For question about algae. Phycology or Algology is discussion about algae
Algae is a heterogenous (polyphyletic) group of primitive-type organisms. It is a set of all primitive-type organisms that can perform oxygenic photosynthesis.
This group includes cyanobacteria and certain group of eukaryotes. The primitive body may be unicellular to few-cellular colonies to large complex plant-like structures such as a sort of brown-algae (kelps).
However among different subsets under algae, there are members that can't do photosynthesis (sometimes called leukophytes), and even parasitic forms found.