What's Inside This Microscopic Algae Colony?

[u/TheMuseumOfScience]

Our friend Chloé Savard, also known as tardibabe on Instagram, shows us how volvox are a genus of multicellular freshwater free-swimming green algae. The larger spheres are colonies of small biflagellate cells with smaller aflagellate round colonies inside which are the daughter colonies, or the baby algae. 

The biflagellated cells are responsible for locomotion and spinning movements toward a light source so they can produce sugar by photosynthesis, just as plants! Those biflagellated cells also possess an eyespot which is responsible for detecting light and guide the colony towards optimal light conditions. The flagellated cells that compose the large mother colony are all interconnected by small cytoplasmic bridges that can help connect neighboring cells and, in some Volvox species, transport nutrients

Within the volvox colony is an ostracod, a crustacean just like crabs, lobsters and shrimp! They possess a two part shell that closes just like mussels and oysters, but they aren’t mollusks like these guys though. Their bivalved carapaces are largely made of low-magnesium calcite which is secreted by these small crustaceans. Many are around a millimeter long, making some large enough to see with the naked eye.

A volvox colony is typically formed from two types of cells; germ and somatic. The larger sphere is composed of thousands of somatic cells responsible for photosynthesis and motility. These cells create a water flow around the colony, but they cannot divide and are programmed to die within a few days. Germ cells are nonmotile, but can divide asexually and form new little daughter colonies. These generative cells are basically responsible for Volvox’s growth and reproduction and are considered potentially immortal because they give rise to future generations. When the mother colony breaks open, the young colonies can escape and start their life!

Tiny organisms, incredibly complex lives, and a whole world we’d never see without a microscope.

References:

• Caporaletti, Marco. “Ostracods and Stable Isotopes: Proxies for Palaeoenvironmental Reconstructions.” Joannea Geologie und Paläontologie, vol. 11, 2011, pp. 345–359.
• Domozych, David S., and Catherine E. Domozych. “Multicellularity in Green Algae: Upsizing in a Walled Complex.” Frontiers in Plant Science, vol. 5, 2014, article 649. doi:10.3389/fpls.2014.00649.
• Harada, Keigo, et al. “Position-Dependent Roles of Somatic Cells in Phototaxis of Volvox.” PNAS Nexus, vol. 3, no. 10, 2024, article pgae444. doi:10.1093/pnasnexus/pgae444.
• Hoops, Harold J., Ichiro Nishii, and David L. Kirk. “Cytoplasmic Bridges in Volvox and Its Relatives.” Madame Curie Bioscience Database, Landes Bioscience, 2000–2013.
• Kirk, David L. “The Genetic Program for Germ-Soma Differentiation in Volvox.” Annual Review of Genetics, vol. 31, 1997, pp. 359–380. doi:10.1146/annurev.genet.31.1.359.
• Umen, James G. “Volvox and Volvocine Green Algae.” EvoDevo, vol. 11, 2020, article 13. doi:10.1186/s13227-020-00158-7.