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Dissolved organic matter (DOM) in the ocean is one of the largest active carbon pools on earth, similar in size to atmospheric CO2 or all land plant biomass. Due to its richness in energy and nutrients it is fundamental for microbial life and for marine food webs. The microbial loop is an essential compartment in the global carbon cycle and is important for the transformation and recycling of organic matter and nutrients in the oceans. Microbial communities shape the molecular composition of DOM and vice versa. Earlier long-term studies have shown that seasonal dynamics in DOM composition and microbial communities exists. The aim of this study was to explore and characterize variations in composition of bacterial communities and DOM over short periods of time, ranging from hours to days. We hypothesize that variations in DOM composition are directly related to variations in the bacterial community and/ or environmental conditions. To test these hypotheses, water samples were taken daily over a time period of 20 days and hourly (over 24 hours) in the open North Sea off Helgoland Island. Sea water was analyzed for environmental variables, molecular DOM composition and the bacterial community structure. DOM was isolated from seawater by solid phase extraction and analyzed via ultrahigh resolution mass spectrometry (FT-ICR-MS, Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry). To investigate bacterial community structure, Automated Ribosomal Intergenic Spacer Analysis (ARISA) fingerprinting was used. The current study did not reveal a direct relation between a bacterial community structure changes and variation in the composition of DOM, neither within daily sampling nor the 24 h time series. However both, bacterial community and DOM composition undergo a characteristic shift during the daily sampling, mainly driven by salinity. The 24 h sampling during this time captured much of the variation in salinity and the microbial community, accordingly. High variations of salinity during the sampling period indicate the presence of changes in different water masses that carry distinct molecular and microbial signatures. For the first time, these changes have been documented in such high temporal and analytical resolution.
