Because the term algae itself comprises of eukaryote algae and cyanobacteria, which are prokaryotes.
We have microalgae and also macroalgae.
It took me quite some times to familiarize myself with those terms.
Macroalgae are basically seaweeds.
Microalgae on the other hand can be divide into 2 categories:
- Eukaryotic microalgae
Because algae has 2 different kingdoms in it, this is where the diversity lies.
For me its like comparing human with ferns, well in terms of genetic.
Say if you have this unknown species, and you don't know whether is it a eukaryote or prokaryote (cyanobacteria), what should you do? I mean of course you would like to know the species or the genus name. You could go for 18S rDNA or 16S rDNA. Why? Because 18S rDNA is for the identification of eukaryotic cells, while 16S rDNA is the identification for prokaryotes. they are homolog.
Oh, by the way, algae can actually live anywhere but most of their members are aquatic majoring with marine algae and freshwater algae.
Why am I writing this?
Just to clear some thoughts.
This is how inorganize my mind is right now.