I’m entering new worldbuilding territory here, because up until now I’ve not dealt with any form of alien life. Even in Shayla’s world, they never encountered another inhabited planet. Everything there has its origins on Earth (even though they’ve long ago forgotten that Earth even existed.)
In The Long Dark, the planet Elysium is girdled by a single plant-like superorganism. The locals call it Sponge.
Like Earthly life, Sponge is based on carbon chemistry, with water as the medium, and an oxygen/nitrogen atmosphere. During the summer season, Sponge photosynthesizes for energy.
But similarities end there. Sponge is not based on cellular structures like our plants and animals, and it has no DNA. I don’t get into these kinds of details in the story, but my basis is that there are other mechanisms possible for self-organization and replication.
Something I am fleshing out, though, is some insight into the inner structure of Sponge as that is important to the story. The plant mass forms a carpet hundreds of meters thick - kilometers in places - that covers the equatorial to mid latitudes, ending about 40 degrees north and south. Given the extreme conditions at the poles, I figured this was a reasonable extent for life to persist.
Map of Elysium, showing the active growth zones north and south,
and the main north-south systems of veins
The name “Sponge” conjures images of a fairly amorphous uniform spongy mass. In the story I try to dispel that with images of specific and detailed structure at all kinds of scales from the microscopic to the gargantuan. There are vast structural ribs holding the plant’s shape, and complex networks of veins, aquifers, and chambers to convey liquid, nutrients, and air to where it’s needed. The mass is riddled with chambers, deep chasms, and vast hollows that would easily swallow a town, along with a variety of specialized structures that make up the living portions of the plant. It is these depths that the colonists harvest for a living.