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  • Alex Cloherty

Mirror, mirror, on the wall, who is the smallest one of all?

This post is inspired by an intriguing question from Althea: "What is the smallest multicellular organism?"


At a first glance, you might think this would be an easy question to answer. Find something that has more than one cell (so it's multicellular), but not very many cells in total (so it's small). However, the answer to this question heavily depends on how you define an "individual".

Let's take for example an organism that is clearly multicellular, and that we typically think of as an "individual": the human. However, we are not only individuals; we are also ecosystems. We are like mini, walking, talking Earths that provide the nutrients, warmth, and fluids that the billions of bacteria living in and on us need to survive. But unless you're the Queen of England, you don't generally refer to yourself as "we" (although I do like to imagine that every time she uses the plural first-person she is including her microbiota).

On the other hand, you can think of an organism that we would typically think of as unicellular: a bacterium like Pseudomonas aeruginosa. Although we do call bacteria like this unicellular, many individual bacteria can also live together in colonies called biofilms. Biofilms are basically layered colonies of bacteria that attach to surfaces like pipes or rocks. Within biofilms, the different individual bacteria can send messages to each other, and even influence each other's behaviour. For instance, if there are multiple layers of cells, the bacteria somewhat arrange themselves and those on the surface behave differently than those underneath. Perhaps we wouldn't exactly call something like a biolayer an "individual", but the fact that it is a collection of cells acting in harmony together does have some of the trappings of individuality about it.

In fact, there is a whole line of research on the evolution of organisms devoted to sorting out what we call individuals. This research focuses on so-called "evolutionary transitions in individuality". What a lovely phrase. But what does it mean?

As this paper so elegantly put it, "diversity originates from evolution of organisms since the origin of life on the Earth, via evolutionary transitions in individuality, in which individuals gathered to become different individuals of higher-level". More simply put, the lines between what we call individuals and what we call groups of individuals are somewhat blurred.

However, for those of you who are less inclined to ponder the philosophy of individuality, of course I found a more scientific answer for you. In a word, algae.

Tetrabaena socialis might not seem that exciting on the surface. And I mean literally on the surface - you'll find it floating about in the average garden pond. But, this specific pond scum, which consists of only four cells as you may have guessed from its name, is indeed the simplest known multicellular organism.

T. socialis is also part of a family of algae that is super interesting to scientists. These so-called volvocine green algae are kind of like a time capsule, or an "evolutionary time machine" for studying how organisms evolved from unicellularity to multicellularity. This family of algae contains species ranging from the unicellular Chlamydomonas to the complex Volvox, which contains more than 500 cells. Studying this family of algae is therefore helpful to scientists that are trying to figure out why and how unicellular organisms evolve to become multicellular, like us humans. Each of these volvocine species is like a time capsule deposited along the steps to increasingly complex versions of multicellularity. Tetrabaena just happens to be one of the earliest time capsules.



So what do you think - what makes an individual an individual? I love how sometimes, these questions that seem so simple can force us to think more deeply about the categorizations that we take for granted.

Until next time,

~ Alex


#algae #multicellular #biofilms #philosophy This edition of Microbial Mondays was heavily inspired by the paper linked below. A big thank you to Arakaki et al. for their elegantly written research paper!

Arakaki Y, Kawai-Toyooka H, Hamamura Y, Higashiyama T, Noga A, Hirono M, et al. (2013) The Simplest Integrated Multicellular Organism Unveiled. PLoS ONE 8(12): e81641. https://doi.org/10.1371/journal.pone.0081641

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