Hardly a day goes by without new media reports on climate change and melting glaciers. We have all seen footage of enormous drifting icebergs, images of rapidly shrinking glaciers and headlines trying to draw public attention to potentially catastrophic consequences of vanishing ice. The times we live in are marked not only by the Great Melt, but also by what is known as the Sixth Mass Extinction, which is a widespread and rapid disappearance of a large number of species due to human activity.

While browsing through photos of increasingly smaller glaciers, many biologists kick themselves for not being quick enough to discover and describe their inhabitants, especially now that global biodiversity is on a steep decline. After all, the world’s glaciers are not only cold, ominous and melting, but also inhabited. By whom? A whole range of creatures, from minute, single-cell organisms, such as bacteria or microscopic plants, to animals consisting of numerous cells and tissues, but still too small to see with a naked eye.

Glacier surfaces crawling with life

In order to be properly understood, glacial ecosystems need to be considered on a microscale. This is why a microscope is an indispensable piece of equipment for every scientist studying biodiversity on glaciers. During the summer, the snow accumulated on glacier surface melts, providing creatures which live on the ice with liquid water and basic nutrients necessary for them to grow. The most common among the creatures are cyanobacteria, which most of us know as blue-green algae. They are one of the most ancient organisms on the planet and one of the pioneers of early oxygen production.

Over the course of Earth’s history, cyanobacteria have skilfully handled a range of adversities and adjusted to highly unpleasant conditions. Some of them have developed a liking for cold places, like ice. Blue-green algae, however, are not the only inhabitants of glacier surfaces. They share their territory with distant cousins of trees, or – to be more specific – microscopic ice algae, which belong to a group of plants known as Zygnematophyceae. Both types of organisms serve a very important role and are often referred to as the engineers of glacial ecosystems. In other words, it is them that produce the organic matter which makes life possible for other creatures, such as bacteria, protozoa, and microscopic animals.

Blue-green algae and ice algae are made up of cells which contain a pigment protecting them from high doses of UV radiation, typical of mountainous and polar regions. Mass algal blooms may lead to ice darkening, which causes increased heat absorption and thus makes glacier surface melt more rapidly. Scientists from different countries observe the phenomenon very closely and try to figure out the impact of these organisms on the rate of glacier melting on a global scale. Paradoxically, by protecting themselves from solar radiation, the tiny plant organisms inhabiting glacier surfaces contribute to the destruction of their icy homes.

Cryoconite and its inhabitants

Contrary to what many of us tend to think, glaciers are not white and immaculate. Looking at their surface, we may notice a dark residue made up of mineral dust, which comes from nearby mountains and the tundra, but also – by means of long-range wind transport – from much more distant places, such as far-away deserts. Beside mineral particles, the residue contains microorganisms (like the above-mentioned types of algae) which live on glacier surface. At first glance, the ice may seem to be polluted, but in most cases it is just how it naturally is. The first person to take notice of the residue was the Finnish traveller and geologist Adolf Erik Nordenskiöld, who called it cryoconite (which is Greek for cold dust).

The interaction between mineral dust and the organisms which use it as an additional source of nutrients reduces the albedo (or the amount of solar radiation reflected off the surface of ice) and causes the ice underneath to melt. Cylindrical melt holes that form as a result are known as cryoconite holes and, despite being only several centimetres in diameter and depth, they constitute one of the most extreme freshwater ecosystems on Earth. The temperature inside them is seldom above 0.1°C, nutrients are scarce, and – if this wasn’t bad enough – they are under constant threat of destruction by heavy rains and warm winds.

