Our pigment analyses from a year‐long study in the coastal Beaufort Sea in the western Canadian Arctic showed the continuous prevalence of eukaryotic picoplankton in the green algal class Prasinophyceae. 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 section, grew optimally at 6°C–8°C, with light saturation at or below 10 μmol photons·m−2·s−1 at 0°C. The 18S rDNA analyses of this isolate and environmental 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 picoprasinophyte 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 containing endemic species, consistent with the biogeography 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.

Marine phytoplankton show complex community structures and biogeographic distributions, the net results of physiological and ecological trade-offs of species responses to fluctuating, heterogeneous environments. We analysed photosynthesis, responses to variable light and macromolecular allocations across a size panel of marine centric diatoms. The diatoms have strong capacities to withstand and exploit fluctuating light, when compared with picophytoplankton. Within marine diatoms, small species show larger effective cross-sections for photochemistry, and fast metabolic repair of photosystem II after photoinactivation. In contrast, large diatoms show lower susceptibility to photoinactivation, and therefore incur lower costs to endure short-term exposures to high light, especially under conditions that limit metabolic rates. This size scaling of key photophysiological parameters thus helps explain the relative competitive advantages of larger versus smaller species under different environmental regimes, with implications for the function of the biogenic carbon pump. These results provide a mechanistic framework to explain and predict shifts in marine phytoplankton community size structure with changes in surface irradiance and mixed layer depth.

Development of the mammalian embryo relies upon nutritive functions fulfilled by the visceral endoderm and then by the liver (1). A part of these functions is accomplished by nutrient carrier proteins of the albumin gene family, a multigene locus expressed by the liver and subject to precise developmental controls. One albumin-related gene, the ␣ 1 -fetoprotein (AFP) 1 gene, is activated at the onset of liver differentiation and operates tightly coupled with liver growth (2, 3). In 1988, our group circumscribed a proximal AFP promoter element essential to AFP gene activity in hepatocytes, and distinct from promoter components regulating the other albumin loci (4). The AFP-specific activator was then identified as orphan receptor fetoprotein transcription factor (5-7), so named for its first identified target locus (genome data base nomenclature, 2 NR5A2 in the nuclear receptor nomenclature, Ref. 9); also referred to as LRH1 or CPF). FTF belonged to a primitive class of nuclear receptors and emerged as a critical lead to connect AFP gene activation with early embryonic growth and differentiation processes.Subsequent studies indicated that developmental FTF functions even preceded its activation of the AFP locus in hepatocytes. In situ hybridization analysis in the mouse at embryonic day 8 -9 showed abundant FTF transcripts in the foregut endoderm, before liver morphogenesis (10). Characterization of the FTF gene promoter also revealed a cluster of regulatory motifs conserved in distant species and potential targets of cell lineage specification factors (11). Among these were three proximal binding sites for GATA factors, known to be essential for visceral endoderm function (12, 13). Furthermore, three HNF genes important to liver differentiation, HNF1␣, HNF4␣, and HNF3␤, were found to each contain double FTF-binding sites in their proximal promoter and to be activated by FTF in transfection assays (11). Thus, a pivotal role was suggested for FTF in a transcriptional cascade using determination factors to activate FTF in prehepatic endodermal cells, and then using FTF to drive AFP and other effectors of the hepatic program.

) were observed in brackish waters of the Beaufort Sea. These results confirm that picophytoplankton can dominate not only in warm oligotrophic waters, but also in a perennially cold ocean during late summer. KEY WORDS:Abundance · Photosynthetic eukaryotes · Picophytoplankton · Biomass · Nanophytoplankton · Microphytoplankton · Canadian Arctic Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 54: [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70] 2009 1988). However, Richardson & Jackson (2007) challenged this view by showing that the share of picophytoplankton in carbon export can match their relative contribution to total net primary production due to the inclusion of small cells into large aggregates that sink rapidly or are grazed by mesozooplankton. Considering the findings of Richardson & Jackson (2007), the conventional view that picophytoplankton contribute little to carbon export should be revisited. Hence, both large and small phytoplankton play a crucial role in the marine biogeochemical cycle.Large phytoplankton cells, including diatoms, prymnesiophytes and dinoflagellates, produce seasonal blooms under specific hydrographic conditions . For instance, the production of large phytoplankton is governed by variations in the vertical stability of the water column, through its effects on nutrient replenishment and the residence time of algal cells in the euphotic zone (e.g. Tremblay et al. 1997). In addition, the duration of the production period is sensitive to the seasonal melt dynamics of sea ice (Fortier et al. 2002). In northern Baffin Bay (BB), an intense diatom bloom characterized by cells > 5 µm begins as early as the end of April when the North Water polynya opens up . In the Canadian Archipelago, particularly in Barrow Strait, the phytoplankton bloom typically develops in July and August, corresponding to the timing of the ice break-up for this region (Michel et al. 2006). In the Chukchi and Beaufort seas, high chlorophyll concentrations are observed in regions along the ice edge and are associated with an overwhelming predominance of diatoms and prymnesiophytes (Hill et al. 2005). In the Barents Sea, largecelled phytoplankton dominate during blooms at the marginal ice zone and are of particular importance for the production of organic matter and the vertical export of carbon .Several studies have shown that small phytoplankton cells (< 5 µm) can also play an important role in carbon fixation in the Arctic Ocean and adjacent seas (Legendre et al. 1993, Gosselin et al. 1997. Picophytoplankton contribute most of the production and biomass in warm and nutrient-poor waters (Agawin et al. 2000). Recent studies have shown that picophytoplankton are often well represented numerically in cold Arctic seawaters. Indeed, eukaryotic cells < 2 µm often dominate the phytoplankton assemblage, reaching up to 28 000 cells ml -1 during the initial spring bloom in the central Arctic Ocean but usually ranging between 1000 and 10 000 cells ml -1 during t...

