Micrographs of an unidentified filamentous cyanobacterium from the Cyanobacteria page of Frankfurt University site, © 1997, Microbial Diversity. Reproduced with the kind permission of the authors. The left photo is phase-contrast, the right one is autofluorescence after illumination with green light. Most of the cells fluoresce in red due to their content of photosynthetic pigments. One cell, called a heterocyst, remains dark. It doesn't participate in photosynthesis because has another function - nitrogen fixation.
(Readers not interested in biology are advised to skip this post.)
While writing my textbook chapter about bacterial structure, I naturally had to touch the question about the uni/multicellularity in prokaryotes. It is known that in some of them, cells not only remain together after completing division but interract and perform "division of labour", i.e. specialize for different functions. An example is shown above. I am totally in love with this photo and because, happily, its authors Tom Lie and Rolf Schauder allow it to be reproduced, I put it in all places where appropriate.
Most biologists don't consider such organisms as truly multicellular and call them with the catch-all term "colonial". To be called a multicellular organism, an aggregate of cell is required not only to be stable and to have "division of labour", but to reproduce as a whole by a mechanism clearly distinct from the division of individual cells and, moreover, to live as a whole. That is, individual cells must die if isolated from the group.
As far as I knew, no prokaryotic organism met these stringent criteria for multicellularity, therefore I used to write confidently that "there are no multicellular prokaryotes". However, I checked that the current school textbook is calling "multicellular" the filamentous cyanobacteria such as the one shown above.
From the viewpoint of true knowledge, this is of little importance because it depends solely on the criteria for multicellularity used. Many scientific terms, including basic ones (in fact, especially basic ones) have no uniform definition. E.g. I cannot name two textbooks using 100% compatible definitions of levels of protein structure. This doesn't bother scientists at all - with or without a definition, they know very well what they are speaking about. However, students suffer much from the pluralism of definitions, because at exams, no error is considered more serious than defining or using a term in a meaning different from the one accepted by textbooks and teachers of the current course (even if the term is used and defined in a way considered quite valid by other experts).
So I decided that whatever I would write, a little googling on the matter would do no harm. And see what I found at allexperts.com:
"I have a question that I have tried to find the answers to myself for quite a while now, but I have failed. I would like to know if it is possible for a prokaryote to have a multicellular form. I know that they can be colonial and undifferentiated or colonial and partially differentiated (such as cyanobacteria), but can they ever be truly multicellular? If so, have we ever found any such organism living or dead? (Or posssibly fossilized?) Or could it even exist, if only in theory?...
Answer (by John Locke): You've preempted the standard response to this question by mentioning the colonial forms of prokaryotes; these are the one of the most familiar multicellular-like prokaryotic organisms. As you probably already know, some of the colonial forms have cells with distinct, though partial, differentiation; that is not the only multicellular form of prokaryotic life, however--enter the multi-celled magnetotactic prokaryote (MMP). This is a multi-celled prokaryote with a somewhat higher level of organization than simple colonial behavior.
Magnetotactic prokaryotes were discovered in the 1970's; they're bacteria capable of orienting themselves according to the earth's magnetic field via the manipulation of ferrous molecules (iron-containing compounds, of course, are affected by the earth's magnetic field). More recently, a unique multi-cellular form of these bacteria has been discovered, the MMP. An MMP exists throughout its lifespan as a motile, hollow ball of cells that is capable of reproducing as a whole--the individual prokaryotes all divide simultaneously, and then two groups of cells separate to form two new MMPs... Prokaryotes that make up the MMP appear to exist only as part of the whole structure and have never been found free-living; indeed, the individual prokaryotes die when removed from the MMP. Like their unicellular magnetotactic counterparts, MMPs are capable of orienting themselves to the earth's magnetic field.
The MMP was only discovered relatively recently, so the references that I give below are necessarily from the scientific literature... There are few resources now available on MMPs for the general public, unfortunately.
Unexpected diversity in populations of the many-celled magnetotactic prokaryote. Simmons SL, Edwards KJ. Environmental Microbiology 9(1):206-15. January 2007.
Multicellular life cycle of magnetotactic prokaryotes. Keim CN, Martins JL, Abreu F, Rosado AS, de Barros HL, Borojevic R, Lins U, Farina M. FEMS Microbiology Letters 15(2):203-8. January 2004.
The MMP continues to be an active area of research, both for its own sake and for its potential applications in electronics and chemical manufacture. While more multicellular prokaryotes may be discovered in the future, the MMP now the only known example of a highly-organized multicellular prokaryote."
The living world will never ceaze to surprise and fascinate us!