All organisms require vitamin B1 (B1) as an essential growth factor, including aquatic bacterio- and phytoplankton, thereby B1 can impact productivity and nutrient cycles. Low environmental concentrations of B1 in the picomolar range can temporarily limit bacterioand phytoplankton growth. Further, higher trophic levels such as birds and fish can be B1 deficient. Bacterioplankton and some marine algae can potentially synthesize B1, but previous research has shown that the inability to synthesize B1 is widespread among plankton. Although B1 is recognized as an important micronutrient in aquatic ecosystems, knowledge about the cycling of B1 within coastal microbial communities is limited. One reason for this is that B1 concentrations are difficult to quantify. In this thesis, B1 and related compounds were quantified across seasons in Roskilde fjord in Denmark and summer/fall in the Neuse River Estuary, North Carolina, United States of America. These measurements together with genomic data allowed investigations on the role of B1 and its cycling within coastal microbial communities. Genomic data indicated that most of the bacterioplankton community was unable to synthesize B1 and relied on few key bacterial groups to potentially produce B1. This thesis presents results on the key components identified in the B1 cycles, core putative B1 producers and consumers and transformers of B1 related compounds. The extensive measurements of B1 and related compounds revealed seasonal dynamics for B1 related compounds and indicated a balance of supply and demand of B1 in Roskilde fjord. Contrasting, B1 and related compounds were highly dynamic in the Neuse River Estuary and shaped community dynamics in bacterio- and phytoplankton. The combination of state-of-the-art methods allowed a novel in depth understanding of B1 dynamics in coastal waters, and how and which microorganisms contribute to the cycling of B1. In summary, this thesis provides a novel holistic view of the coastal vitamin B1 cycle with key bacterioplankton identified, and pools of B1 and related compounds quantified. The findings collectively highlight the importance of B1 related compounds in aquatic B1 cycles and that B1 plays an important role in shaping microbial community composition.