Keystone species determine the "selection mechanism" of multispecies biofilms for bacteria from soil aggregates.

Soil aggregates are integral parts of soil structure and play paramount roles in supporting microbial diversity, nutrient cycling and water retention. The formation of multispecies biofilms is a survival strategy for bacterial adaptation to the environment and help microorganisms access more complex nutrient sources via labor sharing, especially in soil aggregates. However, very little is known about the effect of species richness and composition on bacterial multispecies biofilms formation in different size soil aggregates. A random partition design strategy was used to identify the relative importance of bacterial richness and composition in driving multispecies biofilms. The strategy can separate the effects of species richness and composition from the soil aggregates occurring bacterial assemblage. Increasing species richness was found to be always positively correlated with multispecies biofilms productivity for bacteria from the same aggregate fractions. General linear model analysis revealed that species composition contributed more than species richness to forming multispecies biofilms, suggesting that "selection mechanism" plays a more important role than "complementarity mechanism". This "selection mechanism" relies mainly on culturable keystone species that can significantly enhance the formation of multispecies biofilms. The co-occurrence network was investigated to explore whether the culturable keystone species from the random partitions experiment are consistent with the keystone taxa. Four out of 10 culturable keystone species isolated from soil aggregates were matched the keystone taxa. It is concluded that the culturable keystone species determine the multispecies biofilms formation for bacteria residing in soil aggregates. This study provides insights into the role of culturable keystone species in multispecies biofilms. Understanding the formation of multispecies biofilms is fundamental to decipher how microbes interact with each other in soil aggregates. Meanwhile, it will enhance our knowledge of the quorum behavior of complex bacterial communities.