PO4 is a compund that hogs up the better part of an organic chemestry class right next to why carbon is what it is and why it is essential to all life, but long story short yes algae, plants and other eukarya (includeing us) use it in a number of ways. Everything from makeing ATP/ADP (cellular energy) DNA/RNA etc etc. So keep those PO4 levels down or your bound to have some alage problems eh!