by Roberto
In the early years of STC, Elio wrote two posts on bacterial multicellularity. The first, posted in early 2007, described the unusual lifestyle of some magnetotactic bacteria that are always found as tightly joined aggregates of cells. As the cells grow, the clump splits in two. Their cells are never alone. Moreover, if the cells are forcibly separated in the lab they lose magnetotaxis and die. In his second post on the subject, towards the end of 2008, Elio directed us to a review on the origins of multicellularity. He noted that reading it "may reinforce your view that this phenomenon, as most in biology, had its beginnings in the prokaryotic world." Why the excitement, back then, on bacterial multicellularity? In Elio's words: "We lovers of microbes delight in the complexity of multicellular bacteria such as the actinomycetes, the myxobacteria, and some cyanobacteria." And what's my reason for wanting to broach the subject again now, fifteen years later? Because it is important to recognize that those three bacterial types listed are but the tip of the iceberg when it comes to multicellular bacteria. I'll argue that bacterial multicellularity is the norm and intro biology courses should teach that.
Fig. 1. Transmitted-light photomicrographs of 1.6-billion-year-old microfossils of multicellular eukaryotes. Scale bar, 50µm. Adapted from source.
I've been toying with the idea of writing this post for a while now. A recent report on the age of the oldest multicellular eukaryote and an email I received got me to sit down and write it. The email was from Amy Cheng Vollmer, long-time a friend and STC contributor, expressing concern about a commentary on the report that seemed to "equate multicellularity with being eukaryotic." This is what emerged in my mind...
From analyses on microfossils found in ancient rocks in China, the authors of the report estimate that there were filamentous multicellular eukaryotes 1.6 billion years ago. That age is worth noting for two reasons. First, it is about 600 million years earlier than prior evidence for multicellular eukaryotes. Second, it is pretty much concomitant with the age of microfossils of the first eukaryotes. In other words, multicellularity – in this case chains of cells – seems to have been a rapid evolutionary transition for eukaryotes. Strikingly, the scant microfossil evidence that we have for early multicellular prokaryotes – about 3.5 billion years ago – suggests they arose early in life's natural history. Multicellularity, in the form of cells not separating after growth and division, is likely easy and thus fast to attain. And you can see that there can be numerous advantages to existing as a group of cells. It opens the possibility for cooperative behaviors such as division of labor, and protection from environmental stresses, including predation. It should therefore not be surprising that multicellularity has evolved multiple times during the more than 3.5 billion years of evolution of life on Earth.
Fig. 2. Facultative multicellularity is the norm for bacteria. Image credit: Roberto Kolter
Yet, we still have the tendency to describe prokaryotes as "mostly unicellular," citing as rare examples, oddities even, the myxobacteria, the actinomycetes and some cyanobacteria. But worse, the prevailing thinking still seems to be that eukaryotes are "mostly multicellular." What of the myriad unicellular protists? My sense is that with the current state of knowledge, it is important to recognize that the types of multicellular solutions that have been selected through the course of evolution are enormously diverse. Yet, we must recognize that there are profound differences in the degrees of multicellular complexity. One approach to describing these differences – the one that the authors of the recent report seem to favor – is that there are "simple multicellular organisms, with cell-cell adhesion but limited communication or differentiation among constituent cells" and then there is "complex multicellularity, with greater directed intercellular communication and more pronounced cell and tissue differentiation." However, I think drawing the line between what is simple and what is complex will not always be easy. What is complex for some will be simple for others. I favor dividing multicellularity functionally as obligate or facultative. Seen this way, most bacterial species are facultative multicellular organisms. They can lead a unicellular existence but also form aggregates through the production of an extracellular matrix or forming chains by not separating after septation (or a combination of both); think biofilms! There might a few exceptions, for example the abundant marine cyanobacterium Prochlorococcus appears to be an obligate unicellular organism. Those few exceptions aside, facultative multicellularity is the norm among bacteria. And there are plenty of facultative multicellular eukaryotes as well, for example the familiar yeast Saccharomyces cerevisiae.
Fig. 3. Snowflake yeast. Courtesy of Ratcliff Lab, Georgia Tech. Source
How can we get over the notion, still apparently widely held, that bacteria are largely unicellular and eukaryotes mostly multicellular? Our teaching should emphasize that facultative multicellularity evolved very early, that it might even be "intrinsic to the origin of life," as Ute Römling argued recently. Prokaryotic multicellularity evolved very early and it is very common. And now we know that the earliest known multicellular eukaryotes appeared at pretty much the same time as the last eukaryotic common ancestor (1.6 billion years ago), as the recent study shows. Yet, it took nearly another billion years of evolution before numerous obligate multicellular organisms arose independently; witness the green and red algae, the fungi, and the animals. Yes, it does seem that things make sense in biology in the light of evolution, to paraphrase Theodosius Dobzhansky.
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