What are Algae?

People are talking a lot about "algae". We can make biofuel from algae that are grown on waste water while remediating carbon dioxide - and we can do this without using precious farmland that is needed to grow food. "Algae" are the wonder critters. But what do people mean when they say "algae". As a scientist who works on these things (and loved them even before they were going to save the world), I wince at the broad use of a term like "algae" which is actually representing a huge group of organisms that can be much more different from each other than you are to a mushroom.

What unites things that are called algae is photosynthesis AND the fact that they are not land plants. Some algae - the green algae - gave rise to land plants about 400 million years ago, so they are all in the same family. But other algae, like diatoms, one of the most important photosynthesizers in the ocean, are not at all related to plants. The fact that diatoms do photosynthesis in a similar way to plants is a more a matter of history than of family. So why do these groups all get lumped together into one group that is ostensibly taxonomically useless? Well, that's historical too, resulting from the days when we saw the world as made of up plants, animals, and others. Now, with a century of experience behind us and armed with DNA technology, we now know how diverse the group of organisms called "algae" really is.