CR
Cryospheric Sciences

Subglacial processes

Image of the Week — Microbes munch on iron beneath glaciers

Image of the Week — Microbes munch on iron beneath glaciers

The interface between a glacier and its underlying bedrock is known as the subglacial zone. Here lie subglacial sediments, the product of mechanical crushing of the rock by the glacial ice. Despite their lack of sunlight, nutrients and oxygen, subglacial sediments host active and diverse communities of microorganisms.

What we (don’t) know about subglacial microorganisms

The past few decades have seen major advances in our understanding of these communities, including the role these microbes play in the chemical breakdown of underlying bedrock (chemical weathering reactions). It is now known, for example, that microorganisms in subglacial systems are involved in pyrite oxidation and it certainly seems that bedrock mineralogy influences the composition of these microbial communities.

However, most studies to date have focussed on the biogeochemical cycling of sulfur and nitrogen in these systems. Consequently, the microbial mediation of iron cycling in subglacial systems remains poorly understood, despite the importance of iron in ocean fertilisation and other downstream environments. For instance, phytoplankton in the open ocean are often limited by the amount of iron available, so fluxes of iron to the oceans from glaciers and ice sheets are an important contribution to ocean productivity.

A new study about subglacial iron

In a new paper published in Biogeosciences, we investigate microbial iron reduction in subglacial sediments. Microorganisms that carry out this metabolism are able to harness energy from the reduction of oxidised iron minerals (such as ferrihydrite and other iron oxides).

We wanted to know two things:

  1. are these microorganisms present and alive in subglacial sediment?
  2. are these microorganisms adapted to the cold conditions of these environments?

 

To achieve this, we set up experiments in which we ‘teased out’ the microorganisms that make a living from iron reduction, and measured their rates of iron reduction at two different temperatures: 4°C (blue line in the figure) and 15°C(red line). These temperatures were chosen since truly cold-loving (‘psychrophilic’) microorganisms grow optimally at temperatures below 10-15°C, whereas those that tolerate cold temperatures (‘psychrotolerant’) prefer to grow in higher temperatures.

Microorganisms that can use iron to make a living are amongst the most plausible life to exist on Mars

We found that active iron-reducing microorganisms were present in all of our subglacial sediment samples, which spanned glaciers in the High Arctic, European Alps and Antarctica, and that in almost all cases rates of iron reduction were higher at the lower temperature tested. To get an idea of which microorganisms were carrying out this process, we looked at the DNA from our experiments to identify the microbes present. We found that the microorganisms using iron in our experiments were largely the same, suggesting that the same key players are active in these types of environments worldwide. Overall our paper suggests that microbial iron reduction is widespread in subglacial environments, with implications for the availability of iron for other biogeochemical processes downstream. Subglacial environments are thought to be similar to potentially habitable environments on Mars, and microorganisms that can use iron to make a living are amongst the most plausible life to exist on the Red Planet, now and in the past. Our work therefore strengthens the hypothesis that similar environments beyond Earth could harbour this type of life.

Edited by Sophie Berger


Sophie Nixon is a postdoctoral researcher in the Geomicrobiology group at the University of Manchester. She completed her PhD in Astrobiology in 2014 at the University of Edinburgh, the subject of which was the feasibility for microbial iron reduction on Mars. One essential task in the search for life on Mars and beyond is defining the limits of life in extreme environments here on Earth. It was during her PhD that this study was carried out in collaboration with researchers at the University of Bristol, where Sophie gained her MSci in Geographical Sciences. Sophie’s research interests since joining the University of Manchester are varied, spanning the microbiological implications of anthropogenic engineering of the subsurface (e.g. nuclear waste disposal, shale gas extraction), as well as life in extreme environments and the feasibility for life beyond Earth. 

Image of The Week – Plumes of water melting Greenland’s tidewater glaciers

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Figure 1: Simulation of a plume at a tidewater glacier in a general circulation model (MITgcm). Left – water temperature and right – time-averaged submarine melt rate in metres per day. Shown are face-on views of a tidewater glacier, as if you were under the water in front of the glacier, looking towards the calving front. 250 m3/s of fresh water emerges into the ocean from a channel at the bottom of the glacier, forming a plume. As the plume rises towards the fjord surface it mixes turbulently with warm ocean water, causing the plume to warm with height. Further details of this simulation can be found here: Slater et al. 2015.

Loss of ice from The Greenland Ice Sheet currently contributes approximately 1 mm/year to global sea level (Enderlin et al., 2014). The most rapidly changing and fastest flowing parts of the ice sheet are tidewater glaciers, which transport ice from the interior of the ice sheet directly into the ocean. In order to better predict how Greenland will contribute to future sea level we need to know more about what happens in these regions.


Tidewater glaciers meet the ocean at the calving front (Fig. 2), where ice undergoes melting by the ocean (“submarine melting”) and icebergs calve off into the sea. In recent decades, tidewater glaciers around Greenland have retreated (due to increased loss of ice at the calving front) and started flowing faster. This in turn causes more ice to be released into the ocean, contributing to sea level. Understanding the cause of these changes at tidewater glaciers is an ongoing topic of research.

Figure 2: Kangiata Nunata Sermia, a large tidewater glacier in south-west Greenland. The expression of a plume originating at the base of the calving front is visible on the fjord surface as turbid sediment-rich water. [Credit: Peter Nienow]

Figure 2: Kangiata Nunata Sermia, a large tidewater glacier in south-west Greenland. The expression of a plume originating at the base of the calving front is visible on the fjord surface as turbid sediment-rich water. [Credit: Peter Nienow]

One possible cause of change is an observed warming of the ocean around Greenland (Straneo and Heimbach, 2013). A warming of the ocean is likely to lead to increased submarine melt rates, which may in turn influence iceberg calving if, for example, melting results in instability of the ice at the calving front. Submarine melt rates are thought to be increased further by upwelling of warm water at the calving front (Fig. 1 and Fig. 2).

This upwelling water, called a plume, may be initiated by submarine melting of the ice, or by fresh glacial meltwater from the ice sheet surface. This fresh glacial meltwater penetrates to the base of the glacier and flows into the ocean from beneath the glacier, which may be hundreds of metres underwater. Once in the ocean, the meltwater rises buoyantly because of a density difference between the meltwater and ocean water, forming a plume. In order to better understand the effect of plumes on submarine melting, we can model plumes using a numerical model (e.g. MITgcm). Our image of the week (Fig. 1) shows such a model, which we can use to estimate submarine melt rates. In combination with simpler analytical approaches (Jenkins et al., 2011; Slater et al., 2016), we can estimate how submarine melt rates may change over time and from glacier to glacier (Carroll et al., 2016), and begin to assess the effect of submarine melting on tidewater glaciers and ultimately on future sea level rise.

Edited by Teresa Kyrke-Smith and Emma Smith


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Donald Slater is a PhD student in the Glaciology and Cryosphere Research Group at the University of Edinburgh. His research focusses on understanding the effect of the ocean on the Greenland Ice Sheet. For more information look up his website or follow him on twitter @donald_glacier.

Image of the Week: Under a Glacier

Image of the Week: Under a Glacier

What is happening under a glacier? This is a difficult questions to answer as accessing the glacier bed is usually not that easy. Here, we are getting a rare glimpse of the different processes and materials that are often found at the ice-bed interface. The photograph shows both sediments and hard rock, clear ice and dirty ice, and of course flowing water. No wonder these processes are complicated to say the least!

The photo was taken by Ilkka Matero (University of Leeds, U.K) during the excursion to Hochjochferner at the Karthaus summer school. See also the Image of the Week post from 18th of September to get an outside view of the side of the glacier.