Please find below a file and link. May be of useful. Further, you can also try Standard Methods for the Examination of Water and Wastewater-APHA Book which has some images of algal species in the last few pages.
My samples are always fixed. They belongs to the photic zone of reservoirs and they are for finding the ecological status of the water. (i am only interesting in phytoplancton: biovolum, density, % cyano and algae groups)
In this case because my "problematic organism" is big, means a big biovolum that i need to count (if is an algae) if no,i dont need.
If it was an algae i would thinking in , perhaps, crisophyceae.
I took a nest living sample of the water colum, but there i had'nt find those organims ( I saw Mallomonas, and i was thinking in this possibility, but i am not sure)
The shape, size and sculpture of the wall does show affinity with the tests of Thecamoeba (?Paulinella), except for the green colored endosymbiotic inclusion bodies resembling blue-green algal cells. Scanning electron microscopy would perhaps help.