned as such, the definition of Sanguina did not backdate to other species previously assigned as Chlamydomonas. I also feel that the omission of Chlainomonas from further discussion in this review is an oversight, especially as this is one of the only snow algal species that has it's life cycle documented in detail.
Line 70: Include further references for these other groups, including Ochrophytes responsible for golden-brown snow.
Line 74: The life cycle of S. nivaloides is unknown as, to my knowledge, no cultivable strain exists, and so is theorised. Therefore S. nivaloides should not be named here as a motile flagellated snow algae. It should be made clearer throughout the review that the life cycles of many common snow algal species are only theorised to date, with more care to name the species for which this transition from vegetative motile to encysted cells has been observed.
Line 141: Similar to above, please make clear in this section that not all snow algal species are known to experience this life cycle, and for many it is only theorised.
Line 152: Include Ganey et al 2017 for further information on the contribution of wide scale snow algal blooms to albedo reduction.
Line 159: Please include the species this study is focused on.
Line 163: The Roussel et al study is focused on the analysis of remote sensing data and so is not centred specifically on S. nivaloides. Perhaps rephrase to more general language.
Line 170: Again please include the exact species/genera this study was focused on, or whether there were multiple snow algal species present. This allows for better context throughout the review with much diversity present within the motility and life stages of different snow algal species.
Line 177: It is not clear through this study that these red algal blooms are dominated by S. nivaloides so please rephrase this section accordingly.
Line 206: This short paragraph is lacking an overall point I find. What does it mean for the discussion of the paper that the mean swimming speed is negligible?
Line 235: Please include specific references as to these mechanisms being observed in snow or other microalgae.
Line 335: It may also be worth mentioning here the capacity for snow algal to demonstrate plasticity to the available nutrient conditions as opposed to moving to where they are optimal (see Broadwell et al 2023). Although not focused on movement, it demonstrates the range of mechansims present across motile snow algae.
Citation: https://doi.org/10.5194/egusphere-2026-2167-RC2
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General comment:
This review discusses the potential for active microbial motility in snow algae, a functionally important group of unicellular algae in the cryosphere ecosystems. It provides an updated overview of their ecological context and environmental impact, followed by a concise review of the snow algal diversity and motility in green algae. The remainder and core of the manuscript explores current knowledge and perspectives on active microalgal migration within snowpacks and examines how environmental stimuli influence these movements.
Overall, this work provides valuable insights for the research community. It highlights promising directions for future investigations into the dynamics and assembly of snow algae communities and contributes to improving our understanding of their potential global impacts.
The sections addressing micro-motility and its biophysical basis are very well explained and accessible to snow algae biologists and ecologists.
The quality and clarity of illustration is appreciated.
My comments mainly suggest a better integration of the snow-algal literature to better link the ecological context and integrate the diversity if algal species and behaviours. It seems that several special cases of motile species and behaviour have been omitted, remember that snow algae is a diverse group with diverse life strategies.
I also suggest ensuring that each subsection concludes with a brief synthesis highlighting the ecological relevance of the feature discussed, particularly in relation to motility of snow algae within snowpacks. This would help improving the coherence of the review.
Introduction:
Line 19: I suggest two other relevant and key references for the snow melt rate statement: Ganey et al. 2017 Nature Geosciences, and Roussel et al. 2024 PNAS.
Kahn et al. is very relevant as well.
Line 20: mechanism likely to intensify under climate change -> I would be really careful with this statement. Liang et al. has not used prediction model to evaluate the effect of climate change under different scenarios. This question however is clearly investigated in Roussel et al. 2024 PNAS which does suggest the opposite FOR the Alps (not necessary the case for Antarctica), shorter snow melt duration -> less snow algal bloom
Line 24: About the thermal range and definition, I would suggest to double check what range defines a cryophilic species. It sometimes commonly accepted that cryophilic species optimally grow below 10°C, and temperature above would be lethal (i.e. 15°C) as clearly described in Détain et al. 2025 using TPC (already cited in this review).
“Hoham 1975, Arctic and Alpine Research” is the first original and key reference for these definitions in snow algae.
As an additional reference with a screening of snow algae for thermal preference is Suzuki et al 2023 FEMS, despite not perfect since only 2 temperatures are tested, it shows many species with more cryotolerant-mesophilic traits, a true cryophilic and an absolute mesophilic.
Hulatt et al. 2017 & Leya et al 2009 FEMS are the two great reference for biotech application, alongside Schoeter.
Line 31: This is really COOOL. I feel this is an interesting application honestly. However, I feel this microswimmer application is not essentially related to snow algae. Is it the right place to mention this?
What do you think about the Ice-Binding-Proteins from snow algae that could have biotech application, to add in this section? (if you really want to mention biotech application)
Line 36: Could you make a smoother transition, indicating that snow algae have a motile stage.