Ultraviolet-B (UVB, 280-320 nm) radiation is a natural component of sunlight that harms organisms and disturbs natural communities in surface waters. A natural planktonic assemblage of organisms (Ͻ240 m) was studied in a mesocosm experiment for 7 d under varying conditions of UVB radiation: UVB excluded, natural radiation, and UVB enhanced at two different levels. The dynamics of several populations at different trophic levels comprising heterotrophic bacteria (Ͻ1 m), heterotrophic flagellates (2-10 m), small phytoplankton (Ͻ5 m), large phytoplankton (5-20 m), and ciliates (15-35 m) were monitored during the experiment. Enhanced UVB provoked a significant decrease in the number of ciliates (66%) and large phytoplankton (63%) relative to natural UVB conditions. The severe effects of UVB radiation on ciliates and large phytoplankton communities shown here would strongly limit upward transfer of mass and energy. The decline of predator abundance (ciliates) under UVB stress relative to natural conditions resulted in a positive feedback between enhanced UVB radiation and prey abundances, shown by increased abundances of bacteria (49%), heterotrophic flagellates (up to 300%), and small phytoplankton (41%). Similarly, with respect to carbon partitioning, the decrease in ciliate and diatom carbon biomass (64 and 56%, respectively) under enhanced UVB exposure was balanced by an increase in the carbon biomass of heterotrophic bacteria (48%), heterotrophic flagellates (126%), and autotrophic flagellates (162%). As a manifestation of enhanced UVB at the community level, the ecosystem develops toward a microbial food web in preference to an herbivorous food web. Thus, enhanced UVB radiation can change the structure and dynamics of the pelagic food web.The pelagic planktonic community functions through a web of energy and nutrient exchanges mediated by a diverse array of producers and consumers, which ultimately depend on the energy supplied by sunlight. Following the discovery of stratospheric ozone depletion (Farman et al. 1985) and the resulting increase in intensity of biologically harmful UVB radiation (280-320 nm) reaching Antarctic waters, the majority of UVB studies have focused on phytoplankton be- AcknowledgmentsWe thank F. Rassoulzadegan, F. Azam, and T. Sime-Ngando for comments; C. Lovejoy and L. Bérard for help with the identification of some planktonic species; and D. Bourget and N. Lafontaine for nutrient analyses.This work was supported by NSERC of Canada, Fonds FCAR of Québec, and FODAR (University of Québec). International collaboration was made possible by NATO collaborative research grant (CRG 95139) to S.D. and P.M. This investigation is a contribution to the research programs of the Groupe de Recherche en Environnement Côtier.

By use of biogenic silica as an inert marker, it is shown that chlorophyll a and its derivatives can be destroyed or absorbed during passage through the gut of a herbivorous copepod. This observation is contradictory to the hypothesis that chlorophyll a is converted to pheophorbide a with 100% molar efficiency. The currently used equations for measuring chlorophyll and pheopigment by fluorescence cannot be used to give concentration of pheopigment.

; Institut des sciences de la mer de Rimouski, Université du Québec, Rimouski, Quebec, Canada G5L 3A1 (S.R.); and Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 (M.A., B.R.G.)Diatoms are important contributors to aquatic primary production, and can dominate phytoplankton communities under variable light regimes. We grew two marine diatoms, the small Thalassiosira pseudonana and the large Coscinodiscus radiatus, across a range of temperatures and treated them with a light challenge to understand their exploitation of variable light environments. In the smaller T. pseudonana, photosystem II (PSII) photoinactivation outran the clearance of PSII protein subunits, particularly in cells grown at sub-or supraoptimal temperatures. In turn the absorption cross section serving PSII photochemistry was down-regulated in T. pseudonana through induction of a sustained phase of nonphotochemical quenching that relaxed only slowly over 30 min of subsequent low-light incubation. In contrast, in the larger diatom C. radiatus, PSII subunit turnover was sufficient to counteract a lower intrinsic susceptibility to photoinactivation, and C. radiatus thus did not need to induce sustained nonphotochemical quenching under the high-light treatment. T. pseudonana thus incurs an opportunity cost of sustained photosynthetic down-regulation after the end of an upward light shift, whereas the larger C. radiatus can maintain a balanced PSII repair cycle under comparable conditions.

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