Picozoans are algae after all: study
PIcozoa have puzzled scientists since their startling discovery nearly 15 years ago. These common and globally distributed microbes are little bigger than bacteria, but they belong to the same realm as animals, fungi, and plants, and everything, whatever they eat in their place in the tree of eukaryotic life, has proven difficult to pin down. down.
Now a preprint uploaded to bioRxiv on April 14, claims to have found these puzzled microbes as an evolutionary hotbed. But the article, which is currently undergoing peer review, suggests that picozoans are not surprising scientists. If the authors’ conclusions are correct, then these microbes are indeed algae, even though they seem to lack the most notable feature of the group: plastids, a group of organelles that includes chloroplasts.
Small organizations, big surprises
The 2007 Science paper announcing the discovery of this group of plankton shook the field of microbial ecology. “I still remember exactly where I was when I read the first description of these organisms,” says Fabien Burki, molecular protistologist at Uppsala University in Sweden and lead author of the recent pre-print. At the time, he was a doctoral student working on solving the overall structure of the eukaryotic tree of life. Suddenly there was an entirely new and strange branch to consider.
Initially, the authors of the 2007 study, led by Linda Medlin of the Alfred Wegener Institute for Polar and Marine Research, called the organisms picobiliphytes because they appeared to be small algae photosynthesized using pigments of phycobilin, just like some red algae. A hitherto undocumented alga on its own was surprising, given that algae are arguably the most studied planktonic group, says Burki; their colors have made them popular research subjects for hundreds of years. No one thought there were major lineages of algae awaiting discovery in the 21st century, he says. Yet there, these organisms were overlooked as they slipped through the three-micron filters often used to separate larger eukaryotic microbes such as algae and zooplankton from bacteria.
Once they knew what to look for, the researchers detected these microbes in water samples from disparate locations and in surface and deep ocean waters, suggesting they are everywhere.
In 2011, Michael Sieracki, a microbial oceanographer who was at the time at the Bigelow Laboratory for Ocean Sciences in Maine, and his colleagues discovered that ribosomal gene sequences did not conclusively place picozoans with other algae, and single-cell genomics could not detect photosynthetic genes. Confirming the idea that microbes are heterotrophs, a transient culture of a species by the Medlin team in 2013 allowed detailed microscopy, which conclusively showed that these plankton are consumers that do not contain chloroplasts or plastids of any kind. Thus, the term Picozoa – pico for their small size and zoa for their animal lifestyle of eating other organisms – was proposed and adopted. What they eat is still under investigation, although a 2020 Frontiers in microbiology A study by Sieracki and colleagues suggests that viruses can encompass much of their diet.
Still, the suffix ‘phytes’ denoting the plantation might actually be more appropriate, as Burki’s genomic data suggests that it is a sister group of red algae – and that it is found in the middle of the Archaeplastida, the taxonomic “supergroup” of algae and plants.
Assemble the picozoan puzzle
Another scanning electron micrograph of Picomonas judraskeda
There are no high quality genomic sequences for picozoans. Because no long-term cultures of the organisms have been established, it has been fundamentally impossible to obtain the volume of pure cells needed for deep sequencing, says Burki. So he and his colleagues turned to single-cell genomics, obtaining 11 genomes from single cells and six genomes assembled from multiple cells. When compared with established microbial algal and eukaryotic genomes, the Picozoa were clearly grouped with the archeplastids. In fact, according to phylogenetic analyzes, they separated from their algal parents shortly after the ancestor of all glaucocystid algae (Glaucophyta; a group of unicellular freshwater algae) and green algae and plants terrestrial (Chloroplastida) have done so.
Karolina Fučíková, phycologist at Assumption University of Massachusetts, tells The scientist The group’s phylogenomic analyzes are “robust” and “sophisticated,” but she would not be surprised if future analyzes end up disagreeing with the placement. The branches of the clade are well supported, she notes, but the old, tiny little branches are always subject to a certain degree of uncertainty. “Each phylogeny is a hypothesis, so I think in the future the hypothesis may be supported by additional data, or it will change.”
Sieracki, now a program manager at the National Science Foundation, also finds the document impressive. “It really helps to put [the picozoa] in context, ”he says. And although this is somewhat at odds with the evolutionary relationships proposed by his previous work, he is not terribly surprised by the placement of the non-photosynthetic group within Archaeplastida. Algae and other single-celled eukaryotes have had billions of years to evolve and cobble together survival strategies, he notes, and “the whole idea of plant versus animal is a false construct that we put on these organisms.”
See “Mixing it all up in the web of life”
Picozoan plastids: lost or nonexistent?
In addition to phylogenomic analyzes, Burki’s group combed genomes for evidence that picozoans had previously had plastids, but found none. Even though this matches previous studies, it’s still a bit surprising, says Burki, because it’s unprecedented. “Once you have this organelle, it’s actually extremely difficult to lose it,” he says.
Although plastids are best known for their role in photosynthesis, they are involved in other essential metabolic processes. They make isoprenoids, for example, which form the basis of many key pigments, vitamins, hormones, and membrane molecules. Some of them are considered to be essential nutrients that organisms without plastids get by consuming organisms with plastids. The few groups of archeplastids that lack it are parasites able to compensate for the loss by stealing their hosts.
Picozoans appear to be the only free line of Archaeplastida to have abandoned these organelles, according to Burki.
I still remember exactly where I was when I first read the description of these organisms.
—Fabien Burki, Uppsala University
Fučíková is not entirely convinced that the plastids or their remains are not there; she would feel more confident if there were more data from organism cultures. “Not having a culture limits what you can do and how well you can study these critters,” she notes. She adds that she also wouldn’t be shocked if they really lost them. She points out that scientists don’t really know much about the ecology of organisms that lived over a billion years ago; these organelles may not have been as essential for early plastid-bearing organisms.
There’s also another option, perhaps more controversial: that picozoans never had plastids in the first place.
Like the mitochondria, the plastids arose by endosymbiosis. “The widespread and accepted hypothesis or paradigm is actually that this was a one-time event,” Burki says. “A population of eukaryotes entered into this relationship with a population of bacteria, and that was it.”
If there really is no trace of plastids in the picozoans, then they either removed all the last vestiges or they never had them. And if they never had them, then their common ancestor with red algae did not have them either – which would mean that plastids have to come separately from at least two groups of algae – red algae and all the rest. algae and plants.
Based on everything he has seen, Burki says he leans towards complete loss of plastids, although this is what he describes as “an extremely unlikely event.” The alternative, he adds, “would be much more exciting,” and new findings could tip the balance of evidence in this way. Scientists have barely begun to sequence the diversity of microbial eukaryotes around the world, he notes, and those that live in places such as lakes are particularly under-studied. If work on them revealed algae lines without additional plastids, it could strengthen the case for multiple plastid origins.
Fučíková says that a dual-origin scenario is not impossible. “If there were two endosymbiotic events, but those endosymbionts were from very closely related cyanobacteria, would we even know?” Would we even be able to say it?