The alga provides the food and the fungus provides the water in this mutualistic symbiotic relationship. The alga photosynthesizes (using light, water, and carbon dioxide) to produce food (carbohydrates) for itself and its symbiont (the fungus). In return for its food, the fungus retains liquid water for itself and the alga to avoid dehydration. The fungus also attaches to the rocky substrate. This combination allows both the alga and the fungus to live in an area where nothing else can survive (often directly on a rocky surface). Without each other the fungus would starve and the alga would dehydrate.
Mosses are often thick in the tundra where there is some soil and sufficient water. They can be so thick that they form a soft and squishy cushion in some Arctic areas, making it feel like walking on a soft mattress. A tiny red mite can be found scurrying here and there among the moss plants. If the wind is not blowing you might even find a fly (one of the few insect pollinators for the flowering plants in the Arctic).
Arctic ravens are scavengers in the Arctic because they feed on the carcasses of dead animals and other dead organisms. This helps to break down the carbons in decaying organic materials to help bring the organic materials back into the carbon cycle. Arctic ravens re able to survive in the cold temperatures of the Tundra because their black coloring allows them to absorb heat. In the cold of the winters. they follow around polar bears to eat the remains of anything the polar bear leaves behind.