The summertime plankton community at South Georgia (Southern Ocean): comparing the historical (1926/27) and modern (post 1995) records.

The earliest comprehensive plankton sampling programme in the Southern Ocean was 32 undertaken during the early part of last century by Discovery Investigations to gain a 33 greater scientific understanding of whale stocks and their summer feeding grounds. An 34 initial survey was carried out around South Georgia during December 1926 and January 35 1927 to describe the distribution of plankton during the summer, and to serve as a 36 baseline against which to compare future surveys. We have reanalysed phytoplankton and 37 zooplankton data from this survey and elucidated patterns of community distribution and 38 compared them with our recent understanding of the ecosystem based on contemporary 39 data. Analysis of Discovery data identified five groups of stations with characteristic 40 phytoplankton communities which were almost entirely consistent with the original 41 analysis conducted by Hardy and Gunther (1935). Major groupings were located at the 42 western end of the island and over the northern shelf where Corethron spp. were 43 dominant, and to the south and east where a more diverse flora included high abundances 44 of Nitzschia seriata. Major zooplankton-station groupings were located over the inner 45 shelf which was characterised by a high abundance of Drepanopus forcipatus and in 46 oceanic water >500 m deep that were dominated by Foraminifera, Oithona spp., 47 Ctenocalanus vanus, and Calanoides acutus. Stations along the middle and outer shelf 48 regions to the north and west, were characterised by low overall abundance. There was 49 some evidence that groupings of stations to the north of the island originated in different 50 water masses on either side of the Southern Antarctic Circumpolar Current Front, the 51 major frontal system in the deep ocean close to South Georgia. However, transect lines 52 during 1926/27 did not extend far enough offshore to sample this frontal region 53 3 adequately. Interannual variability of zooplankton abundance was assessed from stations 54 which were sampled repeatedly during 7 recent British Antarctic Survey cruises (1995-55 2005) to the region and following taxonomic harmonization and numerical 56 standardization (ind. m-3), a subset of 45 taxonomic categories of zooplankton (species 57 and higher taxa) from 1926/27, were compared with similar data obtained during the 58 BAS cruises using a linear model. Initially comparisons were restricted to BAS stations 59 that lay within 40 km of Discovery stations although a comparison was also made using 60 all available data. Despite low abundance values in 1926/27, in neither comparison did 61 Discovery data differ significantly from BAS data. Calculation of the percentage 62 similarity index across cruises did not reveal any systematic differences in species 63 composition between 1926/27 and the present. In the light of ocean warming trends, the 64 existence of more subtle changes in species composition is not ruled out, but an absence 65 of finely resolved time-series data make this impossible to determine

SeaWiFs in the southern ocean: spatial and temporal variability in phytoplankton biomass around South Georgia

SeaWiFS was used to examine the spatial and temporal distribution of chlorophyll-a (chl-a) concentrations around the island of South Georgia during four summers between October 1997 and March 2001. A comparison was made between SeaWiFS and ship-collected (in situ) chl-a gathered during three of these seasons. A significant correlation was found between daily SeaWiFS and in situ match-ups (r(2) = 0.62). At low chl-a concentrations ( 5 mg m(-3)). Analysis of SeaWiFS data from two survey boxes (the Western and Eastern Core Boxes) sampled during cruises indicated significant differences between seasons and regions; the 1999 and 2000 seasons were nearly twice as productive as the 1998 and 2001 seasons. In addition, phytoplankton biomass was significantly higher in the Western (similar to1.4 mg m(-3)) than in the Eastern Core Box (similar to0.6 mg m(-3)). These seasonal and regional differences also were reflected in the distribution of copepods, one of the major zooplankton groups around South Georgia. SeaWiFS revealed patterns of growth outside of the survey boxes and cruise periods (January only). Blooms regularly occurred to the southwest and directly north of the island, and occasionally occupied the entire area between South Georgia and the Polar Front. In addition, blooms took place any time between October and March but not continuously. Primary production at South Georgia is mainly a local feature with little growth initiated upstream. However, chl-a may remain elevated downstream of the island (similar to40degreesW) as part of a continuous feature along the Polar Front to the 0degrees meridian. The passage of the Antarctic Circumpolar Current over the ridge and shelf waters in the vicinity of South Georgia may contribute micronutrients such as iron, and could partially explain the elevated chl-a levels associated with the island. We also examine the role of temperature, light and grazing on controlling phytoplankton productivity in this region of the Southern Ocean

Copepod growth and development around South Georgia: relationships with temperature, food and krill

Measurements of egg production rates (EPR) and growth of the early stages of 2 of the Southern Ocean biomass dominant copepods, Calanoides acutus and Rhincalanus gigas, were made over the course of 4 consecutive summer cruises which were carried out in the vicinity of South Georgia. For both species, EPR was found to be weakly but significantly related to chlorophyll a (chl a) although for C. acutus it was below maximum levels recorded in spring. Juvenile mass specificgrowth rates (g) were found to be body mass, stage and species dependent. Mean g for C. acutus stages CII to CIV decreased from 0.24 to 0.14, and for R. gigas stages CI to CIII from 0.06 to 0.04. Overall, values for both species were within the range predicted by recent global models of copepod growth. Neither stage duration nor g varied systematically with either temperature (mean, 0 to 60 m) or food (chl a, 0 to 60 m). However, carbon mass of nearly all species stages was negatively and significantly related to silicate levels (mol m–2, 0 to 60 m), suggesting the positive effect of past production levels. Ordination of zooplankton species occurrence by station across the survey area indicated that changes in abundance were more pronounced than changes in species composition, and that variation in total copepod abundance was also well explained by silicate levels. Our study indicated that changes in EPR, carbon mass and abundance of copepod populations at South Georgia were all strongly regulated by local primary production. Variation of chlorophyll biomass appeared largely dependent on temperature, rather than grazing pressure exerted by either copepods or krill. Krill at South Georgia were more abundant in colder, silicate replete waters and their presence is presumed to be governed by factors operating at the large scale. In contrast, copepod abundance appeared to differ in response to smaller scale variation in the environment and was linked through silicate to factors determining phytoplankton growth. In turn, chl a concentration was strongly and positively related to habitat temperature. This suggests the importance of the physical environment rather than grazing as ultimate factors controlling phytoplankton biomass in this productive ecosystem

