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Meridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (chl a) phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively) four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC) and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of chl a and phaeopigments never exceeded, 0.5 µg l-1 and 0.25 µg l-1 respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, 15 of the nutrients isolines. Mesoscale eddies can bring episodic pulse of nutrients into the photic zone. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area chl a level of 0.4-0.5 µg l-1 distributed homogenously within the mixed layer, and POC and PON 20 accumulated to values up to 10 µM and 1.5 µM, respectively; still indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a postbloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 25 to values up to 1.25 µM at 100m depth centred on the PF and the accumulation of BSi up to 0.5 µM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. Similar southward displacement of the silicic acid depletion beyond the PF has been previously observed throughout the productive period, associated with the development and extension of the seasonal bloom of diatoms. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9±6.5 mmolm-2 d-1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre 5 occasionally displayed relative surface depletion of silicic acid suggesting a late stage of a relatively minor diatom bloom, possibly driven by iceberg drifting releases of iron. An accumulation of BSi up to 0.5 µM was recorded in the top 350m of the southern branch of the ACC and in the Weddell Gyre which may be seen as the presence of heavily silicified diatoms due to lack of iron in this HNLC area. In this domain the estimated range of silica daily net production rate (e.g. 21.1±8.8 mmolm-2 d-1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region. [ABSTRACT FROM AUTHOR]

The production of polyunsaturated fatty acids (PUFA) in Tisochrysis lutea was studied using the gradual incorporation of a 13 C-enriched isotopic marker, 13 CO 2 , for 24 h during the exponential growth of the algae. The 13 C enrichment of eleven fatty acids was followed to understand the synthetic pathways the most likely to form the essential polyunsaturated fatty acids 20:5n-3 (EPA) and 22:6n-3 (DHA) in T. lutea . The fatty acids 16:0, 18:1n-9 + 18:3n-3, 18:2n-6, and 22:5n-6 were the most enriched in 13 C. On the contrary, 18:4n-3 and 18:5n-3 were the least enriched in 13 C after long chain polyunsaturated fatty acids such as 20:5n-3 or 22:5n-3. The algae appeared to use different routes in parallel to form its polyunsaturated fatty acids. The use of the PKS pathway was hypothesized for polyunsaturated fatty acids with n-6 configuration (such as 22:5n-6) but might also exist for n-3 PUFA (especially 20:5n-3). With regard to the conventional n-3 PUFA pathway, Δ6 desaturation of 18:3n-3 appeared to be the most limiting step for T. lutea , "stopping" at the synthesis of 18:4n-3 and 18:5n-3. These two fatty acids were hypothesized to not undergo any further reaction of elongation and desaturation after being formed and were therefore considered "end-products". To circumvent this limiting synthetic route, Tisochrysis lutea seemed to have developed an alternative route via Δ8 desaturation to produce longer chain fatty acids such as 20:5n-3 and 22:5n-3. 22:6n-3 presented a lower enrichment and appeared to be produced by a combination of different pathways: the conventional n-3 PUFA pathway by desaturation of 22:5n-3, the alternative route of ω-3 desaturase using 22:5n-6 as precursor, and possibly the PKS pathway. In this study, PKS synthesis looked particularly effective for producing long chain polyunsaturated fatty acids. The rate of enrichment of these compounds hypothetically synthesized by PKS is remarkably fast, making undetectable the 13 C incorporation into their precursors. Finally, we identified a protein cluster gathering PKS sequences of proteins that are hypothesized allowing n-3 PUFA synthesis.

The synthetic pathways responsible for the production of the polyunsaturated fatty acids 22:6n-3 and 20:5n-3 were studied in the Dinophyte Alexandrium minutum . The purpose of this work was to follow the progressive incorporation of an isotopic label ( 13 CO 2 ) into 11 fatty acids to better understand the fatty acid synthesis pathways in A. minutum . The Dinophyte growth was monitored for 54 h using high-frequency sampling. A. minutum presented a growth in two phases. A lag phase was observed during the first 30 h of development and had been associated with the probable temporary encystment of Dinophyte cells. An exponential growth phase was then observed after t 30 . A. minutum rapidly incorporated 13 C into 22:6n-3, which ended up being the most 13 C-enriched polyunsaturated fatty acid (PUFA) in this experiment, with a higher 13 C atomic enrichment than 18:4n-3, 18:5n-3, 20:5n-3, and 22:5n-3. Overall, the 13 C atomic enrichment (AE) was inversely proportional to number of carbons in n-3 PUFA. C 18 PUFAs, 18:4n-3, and 18:5n-3, were indeed among the least 13 C-enriched FAs during this experiment. They were assumed to be produced by the n-3 PUFA pathway. However, they could not be further elongated or desaturated to produce n-3 C 20 -C 22 PUFA, because the AEs of the n-3 C 18 PUFAs were lower than those of the n-3 C 20 -C 22 PUFAs. Thus, the especially high atomic enrichment of 22:6n-3 (55.8% and 54.9% in neutral lipids (NLs) and polar lipids (PLs), respectively) led us to hypothesize that this major PUFA was synthesized by an O 2 -independent Polyketide Synthase (PKS) pathway. Another parallel PKS, independent of the one leading to 22:6n-3, was also supposed to produce 20:5n-3. The inverse order of the 13 C atomic enrichment for n-3 PUFAs was also suspected to be related to the possible β-oxidation of long-chain n-3 PUFAs occurring during A. minutum encystment.

