Glacier-fed streams, formed by seasonal meltwater from glaciers, supply freshwater to downstream ecosystems and human populations. As these glacier-fed streams are dominated by biofilms, the inhabiting microbial communities suffer from conditions related to climate change. The study “Experimental evidence on the impact of climate-induced hydrological and thermal variations on glacier-fed stream biofilms” in FEMS Microbiology Ecology aimed to understand how microbial biofilms in glaciers respond to climate-induced changes. For today’s World Day for Glaciers, David Touchette explains how microbial activities potentially impact these vital ecosystems, global food webs and water supplies. #MicrobiologyEvents

 

Microbial biofilms thrive in glacier-fed streams

Glacier-fed streams are quite extreme; they often have low nutrient concentrations, very cold temperatures, and high sediment loads. They change rapidly, both daily and seasonally, from being covered by snow in the winter, to being exposed to high ultraviolet radiation in the summer. These streams also experience shifts in water flow due to snowmelt, glacier melt, and groundwater.

Despite these harsh conditions, complex microbial communities build biofilms on streambeds, including stones, pebbles, and sediments. These protective biofilms harbour members from all domains of life, play key roles in nutrient cycling, and form the basis of the food web in stream ecosystems. Their success lies in the microbes’ abilities to adapt and withstand to the dynamic environment.

However, climate change is expected to alter the natural cycles in glacier-fed streams, with warming stream temperatures and more unpredictable flow regimes, including drought events. These shifts threaten water security by disrupting favourable environmental conditions for microbes to grow biofilms. Yet, how these changes affect microbial communities in glacier-fed streams remains poorly understood.

 

Drought-like periods impact glacial stream biofilms

To better understand how biofilms respond to climate-induced changes, the study “Experimental evidence on the impact of climate-induced hydrological and thermal variations on glacier-fed stream biofilms” in FEMS Microbiology Ecology, constructed artificial streams, so-called mesocosms, in the Swiss Alps to grow glacier stream biofilms over a span of 100 days.

The study replicated biofilms as found in natural glacier-fed streams. These harboured the same dominant microbial taxa, including Rhodoferax, Methylotenera, Polaromonas, Hydrurus, and Diatoma, and their typical succession pattern. Moreover, the experimental biofilm communities differed from the nearby stream water communities, indicating a successful establishment of specialised biofilms.

These experimental biofilms were exposed to varying hydrological and thermal conditions to simulate climate change scenarios. Drought-like conditions significantly impacted microbial biofilms and led to considerable shifts in the communities, favouring taxa normally found in soils. Additionally, drought-tolerant species, such as Sphingomonas, Nocardioides, Massilia, and Hymenobacter, were able to successfully compete and grow drought-stressed biofilms.

Their tolerance to drought is likely linked to their abilities to produce extracellular polymeric substances. These substances help retain water within the biofilm matrix, mitigating the negative effects of drought conditions.

 

Climate change scenarios decrease algal biomass in glacier-fed stream biofilms

Under climate change, glacier-fed streams are expected to become “greener” due to reduced glacier meltwater input and turbidity. In this study, both warming and drought conditions led to reduced chlorophyll levels within the biofilms, suggesting a decrease in algal biomass.

This result could be attributed to several mechanisms. Both warming and droughts may stimulate the activity of protozoa predators that feed on algae, leading to smaller algal populations.

Alternatively, these environmental conditions might directly induce algal mortality. For example, drought can induce osmotic stress which alters both the photosynthesis apparatus and the membranes.