| 20 Mar 2026
Distinct Phototrophic Community Structure on a Central Asian Glacier: Predominance of Filamentous Cyanobacteria and Absence of Glacier Algae (Ancylonema spp.)
Abstract. Cold-adapted algae and cyanobacteria are key drivers of snow and ice albedo reduction, yet their dynamics on dust-rich Central Asian glaciers remain poorly understood compared to the well-documented algal blooms in the Arctic. This study investigated the spatio-temporal distribution of phototrophic communities on Urumqi Glacier No.1, eastern Tien Shan, during a two-month melt season. Our findings reveal a distinct seasonal succession where snow-covered surfaces were dominated by snow algae Chloromonadinia species (Chlorophyceae), while the exposure of bare ice led to a sharp increase in biomass dominated by filamentous cyanobacteria (Oscillatoriaceae). Notably, glacier algae such as Ancylonema spp., which drive darkening on Arctic ice, were entirely absent, suggesting a fundamental ecological divergence. Statistical analyses indicated that cyanobacterial proliferation is closely linked to environmental factors, showing significant positive correlations with mineral-derived ions and negative correlations with inorganic nitrogen. These results, supported by recent evidence that specialized cyanobacterial taxa drive the initiation and structural development of cryoconite granules, suggest that high mineral dust deposition from surrounding arid regions facilitates a stable, nutrient-limited niche for cyanobacteria. This "cyanobacteria-mineral synergy" creates a more persistent biological darkening effect than the ephemeral algal blooms observed in polar regions. Our study highlights the necessity of integrating region-specific microbial dynamics, which is characterized by the absence of glacier algae and the dominance of mineral-buffered cyanobacterial communities, into glacier mass balance models to improve the accuracy of future projections for Central Asian water resources.
General comments:
The manuscript presents a study of cyanobacteria on a central Asian Glacier. The authors present biomass values for six sample locations at five time points over the course of the summer on an alpine glacier. While the results are interesting, it is difficult to assess the value of the findings due to the methods lacking clarity, specifically around how the %biomass values were determined and what they represent. Additionally, since cell count data were not included in the manuscript it is difficult to fully interpret the results. The authors attempt to connect the results to both albedo and mineral dust, but neither albedo nor mineral dust were measured in the study. Thus, the related interpretations are overstated and are not supported by the data. Specifically, a “cyanobacteria-mineral synergy” has not been demonstrated by what is presented here. Additionally, there are several statements throughout the manuscript that incorrectly describe information from the literature that must be corrected.
Specific comments:
Line 28 and throughout the discussion: a “cyanobacteria-mineral synergy” has not been demonstrated. There has been no characterization of the mineral dust on the ice (either composition or quantity), nor has any meaningful evaluation of the relationship between the cyanobacteria and mineral dust been completed. Simply comparing against a few dissolved ions is insufficient. The authors should either provide data that supports this claim, or remove this phrase from the manuscript.
Line 36: this first sentence is misrepresenting the estimated 1029 cell count presented in the referenced article. That value is an upper estimate for the total global volume of glacial ice based on microbial counts from deep ice cores. It is not an estimate of cells on glacier and ice sheet surfaces, as is stated in the manuscript. Revise to correctly reflect the literature referenced.
Line 54: This is a broad statement, please identify where in the Arctic this cyanobacteria and cryoconite data is from.
Line 72: “Given their patchy and heterogeneity” is incomplete. Please revise for clarity. “Given their heterogeneity,” is probably sufficient.
Figure 1: what do the thin solid black lines indicate? They are not listed in the legend. I suggest using different colours to improve clarity. The S6 location label is a different font size than the others, and a space is needed between the elevation and units. Please increase the size of the date labels on the photographs.
Section 2.2.2 The manuscript describes cell counting to determine biomass, but it is not clear how the presented community composition was determined. Is it based on the light microscopy? That is what the first few sentences of the results suggest, but this is not clear in the methods and this information must be added to the manuscript. What literature references were used for the identification? Also, where is the cell count data?
Section 3.2 and Figure 4. The text uses units of mLm-2 for the biomass, but the figure is in µL m-2. Please use one unit consistently throughout the manuscript.
Section 3.3 It is not clear what presented % values represent. The terms: community composition, % biomass, % biovolume, and morphological community structure seem to be used interchangeably throughout. What is being presented here? How these numbers were determined and what these data represent must be clarified with consistent and accurate terminology.
Section 3.3 Where are the actual cell counts? Presenting only biovolumes makes it difficult to know how many cells are present at the study site, and therefore the impact of the data. For instance, are the presented ratios present in abundances of 101 cells/mL or 107 cells/mL? This drastically impacts the overall findings.
Section 3.4 It is unusual to see cryosphere geochemical data presented in µEq L-1 and will make it difficult for readers to compare these values to other datasets (typically presented in µg L-1 or mg L-1). Doing so also highlights that there is a strong charge imbalance in the presented data, with cations being disproportionately higher than the anions. Is this actually the case, or are there other dissolved ions present that are not included in the data?
Line 309: without any cell counts presented in the manuscript, the authors cannot claim that cyanobacteria are present in sufficient numbers to contribute to darkening. Please provide the cell count data that supports this statement. This will also require either presenting corresponding albedo measurements, or making comparisons to literature that demonstrates that cyanobacteria present in the reported cell abundances can in fact impact albedo.
Line 314 – 345: while some of the ideas presented in this discussion are interesting, there are no connections made to the literature. Appropriate references to the literature must be added throughout this section to support these points.
Line 346: this is an outdated interpretation. Arctic algal blooms on ice begin within a day or two of snow clearance. Therefore, is minimal difference in the timing of algae vs cryoconite impacting ice albedo.
Line 356-360: this is comparing very different environments. Additionally, this statement does not accurately reflect what is stated in either of the Cook and Hotaling articles referenced. The Hotaling article summarizes a range albedo measurements, which clearly show cryoconite and algae yielding a range of albedo reduction values, that very much overlap. The Cook article does not make any clear statements comparing algal and cryoconite impacts on albedo. Neither describe algal blooms as ‘transient’. Also, T=this interpretation does not account for the spatial extent of these two materials, as outlined in Cook et al. 2020 (https://doi.org/10.5194/tc-14-309-2020 ), while cryoconite on the surface has a greater impact on albedo, it occupies a far smaller surface area on ice sheets compared to ice algae, making the overall impact of cryoconite on albedo negligible. The discussion should be updated to correctly reflect the literature, and to also reflect these nuances of the topic.
Line 368: If assessing the relationship between cyanobacteria and mineral dust was an aim of the study, why was mineral dust not characterized? Simply measuring dissolved ions is insufficient to support the interpretations presented. Such data must be included to support these claims or they should be removed from the manuscript.
Line 381: This statement is incorrect. The McCutcheon 2021 article referenced here specifically characterizes algal biomass nutrient ratios in relation to mineral dust. It does not study cyanobacteria; correct the use of this reference.