Treebeardus Fangorn is a species of big eukaryotes that inhabit the Estuary biome. After the glucose disappearance event, they evolved to became photoautotrophs, relying on light for ATP production. As light constitutes - at least for the moment- a uniform, constant and infinite energy source, they can easily cover all of their energetic needs out of it. Furthermore, they are capable of producing ammonia -to support their own growth- from the dissolved nitrogen in the surrounding environment. Due to their considerable size, their biome can only support a limited number of them.
In order to reproduce, the Treebeardus Fangorn need to slowly navigate -equipped with two flagella- the environment to find and collect scattered clouds of phosphate. During those long voyages across the Estuary, they sometimes meet with potential predators, eager to take over their glucose and ammonia. The rigid double membrane provides them with a relatively good protection, while still keeping the ability to engulf. Indeed, even though the Treebeardus Fangorns are not especially aggressive by nature, they are prepared to engulf and end with any predator that insists too much on hurting them.
However, in some occasions, if attacked by a numerous swarm of fast, agressive, toxin-shooting bacteria, a solitary Treebeardus Fangorn individual can get overwhelmed and perish. That is why, as a sort of evolutionary countermeasure, they evolved, for the first time on the planet, the capability to segregate a special agent that allows them to form colonies. Once a colony of a few Treebeardus Fangorns is formed, they are practically unstoppable, a true force of nature that thrives leaving all competition behind.
However, without any doubt, it is only a matter of time that other species develop into colonies and even multicellular organisms. The evolutionary arms race on the planet has just started, and Treebeardus Fangorn is just a small and temporary link of it.