What is Algae?

What is algae?

This seems like a very simple and easy to answer question. Surprisingly though, many people (even those doing research on specific groups of algae like me), have a tough time answering this question. Mainly, I think this is because we rarely get this question from other people (scientists or laypeople). Also, since science often encourages you to focus your research questions, we rarely have to even think about how a specific group of algae is related to other groups of algae, or how algae fits into the evolutionary scheme of other living organisms (are algae plants or animals?...).

Anabaena, a common genus of cyanobacteria (aka "blue-green algae").
Previously, I blogged about diatoms that live in periphyton in the Everglades, and what I do to periphyton to remove all the other algae that obscure the diatoms I want to study. So far, I haven't explained what algae is at all, and only explained that diatoms are a special group of algae with cell walls made of glass. This summer, I took a Freshwater Algae course at Iowa Lakeside Lab and filled in the gaps of my knowledge about non-diatom algae.

So without further ado, here is the definition of algae:
1. They must have a primarily aquatic life.
2. They must have chlorophyll a and the ability to perform photosynthesis.
3. They must have (relatively) simple reproduction.

That's it.

Now here is the longer version of the answer. Algae is an unnatural group, meaning they do not form a single evolutionary lineage, in which they are related to a common ancestor. Algae includes both prokaryotic cyanobacteria (i.e., blue-green algae) and eukaryotic protists (which are neither plants nor animals!).

Without algae, there would not be life on Earth as we know it. It was because of the photosynthetic activity of cyanobacteria four billion years ago that the Earth's atmosphere built up enough oxygen for aerobic organisms to evolve. Cyanobacteria were the first organisms to contain chlorophyll a, among other pigments, which absorbs sunlight to produce energy, and release oxygen as a waste product.
Graham et al. 2006 Plant Biology

Then, about two billion years ago, a heterotrophic eukaryote swallowed a cyanobacteria and instead of digesting it, obtained a new organelle: the chloroplast. This event is called endosymbiosis, and the evidence is found in the double membrane around the chloroplast in green and red algae. The chloroplast gave the eukaryote the ability to produce its own energy via photosynthesis.

Endosymbiosis: how a eukaryotic cell obtained a chloroplast and the ability to perform photosynthesis

Among the eukaryotes, there are many groups that are considered algae. Actually, the endosymbiotic event happened more than once (secondary and tertiary endosymbiosis) by swallowing up either a green algae, red algae, or even a diatom. That means some algae have three (euglenas, dinoflagellates), four (diatoms, chrysophytes, cryptophytes, brown algae, etc.), or even five (some dinoflagellates) chloroplast membranes!! Some algae can live without sunlight as heterotrophs. Some algae have flagella. Some have cell walls made of glass, rather than cellulose. Some have additional pigments like carotenoids or phycobilins to give them a more orange or brown or reddish color. Accessory pigments help algae absorb light at additional wavelengths to do photosynthesis in dimmer light (such as in deeper depths underwater) and also provides photoprotection in excess light conditions (such as in the intense sunlight of south Florida).

Graham et al. 2006 Plant Biology
Green algae with flagella from left to right: Pandorina, Lepocinclis, and Euglena.
Filamentous green algae from left to right: Mougeotia, Stigeoclonium, and Hydrodictyon.
Diatoms from left to right: a single cell Craticula, a stellate colony of Asterionella, and tube colony of Encyonema.

Both cyanobacteria and eukaryotic algae exhibit an astounding level of diversity in morphology, function, and life history. A conservative estimate of the number of extant species is over a million. Some can produce large blooms with dangerous toxins, which has resulted in bad PR for algae. But algae are beneficial to humans in many ways. To name only a few, algae can be used to remove excess nutrients from wastewater, used to make food products (what is sushi without the red algae used to make nori?), used in the production of antibiotic and anti-cancer pharmaceuticals, and are being used in the development of biofuels. Algae provide food for higher trophic levels like invertebrates that are eaten by small fish, which are in turn eaten by bigger fish.

Algae are important research and environmental monitoring tools. The glass cell walls of diatoms leave permanent records in sediments that paleolimnologists use to understand long-term climate and land use patterns. Algae also play an important role in taking up carbon dioxide from the atmosphere and carrying it down into the depths of the oceans. As sea surface temperatures rise, there is less mixing of the water column, which means less nutrients for algae, and fewer algal cells (especially larger and heavier algae) to sequester carbon dioxide from the atmosphere. Because algae are first responders to environmental change, we really need to learn more about them and pay attention to what they indicate about the future.

If nothing else, there is one reason why we should all be thankful for algae every day: every second breath you take is because of ALGAE.


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