Carbon dioxide forms the basis for many organisms to produce carbon compounds through their metabolism. Plants and certain bacteria use photosynthesis, other bacteria use chemosynthesis. However, animals do not possess this ability and have therefore formed symbioses with protozoa. The bacterium Kentron has also been regarded as a chemosynthetic symbiont of the ciliate Kentrophoros. However, the truth is much more interesting, as researchers from the Max Planck Institute for Marine Microbiology and international colleagues report in the scientific journal "mBio".

Incapable of fixating carbon dioxide

"Con­trary to our ex­pect­a­tions, we could­n’t find any of the known genes for the fix­a­tion of CO2," reports first author Brandon Seah. "From their genes, it seems that Kentron uses small or­ganic com­pounds and turns those into bio­mass," explains Nicole Dubilier, director at the Max Planck Institute for Marine Microbiology and lead author of the study. "In this sense, Ken­tron is up­cyc­ling the garbage. It most prob­ably re­cycles waste products from the en­vir­on­ment and from their hosts into ‘higher value’ bio­mass to feed their hosts." The host, the ciliate, is so dependent on its symbiote that it no longer even has a mouth.

Clear difference to other symbionts

Isotope analyses of the bacterium's proteins confirmed what the genes suggested: Kentron's stable isotope fingerprint differs greatly from that of other chemosynthetic symbionts. "This clearly shows that Ken­tron is get­ting its car­bon dif­fer­ently than other sym­bionts," says Seah. The researchers will now investigate the advantages and disadvantages of this strategy for the bacterium.

Consequences for models of the carbon cycle

However, the discovery could already have far-reaching consequences: "Up­take of or­ganic sub­strates from the en­vir­on­ment and re­cyc­ling waste from their hosts might play a big­ger role in these sym­bi­oses than pre­vi­ously thought," concludes co-author Harald Gruber-Vodicka of the Max Planck Institute for Marine Microbiology. It might therefore be necessary to adapt the ecological models of the carbon cycle.

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