Creature the size of a dust grain found hiding in California's Mono Lake - Berkeley News
Mono Lake, located east of the Sierra Nevada, just outside Yosemite National Park. Dotted with tufa formations, the lake’s salty water is laced with arsenic and cyanide, but is home to unique flies and brine shrimp as well as choanoflagellates.
Nicole King.
What are choanoflagellates and how do they relate to other organisms? Choanoflagellates are a group of free-living, single-celled organisms that are considered to be the closest living relatives of animals (metazoans). They are part of the clade Opisthokonta, which also includes fungi, animals, and some protists. Here’s a closer look at what choanoflagellates are and their relationship to other organisms:And, being unaware of the history of life on a planet that they thought was only a few thousand years old and had been created out of nothing by magic with all the animals and plants fully formed, they could never in a million years, have guessed how multicellular animals evolved several hundred million years ago out of colonies of single-celled organisms.
Characteristics of Choanoflagellates
- Structure: Choanoflagellates are characterized by a distinctive cell structure. Each cell has a single flagellum, which is a whip-like structure used for movement, surrounded by a collar of microvilli (tiny, finger-like projections). The flagellum beats, drawing water through the collar, which traps bacteria and other small particles for the choanoflagellate to feed on.
- Habitat: These organisms are commonly found in aquatic environments, including both freshwater and marine ecosystems.
- Lifestyle: Choanoflagellates can exist as single cells, but some species form colonies. Their colonial forms are of particular interest because they offer insights into the possible evolutionary steps that led to multicellularity in animals.
Relationship to Other Organisms
- Closest Relatives to Animals: Genetic and molecular studies have shown that choanoflagellates are the closest living relatives to animals. This means that they share a common ancestor with animals, which is believed to have lived over 600 million years ago.
- Evolutionary Significance: The relationship between choanoflagellates and animals is crucial for understanding the origin of multicellularity. It is thought that the transition from single-celled to multicellular organisms may have occurred through mechanisms similar to those seen in choanoflagellate colonies. The study of choanoflagellates helps scientists understand the early evolution of animals and the development of complex tissues and organs.
- Shared Genes: Choanoflagellates possess many genes that are also found in animals, including those involved in cell adhesion, signaling, and extracellular matrix formation. These genes are essential for the development and maintenance of multicellular structures in animals, suggesting that the genetic toolkit required for multicellularity was already present in the last common ancestor of choanoflagellates and animals.
In summary, choanoflagellates are not only fascinating in their own right as unique protists, but they also play a key role in understanding the evolutionary steps that led to the rise of the animal kingdom.
And, of course, it's the yawning chasm between what the Bible's authors believed and what we now know, that tells us the Bible can't be the work of an omniscient creator god.
But of course, there are still a few (shrinking) gaps in what we know too. For example, although we have the genetic and structural evidence that multicellular organisms evolved out of single-celled organisms, we don't yet know the precise details, which is why a team of researchers from University of California, Berkeley have been studying a little-known choanoflagellates found in Mono Lake, near the Sierra Nevada, California. choanoflagellates are known to form colonies of organisms so can help shed some light on how related organisms originally got together to form simple multicellular organisms.
An additional discovery of this research was how these colonies incorporate a bacterial biome in a process analogous to the way most multicellular animals have associated biomes like out gut biome, for example.
The Berkely team's work is the subject of an open access paper in the American Society for Microbiology (ASM) journal mBio. It is also explained in a University of California Berkely news release:
Creature the size of a dust grain found hiding in California’s Mono Lake
Colonies of these choanoflagellates — members of a group considered to be the closest living relatives of all animals — have their own unique microbiomes.
Mono Lake in the Eastern Sierra Nevada is known for its towering tufa formations, abundant brine shrimp and black clouds of alkali flies uniquely adapted to the salty, arsenic- and cyanide-laced water.
University of California, Berkeley, researchers have now found another unusual creature lurking in the lake’s briny shallows — one that could tell scientists about the origin of animals more than 650 million years ago.
The organism is a choanoflagellate, a microscopic, single-celled form of life that can divide and develop into multicellular colonies in a way that’s similar to how animal embryos form. It’s not a type of animal, however, but a member of a sister group to all animals. And as animals’ closest living relative, the choanoflagellate is a crucial model for the leap from one-celled to multicellular life.
