All bacteria exist as single cells at some point in their lives – except for one kind, known as multicellular magnetotactic bacteria (MMB), that is.
Scooped from the sulfide-laden sediments of a tidal salt marsh in Massachusetts, scientists think these MMBs could provide clues to our own evolutionary history, as a kind of missing link between simple, single-celled life forms and complex multicellular organisms like ourselves.
Other kinds of bacteria can team up with their fellow cells when needed, sure, but MMBs do everything together, to the point where each individual cell literally cannot survive being separated from its ‘consortium’, the nobbly super-organism it calls home.
The cells in an MMB consortium form a spherical shape with a hollow at the center, not unlike a blastocyst, the stage in embryonic development that comes immediately after the egg and sperm merge.
The fact that this bacteria so closely resembles that transition point, from two single cells with different genetics to one inseparable cluster, is fascinating: embryo comparisons have provided many clues about our evolutionary history.
Unlike a blastocyst, however, each cell in an MMB is its own individual organism. Scientists already knew this, but they had assumed these individuals might be clones of each other, since all the cells sync up to replicate when the entire MMB consortium divides in two.
A team led by environmental microbiologist George Schaible from Montana State University has made headway by mapping the metagenomes of 22 MMB consortia.
This revealed that the cells within a consortium are not clones, and that the variety of genes spread throughout the cluster enables individuals to serve different functions that benefit the whole – similar to organs in a body, or a society that benefits from its members’ diverse skills and interests.
These bacteria survive on a mix of carbon and energy sources, one of which involves reducing sulfate into hydrogen sulfide.
Schaible and his colleagues found the bacteria “are capable of assimilating both inorganic and organic carbon, indicating autotrophic and heterotrophic growth, and that different groups of MMB demonstrate variable affinities for carbon sources.”
This ability to use many sources of energy and carbon may arise from the consortium’s internal diversity, and it could explain why the cells are entirely reliant on each other.
“Individual cells within MMB consortia exhibit dramatically different rates of substrate uptake, indicating metabolic differentiation, as well as localized protein synthesis activity,” the authors write.
It would probably be difficult to survive if you were solely responsible for growing, hunting, and building everything your body needs, so human communities have a division of labor. That’s exactly how the scientists have described the metabolism of an MMB consortium: a ‘division of labor‘. The genetic and metabolic diversity they found in this study sure adds some weight to that theory.
The constant changes of a tidal salt marsh environment could have driven this combination of cooperative and diverse genes, which would allow the consortium to take advantage of whatever the wind, water, and weather throw at it.
“Cells within the consortium could engage in a division of labor by metabolizing specific substrates (e.g., acetate) and then sharing those resources with other cells through the acellular space, possibly by the utilization of membrane vesicles,” they write.
“Modeling approaches could shed light on potential metabolic networks within the consortium, further supporting the hypothesis of a division of labor.”
This research was published in PLOS Biology.