The iconic picture of Arctic marine ecosystems shows an intense pulse of biological productivity around the spring bloom that is sustained while fresh organic matter (OM) is available, after which ecosystem activity declines to basal levels in autumn and winter. We investigated seasonality in benthic biogeochemical cycling at three stations in a high Arctic fjord that has recently lost much of its seasonal ice-cover. Unlike observations from other Arctic locations, we find little seasonality in sediment community respiration and bioturbation rates, although different sediment reworking modes varied through the year. Nutrient fluxes did vary, suggesting that, although OM was processed at similar rates, seasonality in its quality led to spring/summer peaks in inorganic nitrogen and silicate fluxes. These patterns correspond to published information on seasonality in vertical flux at the stations. Largely ice-free Kongsfjorden has a considerable detrital pool in soft sediments which sustain benthic communities over the year. Sources of this include macroalgae and terrestrial runoff. Climate change leading to less ice cover, higher light availability and expanded benthic habitat may lead to more detrital carbon in the system, dampening the quantitative importance of seasonal pulses of phytodetritus to seafloor communities in some areas of the Arctic. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

: The present study sought to characterize the synthesis pathways producing the essential polyunsaturated fatty acid (PUFA) 20:5n-3 (EPA). For this, the incorporation of 13 C was experimentally monitored into 10 fatty acids (FA) during the growth of the diatom Chaetoceros muelleri for 24 h . Chaetoceros muelleri preferentially and quickly incorporated 13 C into C 18 PUFAs such as 18:2n-6 and 18:3n-6 as well as 16:0 and 16:1n-7, which were thus highly 1 3 C-enriched. During the experiment, 20:5n-3 and 16:3n-4 were among the least-enriched fatty acids. The calculation of the enrichment percentage ratio of a fatty acid B over its suspected precursor A allowed us to suggest that the diatom produced 20:5n-3 (EPA) by a combination between the n-3 (via 18:4n-3) and n-6 (via 18:3n-6 and 20:4n-6) synthesis pathways as well as the alternative ω-3 desaturase pathway (via 20:4n-6). In addition, as FA from polar lipids were generally more enriched in 13 C than FA from neutral lipids, particularly for 18:1n-9, 18:2n-6 and 18:3n-6, the existence of acyl-editing mechanisms and connectivity between polar and neutral lipid fatty acid pools were also hypothesized. Because 16:3n-4 and 20:5n-3 presented the same concentration and enrichment dynamics, a structural and metabolic link was proposed between these two PUFAs in C. muelleri ., Competing Interests: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Herbivory of microzooplankton is an emerging key factor of diatom mortality in the ocean. As part of the microbial loop, protozoan grazers also feed on bacteria that accelerate the degradation of diatom detritus. The potentially pivotal effect of microzooplankton grazing on Si(OH)(4) recycling was investigated with cultures of single-celled diatoms, Thalassiosira pseudonana and Chaetoceros gracilis, and heterotrophic protozoans, the dinoflagellate Oxyrrhis marina and the ciliate Strombidium sp. Both grazers ingested diatoms and the bacteria in the non-axenic cultures. C. gracilis, whose frustule is "armed" with setae, was less suitable as a prey than T. pseudonana. Ingestion rates of T. pseudonana were comparable for O. marina and Strombidium, but the dinoflagellate produced two orders of magnitude more detrital bSiO(2) than the ciliate, due to the higher abundance reached by O. marina. Total net release of Si(OH)(4) was lower in the grazing treatments compared to the control possibly due to the reduced bacterial growth by microzooplankton bacterivory, and to the transient protection of detrital bSiO(2) in discarded feeding vacuoles. Over the first 24h, microzooplankton grazing even led to enhanced uptake of Si(OH)(4) by diatoms, confirming the potential of grazing to influence the silicification of diatom frustules. Subsequently however, the Si dynamics in bottles with grazers turned rapidly from net uptake to net Si(OH)(4) release. Protozoan grazers hence tie Si(OH)(4) recycling into the microbial loop by producing detrital bSiO(2)., (Copyright 2009 Elsevier GmbH. All rights reserved.)