Probably, just something like “motile microalgae (like some snow algae) are considered…”
Line 42: about the term “Flagella”. There is no microtubule in a flagellum, flagella are made of flagellin and only fond in bacteria. Cilia are membrane delimited and articulated by microtubule sliding (Kahn & Scholey et al 2018 Current Biology).
I acknowledge that most people understand and still many misuse this term, but I would raise your attention on the fact that a flagella is protein-based structure found in bacteria ONLY (prokaryotes) that propel the particle, while microalgae use cilia (a cellular protrusion membrane delimited) that pull the cell particle in its environment (Kahn & Scholey et al 2018 Current Biology). From a biological perspective as well as biophysical perspective the two structures are clearly different.
I would suggest consistency and either use only flagella or cilia, knowing that “cilia” is the correct biological term, and cells are biciliated as defined in (Raymond et al. 2019 Current Biology for Sanguina aurantia, and Lynn Quarmby’s work.).
Line 41: I could suggest something like “propel themselves through ciliary-beating, a biological process where…”. So, it indicates what kind of movement drives the propulsion mentioned in the next sentence.
Line 43-46: This is very interesting, but considering the target audience, would you have a way to make this statement a bit more digestible for a wider audience? while keeping the message.
Line 58: Cd. reinhardtii
Line 66-70: I think the dynamic in which species names arose from one another is complicated. I would recommend a more straightforward sentence where genetic characterisation and differentiation led to the definition of new genus including Sanguina (Prochazkova et al 2019), Rosetta (Engstrom et al 2024), Chlainomonas (Novis et al. 2008 J. Phycology), Chloromonas (Matsuzaki et al 2019 Plos One, FYI major polyphyletic group of snow algae) and Limnomonas (Tesson & Proschold 2022, Diversity).
Remove Rea & Dial from this section (still relevant for the vertical migration within the snowpack).
Important note: The genus Chlainomonas is a very outstanding group of microswimmer which was omitted in this review. It has a biciliated stage as well and a Quadri-ciliated stage with bulky swimming. This is very outstanding. Please refer to both Novis et al. 2008 and 2023 Journal of Phycology) in this review.
2.1 Use of flagella
Well, as suggested before I would go for “use of cilia”
Line 77: can cilia really rotate?
Line 79-81: to my knowledge, cilia of cells like Chlamydomonas-like are symmetric, beating can be asymmetric to give directional change, like in response to light stimuli. Not sure this specific asymmetry refers to the positioning of cilia on a Chlamydomonas-like cell like snow algae? Or clarify what asymmetry you are specifically talking about in here (maybe an illustration?)
Important note: The genus Hydrurus (golden snow algae Chrysophyceae) have asymmetric cilia, please find a way to describe this heterogeneity in cilia configuration and potential implication in microswimming (this corresponds to the Chrysophyte in Détain et al 2025, and please refer to (Prochazkova et al 2026 Journal of Phycology)
Maybe the numbering of section, 2 and 2.1 is not optimal, you should have a 2. with 2.1 diversity and taxonomy and 2.2 Use of cilia
3.1. Snow
Great section
3.1.1. Microalgal migration in a snowpack at a macroscopic scale
Line 144-147: Yes, but this is hypothetic, it not shown. Figure 4 shows two snow color, but the green cells are maybe not the same as the red cells. So I would suggest phrasing it as: “it is suggested that…”
Line150: UV-protection from astaxanthin is not really sure, astaxanthin is a strong antioxidant that shield the cell from excessive and damaging energy transfer from light to the photosynthetic machinery (Ezzedine et al. 2023 Nature communication).
Line 161: Roussel et al 2024 PNAS suggests that liquid water for at least 46 days is required for bloom formation, shouldn’t be assumed that it is upward migration.
Line 170: No, cysts have not been seen germinating, in this study they only observed the effect of blocking the way between the ground and the surface.
Important note: Please check Matsumoto et al 2024 Journal of Phycology. About Chlainomonas, this study has more convincing evidences of germination, but it is not absolutely proven that it works that way. Mind that this is for a snow-lake environment.
3.1.2 Microalgal migration in a snowpack at the microscopic scale
Line 182: I would suggest the introduce the notion of quasi-liquid layers QLLs before referring to it
Line 194: 40µm for a motile vegetative cell is quite big (except Chlainomonas), I would suggest checking the typical size of vegetative motile cells (more between 6 and 20µm).
Line 200: I am not sure about the interpretation of Ono and Takeuchi 2025. It says non-motile SA (cysts) do not migrate/change layer in the snowpack, and do not seem to be affected by meltwater flow, no significant percolation effect. And this would explain why the cells accumulate on the surface as the snowpack melts. However, cysts can be found within the liquid layer as shown in Ezzedine et al., maybe percolating at a microscale, but not a macroscale. In fine, both study don’t contrast, they are more complementary, one describing the microscale (with a cm2), the other one the macroscale (with dozens of cm2).
