Genes encoding gene transfer agent (GTA) particles are well conserved in bacteria of the order Rhodobacterales. Members of this order are abundant in diverse marine environments, frequently accounting for as much as 25% of the total bacterial community. Conservation of the genes encoding GTAs allows their use as diagnostic markers of Rhodobacterales in biogeographical studies. The first survey of the diversity of Rhodobacterales based on the GTA major capsid gene was conducted in a warm temperate estuarine ecosystem, the Chesapeake Bay, but the biogeography of Rhodobacterales has not been explored extensively. This study investigates Rhodobacterales diversity in the cold subarctic water near Newfoundland, Canada. Our results suggest that the subarctic region of the North Atlantic contains diverse Rhodobacterales communities in both winter and summer, and that the diversity of the Rhodobacterales community in the summer Newfoundland coastal water is higher than that found in the Chesapeake Bay, in either the summer or winter. Approximately one-third of GTA sequences retrieved from the subarctic waters were most closely related to those from bacteria isolated from sea ice or cold regions. Distinguishable diversity patterns were found between the temperate and subarctic waters, providing further support for niche adaptation of specific Rhodobacterales members to unique environments. We also demonstrate that a number of Rhodobacterales strains, from both the subarctic and temperate locations, express the GTA major capsid protein. This provides robust evidence that the widespread conservation of GTA genes in the Rhodobacterales may result in the production of functionally similar and active GTA systems in these bacteria in different environments.