Primary production across the Scotia Sea in relation to the physico-chemical environment

During the austral summer of 2003, a large scale survey of the Scotia Sea was undertaken by the British Antarctic Survey as part of the Southern Ocean Global Ocean Ecosystems Dynamics programme. This cruise provided a unique opportunity to examine the distribution of phytoplankton biomass and primary production in relation to the physico-chemical environment of the Scotia Sea. Phytoplankton were sampled from a range of oceanographic regimes including the open ocean, in the wake of oceanic islands, across major fronts such as the Southern Antarctic Circumpolar Current Front, submarine topographical features such as the South Scotia Ridge and the marginal ice zone. Generally the Scotia Sea was characterised by low biomass ( 28 mmol m− 3) despite favourable environmental conditions for growth caused by shallow mixed layers and deep euphotic depths. Three areas of elevated biomass (77–295 mg chlorophyll-a m− 2) and production (0.73–2.04 g C m− 2 d− 1) as well as substantial macronutrient depletion (e.g., surface nitrate not, vert, similar12 mmol m− 3) were observed during the cruise: to the northwest of the island of South Georgia, to the southwest of South Georgia and further south over the South Scotia Ridge in a region of rapid ice retreat. These productive regions were also characterised by shallow mixed layer depths, although near to South Georgia euphotic depths were reduced due to the high biomass of phytoplankton in the water column. The biomass was composed of over 80% diatoms. We speculate that the contrasting production regimes observed during the cruise were the result of differences in iron availability. Throughout much of the survey area, the eastward flowing Antarctic Circumpolar Current is unlikely to have encountered any shallow bathymetric features that may introduce sedimentary iron into the euphotic zone. However, shallow topographic features may cause upwelling of iron into the euphotic zone. This, together with shallow mixed layers and a favourable light environment, may then account for the dense blooms observed near topographic features during our cruise

An anticyclonic circulation above the Northwest Georgia Rise, Southern Ocean

Data from a variety of sources reveal a warm-core anticyclonic circulation above the Northwest Georgia Rise (NWGR), an similar to2000-m high bathymetric feature north of South Georgia. The sense of the circulation is opposite to the general cyclonic flow in the Georgia Basin. The circulation shows the characteristics of a stratified Taylor column: dimensional analysis shows that the local bathymetry and hydrography are conducive to the formation of such. ERS2 altimeter data show that the column, whilst not fully permanent, is nonetheless a recurring feature. High concentrations of chlorophyll-a are observed at the centre of the circulation, indicating that the modulation of the physical environment has significant consequences for the local biogeochemical system via enhanced primary production. Enhanced chlorophyll-a extends in a long plume from the NWGR along pathways indicated by drifters; this passive redistribution may have consequences for the larger (basin-) scale ecosystem

Crystal Structure of Bacillus anthracis Phosphoglucosamine Mutase, an Enzyme in the Peptidoglycan Biosynthetic Pathway ▿†

Phosphoglucosamine mutase (PNGM) is an evolutionarily conserved bacterial enzyme that participates in the cytoplasmic steps of peptidoglycan biosynthesis. As peptidoglycan is essential for bacterial survival and is absent in humans, enzymes in this pathway have been the focus of intensive inhibitor design efforts. Many aspects of the structural biology of the peptidoglycan pathway have been elucidated, with the exception of the PNGM structure. We present here the crystal structure of PNGM from the human pathogen and bioterrorism agent Bacillus anthracis. The structure reveals key residues in the large active site cleft of the enzyme which likely have roles in catalysis and specificity. A large conformational change of the C-terminal domain of PNGM is observed when comparing two independent molecules in the crystal, shedding light on both the apo- and ligand-bound conformers of the enzyme. Crystal packing analyses and dynamic light scattering studies suggest that the enzyme is a dimer in solution. Multiple sequence alignments show that residues in the dimer interface are conserved, suggesting that many PNGM enzymes adopt this oligomeric state. This work lays the foundation for the development of inhibitors for PNGM enzymes from human pathogens

Discovery of the first genome-wide significant risk loci for attention deficit/hyperactivity disorder

Attention deficit/hyperactivity disorder (ADHD) is a highly heritable childhood behavioral disorder affecting 5% of children and 2.5% of adults. Common genetic variants contribute substantially to ADHD susceptibility, but no variants have been robustly associated with ADHD. We report a genome-wide association meta-analysis of 20,183 individuals diagnosed with ADHD and 35,191 controls that identifies variants surpassing genome-wide significance in 12 independent loci, finding important new information about the underlying biology of ADHD. Associations are enriched in evolutionarily constrained genomic regions and loss-of-function intolerant genes and around brain-expressed regulatory marks. Analyses of three replication studies: a cohort of individuals diagnosed with ADHD, a self-reported ADHD sample and a meta-analysis of quantitative measures of ADHD symptoms in the population, support these findings while highlighting study-specific differences on genetic overlap with educational attainment. Strong concordance with GWAS of quantitative population measures of ADHD symptoms supports that clinical diagnosis of ADHD is an extreme expression of continuous heritable traits