Let’s come together: microbial clumping

Posted on March 10, 2014

The ability of aquatic bacteria to colonise organic particles is known to boost their growth and production. Now new and surprising findings suggest that microorganisms can use their aggregation abilities in flexible ways that depend on the mix of microbial and predator species. In experiments by Swiss and Italian researchers, two species of bacteria (Arthrobacter agilis and Brevundimonas sp.) from Lake Zurich were cultured in isolation and in combination. Additional cultures containing one or both of the bacterial species mixed with a common flagellate predator (Poterioochromonas) were also set up. Starting bacterial concentrations were 1.5 and 0.75 million cells / mL for pure and co-cultures respectively, and predators were added at a rate of 1500 cells/mL. Nutrients were also added and cultures were kept in the dark so that that the flagellates would rely on predation rather than photosynthesis. Samples collected after 168 hours showed that in combined culture, competition from Brevundimonas drove the other bacterial species to extinction – but only in the absence of predators. When predators were present, both bacterial species persisted and clustered together in large coaggregations, presumably influenced by chemicals released by the flagellates. This seemed to be a winning arrangement for all species. By attaching to the aggregations, the flagellates increased their feeding rate and growth efficiency, and they excreted dissolved organic carbon that could be recycled by the bacteria. As a result, the coaggregates became hot spots for bacterial production and the channelling of organic matter into the food chain. In grazed co-cultures, 80% of available organic carbon was incorporated into bacteria (compared to 2-5% in grazed pure cultures), and 43% of it was then transferred to predators. Interestingly, large-scale cell aggregation was a unique product of the interactions between all three species, since it wasn’t seen either when prey species were raised in the absence of predators, or with a single bacterial strain plus predators. These results raise the possibility that coaggregation and emerging interactions between attached bacteria can increase the efficiency of the so-called microbial loop, in which bacteria consume dissolved organic carbon and return it to the food web.

Reference: Corno, G., Villiger, J. & Pernthaler, J. 2013. Coaggregation in a microbial predator–prey system affects competition and trophic transfer efficiency. Ecology 94(4), 870–881. http://www.esajournals.org/doi/pdf/10.1890/12-1652.1