Little Green Spheres

Photo One by By コムケ at Japanese Wikipedia, CC BY-SA 3.0,

Marimo at the bottom of Lake Akan in Japan (Photo One)

Dear Readers, a few weeks ago I mentioned that I was looking into the formation of algal balls for one of my Open University assignments, so today I thought I’d share a bit more about this strange phenomenon. Marimo are round balls of algae that form when a free-floating algae is gently rocked against a lake bottom, which turns them from a mass of threads into a rather delightful green sphere, as seen in the photo above. Japanese people seem to have a particular fondness for ‘cute’ things, and there is something rather endearing about these emerald balls – you could imagine them purring, much like the tribbles in Star Trek.

Photo Two from By Star Trek: The Original Series episode, "The Trouble with Tribbles", Fair use,

Captain James T. Kirk with a pile of tribbles (Photo Two)

Devotees of the TV series might remember that the problem with the tribbles was that they bred with great enthusiasm. Marimo, on the other hand, tend to breed very slowly, living as the algae does in cold shallow lakes not only in Japan, but also in Iceland in Lake Myvatn. The big threat to the ‘ball’ form of the algae seems to be excessive nutrients in the water – the marimo used to be found in lakes throughout Northern Europe, but these days although the alga that forms the balls still exists, it is more often found attached to rocks. I am not quite clear why this might be, but one theory is that the excess nutrients cause an increase in sedimentation at the bottom of the lake – if the algae has nothing to roll against, it won’t form into balls. Plus, too many nutrients may encourage the growth of algae that do not lend themselves to spherification. Many of the lakes that used to have marimo, such as Lake Zell in Austria, have not had the ball form of the algae since 1910, and Lake Myvatn nearly lost all its marimo, though in 2014 the Icelandic government realised what was happened and organised a clean up. Since then, tiny marimo have started to form again.

Photo Three by By Pjt56 --- If you use the picture outside Wikipedia I would appreciate a short e-mail to or a message on my discussion page - Own work, CC BY-SA 4.0,

Lake Myvatn (PJT56 / Wikimedia Commons / CC BY-SA 4.0) (see Photo Three link below)

Very occasionally marimo are found in marine environments – in cases such as this, it might be tidal action that shapes a free-floating algae into green ‘eggs’, such as the ones that washed up on a beach in Sydney in 2014, and again in 2017.

Photo Four from

Green algal balls on Dee Why Beach in Sydney

In this case, what seems to have happened is that the algae was forming dense mats in a lagoon just behind the beach, when there was a period of heavy rainfall. This breached the entrance of the lagoon and the algae was washed out to sea, only to be washed back in when the tide turned. Each turn of the tide (and it was a particularly heavy swell) appears to have turned the threads of algae until they formed a ball. The tides then started to deposit the strange alien spheres onto the beach, much to the fascination of local people and scientists alike. One of the scientists, Julia Cooke, came up with the first real hypothesis, outlined above,  for how the ‘green beachballs’ ended up on the beach.

And so, what can look like a mass of unsightly green ‘stuff’ on the surface of a lake can, in the right circumstances, turn into a tidy little sphere instead. If only I could find a way to persuade my duck weed to do the same thing.

Photo Five from by Ragnar Sigurdsson (, Iceland

Algal balls from Lake Myvatn (Photo Five)

Photo Credits

Photo One by By コムケ at Japanese Wikipedia, CC BY-SA 3.0,

Photo Two from By Star Trek: The Original Series episode, “The Trouble with Tribbles”, Fair use,

Photo Three By Pjt56 — – Own work, CC BY-SA 4.0,

Photo Four from

Photo Five from by Ragnar Sigurdsson (, Iceland

3 thoughts on “Little Green Spheres

  1. Ann Bronkhorst

    On a scruffy beach on the south- east coast of Sicily we found masses of these balls washed up and dry and brown. Had no idea what they were.


Leave a Reply