Insights into the interaction of Marinobacter adhaerens with the diatom Thalassiosira weissflogii by comparative mutant analysis

Interactions between diatoms and heterotrophic bacteria play an important role in the marine biological pump. Diatoms form marine snow aggregates that are used by heterotrophic bacteria as rich nutrient sources. A model system consisting of the γ-proteobacterium, Marinobacter adhaerens HP15, and the diatom, Thalassiosira weissflogii, was used to study diatom-bacteria interactions on a molecular level using various mutants of M. adhaerens HP15. The assessment of the zinc-sensitive mutant HP15 ΔczcCBA.1/2 in diatom co-cultures with and without zinc stress showed that heavy metal resistance helps bacteria to colonize aggregates. Furthermore, addition of ZnSO4 increased the release of exopolymers by M. adhaerens HP15 and led to a higher rate of bacterial aggregate colonization by both, the wild-type and the mutant. Co-cultivation of the non-motile mutant ΔfliC and the non-chemotactic mutant ΔcheA showed that at high exopolymer concentrations motility is more important for bacterial attachment to aggregates than chemotaxis. Proteomic analysis suggested that amino acids present in these aggregates are the preferred nutrient source for M. adhaerens HP15. Amino acid quantification confirmed the presence of especially branched chain amino acids in the attached fractions of co-cultures suggesting that the diatom contributes to the amino acid pool therein. The role of branched chain amino acid uptake for M. adhaerens HP15 was studied with the mutant ΔlivK. M. adhaerens HP15 has five livK genes that are differentially expressed during in vitro growth. The studied livK gene plays only a minor role during in vitro growth despite its previously suggested importance for amino acid uptake in vivo, hinting towards specialized gene expression under different growth conditions. Overall, this study gives important insights into diatom-bacteria interactions on a molecular level and in terms of exchanged nutrients, dynamics of exopolymer release and bacterial aggregate colonization.