Glacier shrinkage will accelerate downstream decomposition of organic matter and alters microbiome structure and function

Abstract

The shrinking of glaciers is among the most iconic consequences of climate change. Despite this, the downstream consequences for ecosystem processes and related microbiome structure and function remain poorly understood. Here, using a space‐for‐time substitution approach across 101 glacier‐fed streams (GFSs) from six major regions worldwide, we investigated how glacier shrinkage is likely to impact the organic matter (OM) decomposition rates of benthic biofilms. To do this, we measured the activities of five common extracellular enzymes and estimated decomposition rates by using enzyme allocation equations based on stoichiometry. We found decomposition rates to average 0.0129 (% d−1), and that decreases in glacier influence (estimated by percent glacier catchment coverage, turbidity, and a glacier index) accelerates decomposition rates. To explore mechanisms behind these relationships, we further compared decomposition rates with biofilm and stream water characteristics. We found that chlorophyll‐a, temperature, and stream water N:P together explained 61% of the variability in decomposition. Algal biomass, which is also increasing with glacier shrinkage, showed a particularly strong relationship with decomposition, likely indicating their importance in contributing labile organic compounds to these carbon‐poor habitats. We also found high relative abundances of chytrid fungi in GFS sediments, which putatively parasitize these algae, promoting decomposition through a fungal shunt. Exploring the biofilm microbiome, we then sought to identify bacterial phylogenetic clades significantly associated with decomposition, and found numerous positively (e.g., Saprospiraceae) and negatively (e.g., Nitrospira) related clades. Lastly, using metagenomics, we found evidence of different bacterial classes possessing different proportions of EEA‐encoding genes, potentially informing some of the microbial associations with decomposition rates. Our results, therefore, present new mechanistic insights into OM decomposition in GFSs by demonstrating that an algal‐based “green food web” is likely to increase in importance in the future and will promote important biogeochemical shifts in these streams as glaciers vanish.

Cross-domain interactions induce community stability to benthic biofilms in proglacial streams

“…Simultaneously, parasitic fungi also foster the release of organic compounds from algae via the 'fungal shunt' 31 . The prevalence of parasitic fungi has been noted previously in GFS 47 and other cryospheric ecosystems 48 ; our analyses further point to the importance of interactions among parasitic fungi and their algal host in proglacial stream biofilms. Along these lines, Mo et al 49 recently suggested that interactions of microeukaryotes between them in the Lena River continental shelf were more stable compared to that of the estuary, potentially explained by variability in salinity.…”

Homogeneous Environmental Selection Structures the Bacterial Communities of Benthic Biofilms in Proglacial Floodplain Streams

“…Understanding the nature of these constraints and their impacts on microbial communities in these ecosystems is particularly relevant in light of climate change and the expected predominance of TRIBs in the future. In line with this, our previous work indicated an important role of eukaryotic phototrophs in explaining the differences between GFS and TRIB communities ( 18 , 25 , 26 ). We expect that organic carbon provided by benthic primary producers can alleviate (to some extent) resource limitation in TRIBs and, thus, reduce the importance of selection therein.…”

Spatial patterns of benthic biofilm diversity among streams draining proglacial floodplains

“…In all three proglacial floodplains, we found significant longitudinal changes in community similarity from the GFSs close to the glacier snout to the outlet of the floodplain (Supplementary Figure 11). These patterns are unexpected given the relatively short distances (hundreds of meters) along the longitudinal chronosequence and the high dispersal of microbial cells in GFSs (Ezzat et al, 2022). We attribute this spatial turnover to downstream changes in the GFS environment that may be partially induced by processes (e.g., sedimentary dynamics) in the GFS channel itself, but also by the cumulative influence of the tributaries discharging into the GFS.…”

“…This network pattern points to a potential role of Chytridiomycota in structuring biofilm communities in proglacial streams. In fact, Chytridiomycota have received increasing scientific interest as common microparasites of marine and freshwater algae ( Grossart et al, 2019 ; Klawonn et al, 2021 ) as well as in the cryosphere ( Brown et al, 2015 ; Lutz et al, 2015 ; Anesio et al, 2017 ; Rojas-Jimenez et al, 2017 ; Kohler et al, 2022 ) where they may shunt algal carbon to fungal biomass, thereby bypassing the typical microbial loop. Given that vascular plant debris is typically absent in proglacial streams, fungi parasitizing on benthic algae seems intuitive.…”