Scientists at the University of California, Berkeley, have found another unusual creature lurking in the lake’s briny shallows. This creature—the size of a dust grain—is a choanoflagellate. It is a microscopic, single-celled form of life that can divide and develop into multicellular colonies.
It’s not an animal, but it’s closely related to them. The choanoflagellate helps us understand how single-celled organisms evolved into multicellular ones. It’s special because it has a stable relationship with bacteria, making it one of the simplest organisms with its microbiome.
Nicole King, a UC Berkeley professor of molecular and cell biology and a Howard Hughes Medical Institute (HHMI) investigator, said, “Very little is known about choanoflagellates, and there are interesting biological phenomena that we can only gain insight into if we understand their ecology.”
Choanoflagellates are visible only through a microscope. Their biology and lifestyle can offer important insight into creatures that existed in the oceans before animals evolved and that eventually gave rise to animals.
This species might help us understand how interactions between animals and bacteria began, eventually leading to the development of the human microbiome.
“Animals evolved in oceans filled with bacteria,” King said. “If you think about the tree of life, all organisms that are alive now are related to each other through evolutionary time. So if we study organisms that are alive today, we can reconstruct what happened in the past.”
About a decade ago, then-UC Berkeley graduate student Daniel Richter went on a trip and collected a vial of Mono Lake water. Under the microscope, it was alive with choanoflagellates. Besides brine shrimp, alkali flies, and different types of nematodes, very few other organisms have been found in the lake’s harsh waters.
The colonies, comprised of about 100 identical choanoflagellate cells, formed a hollow sphere that spun and twirled as each cell moved its flagella. One interesting thing about them is that they have a shape similar to the blastula, a hollow ball of cells that forms early in animal development.
Later, some students revived and stained the choanoflagellates to examine their unique, doughnut-shaped chromosomes. To their surprise, they found DNA inside the hollow part of the colony where no cells should have been. Graduate student Kayley Hake later identified this DNA as coming from bacteria.
The team also detected connective structures, called the extracellular matrix, inside the spherical colony that was secreted by the chaos.
Scientists then realized that the DNA might not be from bacteria the choanoflagellates had eaten but from bacteria living and feeding on substances secreted by the colony.
King said, “No one had ever described a choanoflagellate with a stable physical interaction with bacteria. Our prior studies found that chaos responded to small bacterial molecules floating through the water or [that] the chaos was eating the bacteria. Still, there was no case where they were doing anything that could be a symbiosis. Or, in this case, a microbiome.”
King collaborated with Jill Banfield, an expert in metagenomics, to identify the bacterial species in Mono Lake water and inside the choanoflagellates. Metagenomics involves sequencing all the DNA in a sample to reconstruct the genomes of the organisms present.
Banfield’s lab identified the microbes in the lake water, and Hake used DNA probes to check which ones were also inside the choanoflagellates. The bacterial populations inside the choanoflagellates differed from those in the lake water, suggesting that some bacteria survive better in the low-oxygen environment of the choanoflagellate colony.
They found that these bacteria were not just passing through but were growing and dividing. They wondered if they were escaping the lake’s harsh conditions or if the choanoflagellates were cultivating them for food.
Future experiments should uncover how the bacteria interact with the choanoflagellates. Past work in her lab has already shown that bacteria act like an aphrodisiac to stimulate mating in choanoflagellates and that bacteria can stimulate single-celled choanos to aggregate into colonies.
“I think there’s a great deal more that needs to be done on the microbial life of Mono Lake, because it really underpins everything else about the ecosystem,” King said. “I’m excited about B. monosierra as a new model for studying interactions between eukaryotes and bacteria. And I hope it tells us something about evolution. But even if it doesn’t, I think it’s a fascinating phenomenon.”
Journal Reference:
- K. H. Hake, P. T. West, K. McDonald, D. Laundon, J. Reyes-Rivera et al. A large colonial choanoflagellate from Mono Lake harbors live bacteria. mBio. DOI: 10.1128/mbio.01623-24