Selective degradation of different dissolved organic matter compounds by regionally transplanted bacteria.

Climate change projections indicate that precipitation will increase by ~30% in the Baltic Sea within the next hundred years. This will lead to lowered salinity and increased inputs of dissolved organic matter (DOM) to the sea. The interactive effects of these changes on bacterial communities and DOM degradation are virtually unknown. We studied the selective degradation of different DOM compounds by regionally transplanted bacterial communities. Bacteria from the northern Baltic Sea were transplanted and re-transplanted to the southern Baltic Sea and vice versa. Three fractions of DOM were identified; two allochthonous fractions, originating from terrestrial systems and one autochthononous constituting the protein building blocks tryptophan/tyrosine. The largest decrease of dissolved organic carbon was observed in seawater from the Bothnian Sea (northern Baltic Sea), and the bacteria performing this degradation were those transplanted from the Baltic Proper (southern Baltic Sea). The native bacteria from the Bothnian Sea degraded both allochthonous and autochthonous DOM, while, bacteria from the Baltic Proper consumed mainly the autochthonous part of the DOM. Both autochthonous and allochthonous components of the DOM were found to shape the bacterioplankton community, Cyanobacteria and γ-proteobacteria were favored by all three DOM components, while α-proteobacteria and Bacteroidetes were favored by autochthonous DOM and β-proteobacteria by terrestrial DOM. However, no clear connection between different DOM components, specific bacterial groups and metabolic processes could be identified. Our study thus indicates that climate change can cause unforeseen adjustments of the bacterial community composition and function, governed by complex interactions between bacteria and their chemical environment.