Frontiers for Young Minds

Frontiers for Young Minds
Core Concept Collection Article Published: November 26, 2020

Freezing in the Sun


When the air is very cold, water at the surface of the ocean freezes, forming sea ice. Parts of the Arctic Ocean are covered by sea ice during the entire year. Often, snow falls onto the sea ice. Despite the cold, many plants and animals can live in the Arctic Ocean, some in the water, and some even in the sea ice. Particularly, algae can live in small bubbles in the sea ice. Like other plants, algae need energy to grow. This energy comes from food and sunlight. But how can the sunlight reach these little algae living inside the sea ice? From the sun, the light must pass through the atmosphere, the snow, and finally the sea ice itself. In this article, we describe how ice algae can live in this special environment and we explain what influences how much light reaches the algae to make them grow.

Sea Ice: The House of Algae

If you think about an ice cube, like the ones in a cold drink in summer, you can hardly believe that something could live inside it. But the sea ice, which is ice formed from freezing sea water, can indeed be the home for little organisms. How is this possible, and where do these organisms live? Ocean water contains both salt and algae. When the water freezes, some of these salt grains, and some algae, remain trapped in the ice. The salt grains melt some of the ice around them and create little bubbles filled with salty water [1]. These bubbles are called brine pockets. The brine pockets can be up to 5 mm in size, the diameter of a pencil, and they grow larger during the spring and summer seasons. Since algae are smaller than 1 mm, they can comfortably live inside the brine pockets (Figure 1). As the sea ice grows thicker, the brine pockets are pushed toward the bottom of the sea ice. Thus, algae live mainly in the bottom layer of the sea ice (Figure 1).

Figure 1 - The left panel shows a picture of a sea ice block, turned horizontally.
  • Figure 1 - The left panel shows a picture of a sea ice block, turned horizontally.
  • A brownish layer of algae is visible at the bottom. The middle panel shows the small bubbles, connected by veins, that make up the brine pockets that the algae live in. The right panel shows one pocket full of algae, which create a greenish-brown color.

How Big is Algae’s House?

An ice cube in your drink is relatively small; you can easily take it in your hand. But how big is sea ice? Sea ice covers large parts of the Arctic Ocean. It starts to form in autumn, when the air gets increasingly colder, and it reaches its maximum extent in late winter (February to March). The sea ice starts to decrease its extent again in late spring when the sun warms up the atmosphere and the surface of the ice, causing the ice to melt. We can imagine sea ice as a large blanket covering the Arctic Ocean. This blanket gets larger when it is cold and shrinks again when it is warmer. The sea ice also becomes thicker in winter and thinner again in summer. Sometimes the ice wrinkles and it can also break into pieces of varying sizes, from small pieces with a diameter of just a few meters up to large pieces of several kilometers. In winter the ice is the thickest, the blanket is the largest, and the most wrinkles are present.

The extent of sea ice in winter can reach 14 million km2, an area of about 2 billion football fields, and it can easily be 2 m thick, same as the height of a basketball player. When it crumples, the ice can create wrinkles that are 3 or 4 m high above the water and that reach down 10–15 m into the water. Moreover, in winter, snow accumulates on top of the sea ice. The snow is usually 20–30 cm thick. If you imagine the Arctic sea ice as a house inhabited by algae, the algae would live in the basement, at the bottom of the sea ice, often covered by a “roof” of snow.

Algae Need to Eat… and to be Eaten

Having a house, of course, is not all that algae need to live; they also need food. Algae are plants, so they need sunlight and carbon to perform photosynthesis [2], to create the energy they need to grow. Carbon is usually contained in sea water, so each brine pocket has a bit of carbon that can be used by the algae. What about sunlight? In the Arctic during the summer, days become so long that there is no night for about 2 months. On the other hand, in winter, the nights are so long that there is no light at all. Thus, algae can grow only in summer, when there is sunlight, and they die in winter, when it is dark. Dead algae are broken down and their nutrients are recycled, the same way plants that grown on land are. Algae are important because they are food for animals. As on land, the ocean is populated by small animals that eat plants. These animals are similar to shrimps or insects, but very small. The animals that eat plants are then eaten by bigger animals, like fish. Fish are eaten by birds and seals, which are then eaten by the biggest predator in the Arctic: the polar bear. Thus, sea-ice algae are important in the Polar regions because they are the first link in the Arctic marine food chain.

