They鈥檙e possibly the tiniest, most ancient gardeners in the world. A type of marine bacteria tends algae, using pesticides to keep other microbes away.
Understanding how Roseobacter does this could help us better understand nutrient circulation in the world鈥檚 oceans, where the bacteria and their microalgae 鈥渃rops鈥� are abundant.
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鈥淭hey鈥檙e key players in global nutrient recycling,鈥� says of the Technical University of Denmark in Lyngby, who last month at the annual meeting of the American Society for Microbiology in Boston. On average Roseobacter account for 3 per cent of bacterial diversity globally, and as much as 20 per cent in places.
Sonnenschein鈥檚 work builds on a 2011 discovery by Mohammad Seyedsayamdost, now at Princeton University. He found that in times of plenty, the algae produce nutrients that help Roseobacter microbes thrive. In return, the bacteria make antibiotics which serve as pesticides, protecting the algae from rival bacterial strains.
Both the bacteria and the algae appear to benefit from the arrangement, reminiscent of ants farming aphids. 鈥淚 suspect it鈥檚 mutualism,鈥� says Rita Colwell of the University of Maryland at College Park. 鈥淭hey wouldn鈥檛 be there if it wasn鈥檛 beneficial to both parties.鈥�
When times get tougher, though, and the algae begin competing with the bacteria for resources, the 鈥渇armers鈥� stop making antibiotics and instead produce a substance that kills off the algae, harvesting the rich algal decay products.
Microbes with tools
Sonnenschein鈥檚 team has now investigated the 鈥渢ools鈥� that the bacteria use to manage their algal gardens. These include a 鈥渉erbicide鈥�, tropodithietic acid (TDA), which protects the algae from other bacteria, and roseobacticides, the substances that kill off the algae for harvest.
The team studied samples from aquaculture facilities in Denmark, Spain and Greece and from seawater off the coast of Denmark, Germany and Australia. They identified and studied strains of a particular species of Roseobacter called Phaeobacter inhibens, most of which produced roseobacticides.
By comparing their genomes with those of the strains that did not, the researchers could home in on genes likely to be responsible for their production. They found that the microbes that make TDA aren鈥檛 always also able to produce the compounds that kill algae. Indeed, they identified 41 gene clusters that are unique in those bacteria that can make roseobacticides.
It is thought that algal breakdown products induce more production of roseobacticides, creating a positive feedback loop that causes more algae to be killed off.
Sonnenschein and her colleagues are now trying to gauge the environmental impact of the algal farming, and the factors that dictate whether the bacteria protect or harvest the algae. 鈥淲e鈥檙e doing experiments battling the two against each other,鈥� she says.
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