Mono Lake in the eastern Sierra Nevada is known for its towering tuff formations, abundant brine shrimp and black cloud alkali flies that are uniquely adapted to the salty, arsenic and cyanide-rich water.
Researchers from the University of California, Berkeley have now found another unusual creature lurking in the salty shallow waters of the lake – one that could tell scientists about the origins of animals more than 650 million years ago.
The organism is a choanoflagellate, a microscopic, single-celled life form that can divide and develop into multicellular colonies in a manner similar to the way animal embryos form. However, it is not a species of animal, but a member of a sister group of all animals. And as the closest living relative of animals, the choanoflagellate is a crucial model for the leap from single-celled to multicellular life.
Surprisingly, it harbors its own microbiome, making it the first choanoflagellate known to establish a stable physical relationship with bacteria, rather than just eating them. As such, it is one of the simplest organisms known to have a microbiome.
“Very little is known about choanoflagellates, and there are interesting biological phenomena that we can only gain insight into if we understand their ecology,” says Nicole King, professor of molecular and cell biology at UC Berkeley and a Howard Hughes Medical Institute (HHMI) . researcher who studies choanoflagellates as a model for what early life was like in the ancient oceans.
Choanoflagellates are typically only visible through a microscope, but are often ignored by aquatic biologists, who focus instead on macroscopic animals, photosynthetic algae, or bacteria. But their biology and lifestyle may provide insight into creatures that existed in the oceans before animals evolved and from which animals eventually evolved. This species in particular could shed light on the origins of the animal-bacteria interactions that led to the human microbiome.
“Animals evolved in oceans that were filled with bacteria,” King said. “If you think about the tree of life, all the organisms that are alive today are connected through evolutionary time. So if we study organisms that are alive today, we can reconstruct what happened in the past.”
King and her colleagues at UC Berkeley described the organism — and gave it a name Barroeca monosierra, after the lake – in an article published online in the magazine on August 14 mBio.
A beautiful colony
Nearly a decade ago, then-UC Berkeley student Daniel Richter returned from a climbing trip in the eastern Sierra Nevada with a bottle of Mono Lake water he had casually collected along the way. Under the microscope it lived with choanoflagellates. Apart from brine shrimp, alkali flies and several species of nematodes, few other life forms are said to inhabit the inhospitable waters of the lake.
“It was just full of these big, beautiful colonies of choanoflagellates,” King said. “I mean, they were the biggest we’d ever seen.”
The colonies of what appeared to be nearly a hundred identical choanoflagellate cells formed a hollow sphere that rotated as each individual cell kicked its flagella.
“One of the things that’s interesting about them is that these colonies have a shape similar to the blastula — a hollow ball of cells that forms early in animal development,” King said. “We wanted to know more about it.”
At the time, however, King was working on other types of choanos, as she calls them, so the Mono Lake choanos languished in the freezer until some students revived them and stained them to look at their unusual, donut-shaped chromosomes. Surprisingly, there was also DNA in the hollow colony where no cells should have been. After further investigation, graduate student Kayley Hake determined it was bacteria.
“The bacteria were a big surprise, that was just fascinating,” King said.
Hake also discovered connecting structures called the extracellular matrix in the spherical colony that were secreted by the choanos.
Only then did it dawn on Hake and King that these might not be the remains of bacteria that the choanos ate, but bacteria that lived and grazed on stuff secreted by the colony.
“No one had ever described a choanoflagellate with a stable physical interaction with bacteria,” she said. “In our previous studies, we found that choanos responded to small bacterial molecules floating through the water [that] the choanos ate the bacteria, but there was no instance where they did anything that could possibly be a symbiosis. Or in this case, a microbiome.”
King worked with Jill Banfield, a pioneer of metagenomics and professor of environmental science, policy and management, and earth and planetary sciences at UC Berkeley, to determine which bacterial species were in the water and in the choanos. Metagenomics involves sequencing all the DNA in an environmental sample to reconstruct the genomes of the organisms that live there.
After Banfield’s lab identified the microbes in the Mono Lake water, Hake created DNA probes to determine which ones were also in the choanos. The bacterial populations were not identical, King said, so apparently some bacteria survive better than others in the oxygen-poor lumen of the choanoflagellate colony. Hake determined that they were not there by accident; they grew and divided. Perhaps they escaped the lake’s toxic environment, King mused, or perhaps the choanos grew the bacteria to eat them.
Much of this is speculation, she admits. Future experiments should show how the bacteria interact with the choanoflagellates. Previous work in her lab has shown that bacteria act as an aphrodisiac to stimulate mating in choanoflagellates, and that bacteria can stimulate single-celled choanos to aggregate into colonies.
For her, the Mono Lake choanoflagellate will become a model system in which she can study evolution, much like the choanos that live in plunge pools on the island of Curacao in the Caribbean – her main focus at the moment – and the choanos in pools. at the North and South Poles. However, getting more samples from Mono Lake can be a challenge. On a recent visit, only six out of a hundred samples contained these energetic microorganisms.
“I think there’s a lot more work to be done on Mono Lake’s microbial life, because it really supports everything else about the ecosystem,” King said. “I’m excited about it 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 that’s not the case, I find it a fascinating phenomenon.”
In addition to King, Banfield, Hake and Richter, UC Berkeley co-authors on the paper include former doctoral student Patrick West, electron microscopist Kent McDonald and postdoctoral fellows Josean Reyes-Rivera and Alain Garcia De Las Bayonas. The work is supported by HHMI and the National Science Foundation.