Surprisingly, it harbors its own microbiome, making it the first choanoflagellate known to establish a stable physical relationship with bacteria, instead of solely eating them. As such, it’s 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.
Professor Nicole King, senior author
Professor of molecular and cell biology
Howard Hughes Medical Institute and Department of Molecular and Cell Biology
University of California, Berkeley, California, USA.
Globular colonies of the choanoflagellate B. monosierra seen under a microscope. As indicated by the 50-micron scale bar, these colonies are at the limit of what’s visible to the naked eye.Alain Garcia De Las Bayonas, Nicole King lab.
Typically visible only through a microscope, choanoflagellates are often ignored by aquatic biologists, who instead focus on macroscopic animals, photosynthetic algae or bacteria. But their biology and lifestyle can give insight into creatures that existed in the oceans before animals evolved and that eventually gave rise to animals. This species in particular could shed light on the origin of interactions between animals and bacteria that led to the human microbiome.
Animals evolved in oceans that were filled with bacteria. 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, then we can reconstruct what happened in the past.
Professor Nicole King.
King and her UC Berkeley colleagues described the organism — which they named Barroeca monosierra, after the lake — in a paper published online Aug. 14 in the journal mBio.
A beautiful colony
Nearly 10 years ago, then-UC Berkeley graduate student Daniel Richter came back from a climbing trip in the Eastern Sierra Nevada with a vial of Mono Lake water he’d casually collected along the way. Under the microscope, it was alive with choanoflagellates. Aside from brine shrimp, alkali flies and various species of nematode, few other forms of life have been reported to live in the inhospitable waters of the lake.
The newly named species Barroeca monosierra discovered in Mono Lake. Colonies of these organisms consist of numerous identical cells (cyan), each with flagella (green) that allow them to propel themselves through the water. This choanoflagellate colony hosts its own microbiome (red), something never before seen in these organisms. The extracellular matrix with which the bacteria interact is shown in white.Video credit: Davis Laundon and Pawel Burkhardt, Sars Centre, Norway; Kent McDonald and Nicole King, UC Berkeley.It was just packed full of these big, beautiful colonies of choanoflagellates. I mean, they were the biggest ones we’d ever seen.
Professor Nicole King.
The colonies of what seemed to be close to 100 identical choanoflagellate cells formed a hollow sphere that twirled and spun 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. We wanted to learn more about it.
Professor Nicole King.
At the time, however, King was occupied with other species of choanos, as she calls them, so the Mono Lake choanos languished in the freezer until some students revived and stained them to look at their unusual, doughnut-shaped chromosomes. Surprisingly, there was also DNA inside the hollow colony where there should have been no cells. After further investigation, graduate student Kayley Hake determined that they were bacteria.
A colony of choanoflagellates stained to show its features. Cyan indicates DNA — the doughnut-shaped DNA of the choanoflagellate cells and a cloud of bacterial DNA inside the colony — while flagella are white and microscopic hairs (villi) on each cell are red.Kayley Hake, Nicole King labThe bacteria were a huge surprise. That just was fascinating.
Professor Nicole King.
Hake also detected connective structures, called extracellular matrix, inside the spherical colony that were secreted by the choanos.
Only then did it occur to Hake and King that these might not be the remains of bacteria the choanos ate, but bacteria living and grazing on stuff secreted by the colony.
No one had ever described a choanoflagellate with a stable physical interaction with bacteria. In our prior studies, we found that choanos responded to small bacterial molecules that were floating through the water, or [that] the choanos were eating the bacteria, but there was no case where they were doing anything that could potentially be a symbiosis. Or in this case, a microbiome.
Professor Nicole King.
King teamed up with Jill Banfield, a pioneer in metagenomics and a UC Berkeley professor of environmental science, policy and management and of earth and planetary science, to determine which bacterial species were in the water and inside the choanos. Metagenomics involves sequencing all the DNA in an environmental sample to reconstruct the genomes of the organisms living there.
