Cooperation fundamentally contributes to the success of life on earth, but its persistence in diverse communities remains a riddle, as selfish phenotypes rapidly evolve and may spread until disrupting cooperation. Here we investigate how evolutionary history affects the emergence and spread of defectors in multispecies communities. We set up bacterial communities of varying diversity and phylogenetic relatedness and measure investment into cooperation (proteolytic activity) and their vulnerability to invasion by defectors. We show that evolutionary relationships predict the stability of cooperation: phylogenetically diverse communities are rapidly invaded by spontaneous signal-blind mutants (ignoring signals regulating cooperation), while cooperation is stable in closely related ones. Maintenance of cooperation is controlled by antagonism against defectors: cooperators inhibit phylogenetically related defectors, but not distant ones. This kin-dependent inhibition links phylogenetic diversity and evolutionary dynamics and thus provides a robust mechanistic predictor for the persistence of cooperation in natural communities.
The dissolved oxygen concentration is a crucial parameter in aerobic bioprocesses due to the low solubility of oxygen in water. The present study describes a new method for determining the oxygen transfer rate (OTR) in shaken-culture systems based on the sodium sulfite method in combination with an electrochemical oxygen sensor. The method replaces the laborious titration of the remaining sulfite by an on-line detection of the end point of the reaction. This method is a two-step procedure that can be applied in arbitrary flasks that do not allow the insertion of electrodes. The method does not therefore depend on the type of vessel in which the OTR is detected. The concept is demonstrated by determination of the OTR for standard baffled 1-L shake flasks and for opaque Ultra Yield™ flasks. Under typical shaking conditions, k(L) a values in the standard baffled flasks reached values up to 220 h(-1) , whereas the k(L) a values of the Ultra Yield flasks were significantly higher (up to 422 h(-1) ).
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