| Summary: | Bacterial consumption of dissolved organic matter (DOM) drives much of the movementof carbon through the oceanic food web and the global carbon cycle. Understandingcomplex interactions between bacteria and marine DOM remains an importantchallenge. We tested the hypothesis that bacterial growth and community successionwould respond differently to DOM additions due to seasonal changes in phytoplanktonabundance in the environment. Four mesocosm experiments were conducted thatspanned the spring transitional period (August–December 2013) in surface waters of theWestern Antarctic Peninsula (WAP). Each mesocosm consisted of nearshore surfaceseawater (50 L) incubated in the laboratory for 10 days. The addition of DOM, in theform of cell-free exudates extracted from Thalassiosira weissflogii diatom cultures led tochanges in bacterial abundance, production, and community composition. The timingof each mesocosm experiment (i.e., late winter vs. late spring) influenced the magnitudeand direction of bacterial changes. For example, the same DOM treatment appliedat different times during the season resulted in different levels of bacterial productionand different bacterial community composition. There was a mid-season shift fromCollwelliaceae to Polaribacter having the greatest relative abundance after incubation.This shift corresponded to a modest but significant increase in the initial relativeabundance of Polaribacter in the nearshore seawater used to set up experiments. Thisfinding supports a new hypothesis that starting community composition, through priorityeffects, influenced the trajectory of community succession in response to DOM addition.As strong inter-annual variability and long-term climate change may shift the timingof WAP phytoplankton blooms, and the corresponding production of DOM exudates,this study suggests a mechanism by which different seasonal successional patterns inbacterial communities could occur.
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