After Banfield’s lab identified the microbes in Mono Lake water, Hake created DNA probes to determine which ones were also inside the choanos. The bacterial populations were not identical, King said, so evidently some bacteria survive better than others inside the oxygen-starved lumen of the choanoflagellate colony. Hake determined that they were not there accidentally; they were growing and dividing. Perhaps they were escaping the toxic environment of the lake, King mused, or maybe the choanos were farming the bacteria to eat them.
Much of this is speculation, she admits. 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.
For her, the Mono Lake choanoflagellate will become another model system in which to study evolution, just like the choanos that live in splash pools on the island of Curaçao in the Caribbean — her main focus at the moment — and the choanos in pools at the North and South Poles. It might be a challenge to get more samples from Mono Lake, however. On a recent visit, only six of 100 samples contained these energetic microorganisms.
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,. 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.
Professor Nicole King.
In addition to King, Banfield, Hake and Richter, UC Berkeley co-authors of 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.
ABSTRACTThis discovery shows a plausible route to multicellularity from a related single-celled eukaryote organism and offers an explanation for the early stages of embryo development where a single-cell zygote gives rise early on to a hollow gastrula. This could have formed in an ancestral multicellular metazoan as somewhere for a symbiotic colony of bacteria to reside and provide nutrients in return for protection. It also offers a plausible explanation for our commensal, symbiotic microbiome in our gut which has its embryonic origins in the hollow gastrula.
As the closest living relatives of animals, choanoflagellates offer insights into the ancestry of animal cell physiology. Here, we report the isolation and characterization of a colonial choanoflagellate from Mono Lake, California. The choanoflagellate forms large spherical colonies that are an order of magnitude larger than those formed by the closely related choanoflagellate Salpingoeca rosetta. In cultures maintained in the laboratory, the lumen of the spherical colony is filled with a branched network of extracellular matrix and colonized by bacteria, including diverse Gammaproteobacteria and Alphaproteobacteria. We propose to erect Barroeca monosierra gen. nov., sp. nov. Hake, Burkhardt, Richter, and King to accommodate this extremophile choanoflagellate. The physical association between bacteria and B. monosierra in culture presents a new experimental model for investigating interactions among bacteria and eukaryotes. Future work will investigate the nature of these interactions in wild populations and the mechanisms underpinning the colonization of B. monosierra spheres by bacteria.
IMPORTANCE
The diversity of organisms that live in the extreme environment of Mono Lake (California, USA) is limited. We sought to investigate whether the closest living relatives of animals, the choanoflagellates, exist in Mono Lake, a hypersaline, alkaline, arsenic-rich environment. We repeatedly isolated members of a new species of choanoflagellate, which we have named Barroeca monosierra. Characterization of B. monosierra revealed that it forms large spherical colonies containing diverse co-isolated bacteria, providing an opportunity to investigate mechanisms underlying physical associations between eukaryotes and bacteria.
OBSERVATION
A newly identified choanoflagellate species forms large spherical colonies
Choanoflagellates are the closest living relatives of animals and, as such, provide insights into the origin of key features of animal biology (1, 2). Over five sampling trips to Mono Lake, California (Fig. 1A; Table S1), we collected single-celled choanoflagellates and large spherical choanoflagellate colonies, many of which were seemingly hollow (Fig. 1B). In colonies and single cells, each cell bore the diagnostic collar complex observed in other choanoflagellates: an apical flagellum surrounded by a collar of microvilli (1, 2). In the spherical colonies, each cell was oriented with the basal pole of the cell body facing inwards and the apical flagellum facing out (Fig. 1B). We know of no prior reports of choanoflagellates having been isolated and cultured from any hypersaline alkaline lake, including Mono Lake.Fig 1
Fig 1 A colonial choanoflagellate isolated from Mono Lake. (A) Choanoflagellates were collected from three sampling sites (asterisks) near the shore of Mono Lake, California (modified from a map in the public domain, formatted as USGS Imagery Only; https://www.usgs.gov/volcanoes/long-valley-caldera) (B) B. monosierra forms large colonies (differential interference contrast image). Scale bar = 50 µm. (C) B. monosierra (shown in bold) is a craspedid choanoflagellate closely related to S. rosetta and Microstomoeca roanoka. Phylogeny based on sequences of three genes: 18S rRNA, EFL, and HSP90. Metazoa (seven species) were collapsed to save space. Bayesian posterior probabilities are indicated above each internal branch, and maximum likelihood bootstrap values are below. (A “—” value indicates a bifurcation lacking support or not present in one of the two reconstructions.) Also see Fig. S1C for further phylogenetic analyses. (D and E) Two colonies from the ML2.1G culture (Fig. S1, Box2) reveal the extremes of the B. monosierra colony size range (D, 58 µm diameter; E, 19 µm diameter; scale bar = 20 µm). In B. monosierra colonies, each cell is oriented with its apical flagellum (white; labeled with anti-tubulin antibody) and the apical collar of microvilli (red; stained with phalloidin) pointing out. Nuclei (cyan) were visualized with the DNA-stain Hoechst 33342. (F) Colonies of B. monosierra span from 10 µm in diameter, a size comparable to that of small S. rosetta colonies, to 120 µm, over an order of magnitude larger. The diameters of B. monosierra and S. rosetta colonies were plotted as a violin plot; the median is indicated as a thick black line. Diameters of the colonies in panels D and E are indicated as colored bars behind the violin plot (D, red bar; E, blue bar).