Line 214-238: This is a really great section, maybe the most valuable and novel information/perspective you bring in this review. Do you think it would be possible to illustrate i) comparison between narrow and wider channel in terms of physical forces, ii) how the interfacial pre-melting could enhance migration, or whatever you think is worth illustrating.
4 Tactic behaviour
Line 240: agree with the taxis definition but I could suggest improving with something more like: “… a stimulus from biological, chemical or physical origin” so it includes all components of taxism, including chemo-, photo-, thermos- and gravi-.
For the equation of the different taxes, it is a bit confusing how the different equations build. What is preceding the “+” ? (this maybe due to my ignorance in equation syntaxes)
4.1 Phototaxis
Line 285-286: I would rephrase with something explaining like: “light sensing by photoreceptor triggers modulation of intracellular calcium current (Ca2+ signature), which in turn induces an asymmetric ciliary beating followed by a movement toward or away from the light source.”
Line 289: Phototaxis was examined at only 1 temperature (except for Cd. reinhardtii) in Détain et al. 2025.
Following this, I invite you to discuss the discussion (section “Occurrence of stigmas in snow Chloromonads”) in Novis et al. 2024 Phycologia (on Chloromonas fuhrii) regarding the role and evolution of the eyespot in Chloromonas species. Their perspectives are interesting and worth mentioning at the end of this section. Especially that not having an eyespot in snow algae is to avoid confusing signals from light reflecting from numerous directions off snow crystals.
Line 306: What species are you referring to with Prochazkova 2019. Cd. nivalis is not Sanguina. According to the literature, Sanguina species are only available in culture in two laboratories so far, S. aurantia from 2022 in Canada and now in France. And S. nivaloides from 2025 in Norway.
Important note: Light quality is also affecting mating/sexual reproduction in snow algae (Hoham et al. 1998, Hydrol Process). It is not directly related to phototaxis, but somewhere in this review, it would be necessary to highlight that the motile stage of snow algae is suggested to play a key role in the life cycle and strategy of snow algae, through sexual reproduction by cilia pairing. Refer to other Hoham et al. paperS and Matsumoto et al 2024 Journal of Phycology for the life cycle of Chloromonas snow algae.
Maybe introduce the notion in introduction, since mating is also discussed in the chemotaxis section
4.2 Chemotaxis
Line 326: other molecules than ions are sensed for chemotaxis
Line 328-333: Not convinced the results from Almela et al. 2024 needs to be developed that much. Results from different regions or bloom types suggest different scenario, where there is not much of a consensus. Davey 2019 New Phytol highlights the metabolic difference between nutrient deprived red blooms and green metabolically active blooms. Ezzedine et al. 2023 Nat comm and Suzuki et al. 2025 The Plant Journal highlight the adaptability of snow algae to low Phosphorus condition with an adapted lipidome. Suzuki et al. 2023 FEMS shows that biciliated motile Chloromonas reticulata accumulate lipid droplets and specific lipids under N-starvation in the same way as Sanguina nivaloides does (Ezzedine et al. 2023).
Therefore, green bloom supported by higher nutrient content in Davey 2019, as well Ono and Takeuchi 2025 in the wood, are metabolically active and can fuel motility, enhanced by chemotaxis for better success??
I suggest to better introduce the context or relationship between nutrients and snow algae
Figure 7: Great figures, I would only suggest using NH4+ instead of K+ as your example, to better match the key nutrients discussed layer.
I would suggest looking into Quorum sensing behaviour in Chlamydomonas reinhardtii from Folcik et al 2020, iScience
4.4 Gyrotaxis
Line 446: I think it should be: “orient (Pedley and Kessler 1992)”
In this paragraph, would you have anything to suggest about the effect of gyrotaxis and bioconvection pattern for snow algae, in the snowpack? What about snow-covered alpine lakes with Chlainomonas species (Matsumoto et al) ??
Important note: Hulatt et al 2024 G3 (genome paper on Limnomonas) has a nice illustration of the culture with accumulation of biomass at the surface (very gravitactic) and the cells (biciliated with very long cilia (Tesson & Pröschold 2022)) are even climbing the walls of the flasks. You may have a comment on this behaviour and the ecological relevance in snow in term displacement?
4.5 Thermotaxis and temperature sensitivity
Line 490: Cryotolerant species are somehow mesophilic but tolerate the cold, do they necessarily have lower speed compared to the pure mesophilic? maybe not? The list of species in Détain et al. is too small to compare cryophilic vs. cryotolerant vs. mesophilic. But it does contrast pure cryophilic from mesophilic with or without cryotolerance.
Line 496: As previously suggested, the range for cryophilic is probably 0 to 10°C; most cryophilic are happy between 5 and 10°C, but do not survive above 15°C. In the table, the Chrysophyte has a Topt of 9.9°C, but it doesn’t survive much above 12°C.
Comment: would you have any thoughts about the effect of temperature on ciliary beating and consequently trajectory? Have a look at Geyer et al. 2022 Nature physics Fig 2. And 3.