No one knows what it is and they are calling it “protozoa” for now.
From “Rare Protozoan from Sludge in Norwegian Lake Does Not Fit On Main Branches of Tree of Life” (ScienceDaily, Apr. 26, 2012), we learn of a micro-organism from south-Norway, one of the smallest known species, that “may provide an insight into what life looked like on earth almost one thousand million years ago”:
When researchers from the University of Oslo, Norway compared its genes with all other known species in the world, they saw that the protozoan did not fit on any of the main branches of the tree of life. The protozoan is not a fungus, alga, parasite, plant or animal.
There are not many of them. And the University of Oslo biologists have not found them anywhere else other than in this lake.
“We are surprised. Enormous quantities of environmental samples are taken all over the world. We have searched for the species in every existing DNA database, but have only found a partial match with a gene sequence in Tibet. So it is conceivable that only a few other species exist in this family branch of the tree of life, which has survived all the many hundreds of millions of years since the eukaryote species appeared on Earth for the first time.”
Apparently, they eat both algae and each other.