7 páginas, 2 tablas, 3 figuras.The winter Arctic Ocean is one of the most unexplored marine environments from a
microbiological perspective. Heterotrophic bacteria maintain their activity at a baseline
level during the extremely low-energy conditions of the winter, but little is known
about the specific phylotypes that have the potential to survive and grow in such harsh
environment. In this study, we aimed at identifying actively growing ribotypes in winter
Arctic Ocean seawater cultures by experimental incubations with the thymidine analog
bromodeoxyuridine (BrdU), followed by immunocapturing, terminal restriction fragment
length polymorphism fingerprinting, cloning, and sequencing the 16S rRNA gene. We
incubated water collected at different months over the Arctic winter and showed that
the actively growing bacterial fraction, taking up BrdU, represented only a subset of the
total community. Among the BrdU-labeled bacterial taxa we identified the Flavobacteria
Polaribacter, the Alphaproteobacteria SAR11, the Gammaproteobacteria Arctic 96B-16
cluster and, predominately, members of Colwellia spp. Interestingly, Colwellia sequences
formed three clusters (93 and 97% pairwise 16S rRNA identity) that contributed in
contrasting ways to the active communities in the incubations. Polaribacter, Arctic 96B-16
and one cluster of Colwellia were more abundant in the active community represented
by the BrdU-labeled DNA. In contrast, SAR11 and two other Colwellia clusters were
underrepresented in the BrdU-labeled community compared to total communities. Despite
the limitation of the long incubations needed to label slow growing arctic communities, the
BrdU approach revealed the potential for active growth in low-energy conditions in some
relevant groups of polar bacteria, includingPolaribacter and Arctic 96B-16. Moreover, under
similar incubation conditions, the growth of differentColwellia ribotypes varied, suggesting
that related clusters of Colwellia may have distinct metabolic features.Thisworkisacontribution
totheInternationalPolarYear–CircumpolarFlawLeadsys-
temstudy(IPY-CFL2007/2008)andArcticNet.PierreE.Galand
and LauraAlonso-SáezweresupportedbyMarieCurieIntra-
EuropeanFellowshipgrantsCRENARCMEIF-CT-2007-040247
and CHEMOARCPIEF-GA-2008-221121,respectively.Connie
LovejoywassupportedbytheNaturalSciencesandEngineering
ResearchCouncilofCanada(NSERC)SROandDiscoverygrants
and ArcticNet.TheworkdoneinEmilioO.Casamayorlabwas
supportedbygrantPIRENACGL2009-13318fromtheSpanish
MinisteriodeCienciaeInnovación(MICINN)andthemolecu-
lar workcarriedoutinUppsalawasfundedbygrantsfromthe
SwedishResearchCounciltoStefanBertilsson.Peer reviewe