a large and diverse group of procaryotic and eucaryotic organisms that have in common the production of energy by photosynthesis. They range in size from microscopic to several metres-most are free living, but some are symbiotic. Most live in aquatic environments or in moist environments on land-though some blue green "algae" (actually bacteria) can withstand dessication for decades, while retaining the ability to germinate. It is now recognized that very closely related organisms can be photosynthetic or not, and thus the "algae" are now considered to be an artificial group.

Alternative forms for alga : algae, algal, algoid.