A Long Way From the Sun to the Earth

The light that we receive every day on the Earth originates from the Sun, our very own star. This light, which is sent permanently by the Sun in the form of rays, has to make a long journey through the solar system to reach the Earth. Once the sun rays reach our planet, they still need to get through the atmosphere before they can provide us with energy and warm us up. The atmosphere is the huge layer of air around the Earth’s surface that allows us to breathe. It is composed of a lot of tiny molecules of different gases. When the sun’s rays cross the atmosphere, some of them are reflected back into space by the gas molecules and also by the little water droplets that form the clouds (Figure 2). Other rays are absorbed by the particles. So, only a little more than half of the sun’s rays (~55%) will reach the Earth’s surface [3].

Figure 2 - Light has a long and difficult journey from the sun to the sea-ice algae.
  • Figure 2 - Light has a long and difficult journey from the sun to the sea-ice algae.
  • The yellow arrows represent the sunlight. The width of the arrows represents the amount of light: the thinner the arrow, the less sunlight available. Some of the sun’s rays are reflected back into the atmosphere by the snow layer, and some of the rays are absorbed by the snow and the ice. The amount of light that reaches the algae is very small compared to the amount of light that originally hit the snow.

The Difficult Journey of Sunlight Through the Sea Ice

We learned that algae live at the bottom of sea ice, which can be covered by snow. We also learned that, as plants, algae need sunlight to grow. But how is the sunlight capable of making it through the thick ice and snow to finally reach the little algae, to give them energy? The layer of snow is made of a large number of snowflakes packed together. Most of the sunlight that hits the snow is reflected back to the atmosphere, because the snow almost acts like a mirror. This is why it is always hard to look at the bright snow when the sun shines. Just like in the atmosphere, some of the sunlight is absorbed by the snowflakes, warming up the snow and contributing to its melting. It is much more difficult for the sunlight to pass through snow than to pass through the atmosphere though, because the snowflakes are more densely packed than the tiny gas molecules in the atmosphere. However, some rays find their way through. Those sun rays that make it through the snow layer will then encounter the sea ice below. Sea ice is ten times easier for the sun rays to cross than the snow is, since the ice is usually clearer than snow, with fewer particles and no snowflakes. But the sea ice is usually thicker than the snow, which again makes it harder for the sunlight to reach the algae’s living place. Eventually, some of the sun’s rays reach the bubbles where the algae live, making it possible for the algae to enjoy a bit of sunlight. But that is not the end! Once the sun’s rays have filled the algae with energy, some leftover rays continue their journey deeper into the ocean water, giving their energy to other plants and animals until all the light rays are used up (Figure 2).

What About the Future of Algae?

Many activities that we do as humans have led to pollution, resulting in a heating of the atmosphere called climate change or climate warming. Climate warming can already be measured today and will continue for a long time into the future. It is important for us to understand what this warming will mean for the different regions of the planet, including the Arctic. As it turns out, the Arctic is warming faster than the rest of planet, leading to less snow cover and thinner sea ice. As a consequence, it is easier for the sunlight to find its way through the ice to reach the algae. For the algae, this means that, in the future, there will most likely be more light available, and thus more energy to use to grow. But, scientists from all around the world have calculated that, sometime in the future, the sea ice will very likely disappear completely each summer. If the sea ice disappears each summer, there will be no housing for the sea-ice algae to live in. This means that climate change is a big threat to sea-ice algae.

Arctic research tries to understand the consequences of climate change for the Arctic, the atmosphere, the sea ice, the ocean, the plants and animals, and the humans. Our focus is on the sea-ice algae and the place they live, which is the sea-ice cover of the Arctic Ocean. We tell many people about our work and what we have discovered about the changes in the Arctic, so that people, including politicians, can have a better idea of what is happening now and what might happen in future. This knowledge will help us to make the proper decisions for the future of our planet.


Sea Ice: Frozen ocean water.

Brine Pockets: Small bubbles of salty water inside the sea ice.

Photosynthesis: A process by which plants use sunlight to produce food and energy.

Molecules: Very small particles that are the material from which every solid, liquid, or gaseous material is made.

Climate Change/Climate Warming: Worldwide increase in temperature and change in climate as response to human activities.

Conflict of Interest

FK and MK were employed by company Ocean Atmosphere Systems GmbH.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


[1] Glessmer, M. 2019. How does ice form in the sea? Front. Young Minds 7:79. doi: 10.3389/frym.2019.00079

[2] Ghosh, T., and Mishra, S. 2017. How does photosynthesis take place in our oceans? Front. Young Minds 5:34. doi: 10.3389/frym.2017.00034

[3] Fondriest Environmental, Inc. 2014. Solar Radiation and Photosynethically Active Radiation. Fundamentals of Environmental Measurements. Available online at: