One of my new obsessions is bacterial multicellularity. Single celled micro-organisms are constantly interacting with their neighbors, from individuals of the same species to cells from different domains of life, forming complex biofilm patterns, complex nutritional symbioses, and complex clumps.
Clumps and granules consisting of multiple different species form spontaneously in many interesting environments. In anaerobic wastewater sludge reactors, large organic molecules are broken down by aggregates of bacteria. Many of the chemical reactions would be unfavorable without the close association of the bacteria in these clumps, where metabolites can be freely exchanged between different species. The bacteria on the outside of the granule break down larger molecules into acids, which are passed onto different species that consume the acids and produce hydrogen and carbon dioxide. In the center of the granule, methanogenic archaea consume the hydrogen and carbon dioxide to produce methane gas. Similar microbial relationships occur in the rumen of cows, except that the methanogens live on the outside of larger protozoa cells, producing the methane gas of cow farts, a significant source of greenhouse gasses.
Clumps of many different microbial species are also found in kefir, an effervescent yogurt-like drink. Kefir is fermented by grains of yeasts and lactic acid bacteria that form on a spongy fibrous matrix. A beautiful essay by Lynn Margulis for Scientific American speculates on the evolutionary origin of multicellular life and death in kefir granules:
Kefir, which looks like large-curd cottage cheese, grows by division. It ferments milk sugars and proteins, making the yogurtlike drink. When the active metabolism that assures individuality ceases, kefir curds dissolve and die without aging. After the curds die, kefir individuals become an arbitrary mix of fermenting microbes rather than the specific combination of bacteria and yeast that forms each curd. Like our protoctist ancestors that evolved from symbioses among bacteria, kefir individuals arose from the physical association of 30 different kinds of microbes. These yeasts and bacteria remain together in precise relationships as each divides, maintaining the integrity of the individual curd.
For many more beautiful examples of bacterial multicellularity, check out the 1988 Scientific American article by James Shapiro, "Bacteria as Multicellular Organisms" (PDF).