DISTRIBUTION, PHYLOGENY, AND GROWTH OF COLD-ADAPTED PICOPRASINOPHYTES IN ARCTIC SEAS 1 Connie Lovejoy 2 , Warwick F. Vincent De ´partement de Biologie, Universite´ Laval, Que ´bec City, Que ´bec, Canada, G1K 7P4 Sylvia Bonilla Seccio ´n Limnologı ´a, Facultad de Ciencias, Universidad de la Repu ´blica, 11400 Montevideo, Uruguay Suzanne Roy Institut des Sciencesde la Mer, Universite´ du Que´bec a` Rimouski, 310 Alle´e des Ursulines, Rimouski, Que ´bec, Canada, G5L 3A1 Marie-Jose´e Martineau, Ramon Terrado, Marianne Potvin De ´partement de Biologie, Universite´ Laval, Que ´bec City, Que ´bec, Canada, G1K 7P4 Ramon Massana, and Carlos Pedro´s-Alio´ CSIC CIMIMA, Departament de Biologia Marina i Oceanografia, Institut de Cie `ncies del Mar, CMIMA, CSIC, Passeig Marı ´tim de la Barceloneta, 37-49 Barcelona, Spain Our pigment analyses from a year-long study in the coastal Beaufort Sea in the western Canadian Arctic showed the continuous prevalence of eukar- yotic picoplankton in the green algal class Pra- sinophyceae. Microscopic analyses revealed that the most abundant photosynthetic cell types were Micromonas-like picoprasinophytes that persisted throughout winter darkness and then maintained steady exponential growth from late winter to early summer. A Micromonas (CCMP2099) isolated from an Arctic polynya (North Water Polynya between Ellesmere Island and Greenland), an ice-free sec- tion, grew optimally at 61C–81C, with light satur- ation at or below 10 lmol photons . m  2 . s  1 at 01C. The 18S rDNA analyses of this isolate and environ- mental DNA clone libraries from diverse sites across the Arctic Basin indicate that this single psychrophilic Micromonas ecotype has a pan-Arctic distribution. The 18S rDNA from two other pico- prasinophyte genera was also found in our pan- Arctic clone libraries: Bathycoccus and Mantoniella. The Arctic Micromonas differed from genotypes elsewhere in the World Ocean, implying that the Arctic Basin is a marine microbial province con- taining endemic species, consistent with the bioge- ography of its macroorganisms. The prevalence of obligate low-temperature, shade-adapted species in the phytoplankton indicates that the lower food web of the Arctic Ocean is vulnerable to ongoing climate change in the region. Key index words: Arctic Ocean; biogeography; climate change; phytoplankton; picoeukaryotes; picoplankton; pigments; polar; prasinophytes; psy- chrophiles Abbreviations: FISH, fluorescent in situ hybridization A widely held oceanographic paradigm is that photosynthetic picoplanktonic cyanobacteria, such as Synechococcus or Prochlorococcus, are continuously abun- dant in the sea (Scanlan and West 2002). Larger-celled eukaryotes including diatoms, prymnesiophytes, and dinoflagellates rise above this phototrophic background and produce seasonal blooms under specific hydro- graphic conditions (Smetacek et al. 1990, Li 2002). An unusual feature of Arctic and Antarctic marine ecosys- tems is that the background population of picocyano- bacteria is conspicuously absent or sparse (Li 1998, Vincent 2000), suggesting the existence of ecological filters or geographic barriers that give rise to biogeo- graphic patterns in the global distribution of marine microbes. In the Arctic Ocean, high concentrations of minute, chl-containing eukaryotes (picoeukaryotes; nucleus-containing cells less than 2–3 mm in diameter) have been reported in the picoplankton (Booth and Horner 1997, Booth and Smith 1997, Sherr et al. 2003). Earlier studies (Throndsen and Kristiansen 1991) using dilution culture assays noted that the small prasinophyte identified as the monotypic species Micromonas pusilla (Butcher) Manton et Parke was numerically abundant in European Arctic waters. More recently, the application of genera-specific fluorescence in situ hybridization (FISH) nucleotide probes confirmed that Micromonas was dominant in late summer in these waters along with another monotypic picoprasinophyte, Bathycoccus prasinos Eikrem et Throndsen (Not et al. 2005). 1 Received 17 April 2006. Accepted 2 November 2006. 2 Author for correspondence: e-mail connie.lovejoy@bio.ulaval.ca. 78 J. Phycol. 43, 78–89 (2007) r 2007 by the Phycological Society of America DOI: 10.1111/j.1529-8817.2006.00310.x