Fig 2
Fig 2 Bacteria reside in the lumina of B. monosierra colonies. (A and A′) The center of a B. monosierra colony from culture ML2.1G (Fig. S1, Box 2), shown as a maximum intensity projection (A) and optical z-section (A′), contains DNA (revealed by Hoechst 33342 staining; cyan). Apical flagella were labeled with anti-tubulin antibody (white); microvilli were stained with phalloidin (red). Hoechst 33342 staining (cyan) revealed the spherical choanoflagellate nuclei along the colony perimeter and an amorphous cloud of DNA sitting within the central cavity formed by the monolayer of choanoflagellate cells. (B, B’’ and B′) A thin section through a B. monosierra colony, imaged by transmission electron microscopy (TEM), revealed the presence of small cells in the central cavity. (B′) Inset (box from panel B) reveals that the interior cells are each surrounded by a cell wall. (C, C’’, C’’’, and C″′′) The small cells inside B. monosierra colonies (grown from cultures ML2.1E/ML2.1EC) are bacteria, as revealed by hybridization with a broad-spectrum 16S rRNA probe (C, green) and a probe targeting Gammaproteobacteria (C′, red). Choanoflagellate nuclei and bacterial nucleoids were revealed by staining with Hoechst (C″, cyan). (C″′) Merge of panels C, C′, and C″. Scale bar for all = 5 µm. (D, D’ and D″) 3D reconstruction of a 70-cell B. monosierra colony from transmission electron micrographs of serial ultrathin sections revealed that the bacteria are closely associated with and wrapped around the extracellular matrix (ECM) inside the colony. (D) Whole colony view. (D′) Cut-away view of colony center. False colors indicate cell bodies (cyan), microvilli (orange), flagella (green), bacteria (red), ECM (white), intercellular bridges (yellow; see also Fig. S5), and filopodia (purple). (D″) Reducing the opacity of the choanoflagellate cell renderings revealed the presence of bacteria positioned between the lateral surfaces of choanoflagellate cells (brackets; see also Fig. S7). (E) Representative B. monosierra colony from an environmental sample shown as an average intensity projection (planes 17–27 from 1-µm optical sections). Choanoflagellate nuclei and bacterial nucleoids (examples indicated by arrowheads) were revealed by staining with Hoechst (cyan). To allow visualization of the much smaller bacterial nucleoids, the imaging of the choanoflagellate nuclei was saturated. Concanavalin A staining (magenta) revealed the branched extracellular matrix. (F and G) Optical sections 19 and 24, respectively, of the B. monosierra colony shown in panel E. (F′ and G′) Higher magnification Hoechst-stained DNA from the boxed regions in F and G, highlighting the resident bacteria (examples indicated by arrowheads). Scale bars in panels E–G = 10 μm.
Creationists might also like to ignore the fact that the research team believe they are explaining a fundamental step in the evolution of multicellularity 650 million years ago, and have found no need to abandon science and the scientific method and turn to creationism with its magic and unproven entities for a more rational explanation of the facts.
And of course, as with 99.9975% of all history, this stage in the evolution of multicellular life happened in that long, pre-'Creation Week', period of Earth's history.
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