Creature Feature: Freshwater Algae

by Jason Lopiccolo

Freshwater algae gets a bad rap. It can be slimy, unsightly, and heck… sometimes even toxic. During the summer months where the temperatures rise and water flow drops, we sometimes get cyanobacteria blooms in our local freshwater bodies. The cyanobacteria (sometimes called blue-green algae) produce cyanotoxins that are normally not abundant enough to produce any ill effects, but when a bloom happens it can make swimming in and drinking water from one of these bodies of water a risky proposition. Microcystis sp. is one of the most abundant cyanobacterial species responsible for these harmful algal blooms (HABs). While it’s a very small celled alga, they clump together in mucus and grow and grow in population.

microcystis Sergei Shalygin

The thing is, freshwater algae are so cool! I don’t want to downplay the magnificence of seaweeds and other macroalgae, but as far as visual stunners are concerned… freshwater algae are some of the tops. Probably some of the showiest specimens come from the Chlorophyta (the green algae). Problem is, they’re tiny and you need a microscope to really appreciate them. Lucky for you, our plucky reader, I spend hours and hours under a microscope looking at piles of freshwater muck for algae.

Spirogyra, as its name suggests, has beautiful coiling chloroplasts running through its cells. Chloroplasts are organelles within a cell where photosynthetic pigments are held. These are punctuated by clearish round structures called pyrenoids that are the site of carbon fixation. Spirogyra can be found in ponds, gutters, and slow-moving streams. 



Another alga in the same group as Spirogyra is Zygnema. Where the former has spiral chloroplasts, the latter has a pair of starbursts. It can be found in a variety of freshwater habitats around the world.



An interesting group of freshwater green algae are the coenobial (a colony whose shape and number of cells are genetically fixed) colonies such as Pediastrum. The colony is formed of many cells that create a gorgeous spiked star. The curious thing is that each of the cells, when undergoing asexual reproduction, will produce another colony with the same shape and number of cells as the original. The new colony bursts forth from the cell as a miniature facsimile of the mother colony and will then balloon up to the normal size. Pediastrum can be found floating in the freshwater plankton where its numerous projections help to keep it from sinking. 


Volvox whole colony at 100x magnification
Volvox daughter colony







Surely one of the most mesmerizing of the green algae are the colonies of Volvox. These green orbs will dance around the water under a microscope with all of their many flagella beating in concert. An interesting fact about Volvox is that it is a model organism for research on the evolution of multicellularity. The sphere of cells will begin growing folded in pockets of daughter colonies within itself that eventually evert and separate from the mother colony. Volvox is widely distributed in freshwater environments.


So next time you see a little green pile of muck down in a gutter, remember that there’s a whole world of delightful, myriad, little algae in there. One need only look under a microscope.

You can find out more about cyanobacterial blooms here ( and check the current status of harmful algal blooms (HABs) in California here (