So who would ever want to live there? The above-mentioned organisms and, believe it or not, a good few more. At the bottom of cryoconite holes lies a layer of residue (cryoconite), which is home for tardigrades and rotifers. Despite being invisible to the naked eye, they are the most dominant and numerous of all the animals inhabiting the little ecosystems. Tardigrades are colloquially referred to as water bears and they do indeed resemble clumsy bears (or little sausages with four pairs of limbs, if you will). Rotifers, on the other hand, are more like tiny swimming bags. Their name comes from a Latin word meaning wheel-bearer, and was inspired by the circular structure found on top of their body. It is called the corona (crown) and is used for feeding and movement. Rotifers and tardigrades measure typically about 0.3 mm but, despite their diminutive size, it is them that are the greatest glacial predators, feeding on bacteria and algae.

Both tardigrades and rotifers can be found in the soil, mosses and lichens, as well as lakes and seas. Those that live on glaciers, however, endure serious thermal hardships and – from the point of view of any biologist studying glacial ecosystems – they are absolutely extraordinary. They survive in temperatures oscillating around 0°C and may be repeatedly frozen, defrosted, and – in some cases – even dried. They freeze for many months, without even knowing if the place they froze in would still be covered in ice the following summer. They do struggle in high temperatures, however, which means they are probably well impressed by their savannah-dwelling cousins.

The little creatures inhabiting glacier surfaces have another distinctive feature – they are exceptionally cute. The most interesting of them have unique morphological characteristics which distinguish them from their relatives in lakes or in the soil. For instance, some tardigrades found on glaciers are black. It is likely that the dark pigment helps them survive and protects them against high doses of UV radiation they are exposed to once they have been washed out from the relative safety of their cryoconite hole and deposited directly on glacier surface. Tough life, right?

Mysterious organisms in the ice

Apart from tardigrades and rotifers, the inhabitants of glacier surfaces include little arthropods known as springtails (Collembola), which feed on organic matter. Their name is far from accidental. Springtails are equipped with a furcula, or a forked, tail-like appendage, which enables them to jump. On a glacier, they look like tiny, dark fleas. But the greatest surprise awaits those who study glacial ecosystems along the western coast of the USA and Canada, where in the ice (yep, right inside it) live the so-called ice worms. Measuring up to 3 mm in length, they are classified as the annelids (Oligochatea) and are thus related to earthworms. Due to a long series of events, however, they have come to inhabit the inside of glaciers rather than the good old soil.

Ice worms live inside glaciers found along the coast. This is because these glaciers contain microchannels with liquid water, which do not freeze even during the winter. It is these channels that ice worms squeeze through. At night, they crawl to the surface and feed on algae until dawn, when they withdraw back into the ice to hide from the sun. They serve as an inspiration for astrobiologists who dream of discovering macroscopic life on ice-covered planets and moons. After all, if earthly ice can be inhabited, it does not take much to imagine life thriving inside glaciers enveloping other celestial bodies. For the time being, however, such concepts are still pure science fiction.

Biodiversity yet to be discovered

Even though glacial ecosystems have been studied for many years, a vast majority of creatures inhabiting these extraordinary places is still completely unknown to science. It is a real pity, as organisms which live in such extreme environments might provide insights invaluable for astrobiologists dealing with extraterrestrial life or biotechnologists interested in cold-active enzymes. Moreover, understanding life on glaciers is crucial for regular environmental studies. Why? Because it is cryoconite and the organic matter produced by the engineers of glacial ecosystems that reach glacier forefields and take part in the initial stages of soil formation. It is pigmented organisms that accelerate glacier melting. And finally, because our inherent curiosity forces us to keep learning about life on Earth. And it is that curiosity that makes us human.

For many biologists, glaciers and the organisms found inside them and on their surface constitute natural heritage which deserves a thorough scientific examination, especially now that climate change is right upon us. Unfortunately, when it comes to biodiversity, the best studied glaciers are those located in easily accessible areas or in the vicinity of research stations. On a global scale, it is just a handful of places. What biological marvels are still to be found further afield is anybody’s guess.

Intrigued by the topic? Pop into our GALLERY to find photographs of cryoconite holes and various ice creatures.

Author: Krzysztof Zawierucha, PhD