Bacterial control of silicon regeneration from diatom detritus: Significance of bacterial ectohydrolases and species identity

Bacteria (and possibly archaea) accelerate silica dissolution in the sea by colonizing and enzymatically degrading the organic matrix of diatom frustu les. We tested whether colonizer species composition and ectohydrolase prof iles critically control silicon regeneration by allowing diatom (Thalassios ira weissflogii and Chaetoceros simplex) detritus to be colonized by natura l bacterial assemblages and 12 phylogenetically characterized marine isolat es. We characterized the colonizers' ectohydrolase profiles and rates of si licon regeneration. The colonizers' cell-specific protease activity was con sistently the dominant ectohydrolase, and it strongly correlated with silic a dissolution rates. Cell-specific glucosidase, lipase, and chitinase activ ities showed no correlation with silicon regeneration. Denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes was used to moni tor colonization of detritus by natural microbial assemblages and to identi fy colonizing phylotypes. Representatives from gammaproteobacteria and sphi ngobacteria-flavobacteria classes dominated colonizer populations by compri sing 65% and 25% of detected phylotypes, respectively. Archaea were not det ected among colonizer populations. All bacterial isolates accelerated silic a dissolution, but individual rates varied by >300%. Significant variabilit y was observed within the Alteromonadaceae, which indicates different abili ties to process diatom organic matter. Isolates that displayed enhanced col onization and protease activities were the most effective at regenerating s ilicon. The most effective isolate belonged to the sphingobacteria-flavobac teria, a group specialized in colonizing marine particles. Other effective isolates grouped with Pseudoalteromonas, Alteromonas, and Vibrio genera. On e isolate caused intense aggregation of diatom detritus, significantly redu cing silicon regeneration. Our results indicate that bacterial species iden tity strongly controlled silicon regeneration by influencing the colonizati on potential and ectohydrolytic profiles of bacteria as well as aggregate f ormation. Mechanistic models of oceanic silica cycling should incorporate s pecies composition and ectohydrolase profiles of bacteria involved in silic on regeneration.