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Heavy metal resistance in Marinobacter adhaerens HP15 supports colonization of transparent exopolymer particles during its interaction with diatoms

Veronika Will*, Antje Stahl, Matthias S. Ullrich

*Corresponding author:

ABSTRACT: Marine diatoms and heterotrophic bacteria play a crucial role in the oceanic carbon cycle as they form aggregates that sink and transport biomass down in the water column. Polysaccharides are a major component of these aggregates and can bind heavy metal cations. Therefore, both organism groups might also be important for the vertical or micro-scale distribution of heavy metals in the ocean. Our model organism for diatom–bacteria interactions, Marinobacter adhaerens HP15, possesses 2 operons coding for heavy metal efflux pumps of the czcCBA type that are responsible for zinc resistance in this bacterium. The goal of this study was to elucidate the role of zinc resistance in M. adhaerens HP15 during its interaction with the diatom Thalassiosira weissflogii. For this, a HP15 mutant deficient in both czcCBA gene clusters was used in diatom co-cultivation experiments with or without zinc stress. When zinc was added to the co-cultures, the release of Alcian Blue-stainable particles increased 10-fold compared to assays without zinc. Both M. adhaerens HP15 wild-type and ΔczcCBA.1/2 mutant colonized the aggregates and the percentage of bacteria attaching to transparent exopolymer particles (TEP) increased significantly under zinc stress. In the assays with zinc, HP15 wild-type had a higher cell density inside the aggregates than the mutant, although the zinc concentrations inside TEP were comparable. As TEP can bind and thus enrich heavy metals, resistance against them may help M. adhaerens HP15 to colonize aggregates. These results suggest that heavy metal resistance can be beneficial for bacteria during TEP- and aggregate-associated lifestyles.