Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the -Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the +Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency

Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special_issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue

A benthic Si mass balance on the Congo margin: Origin of the 4000 m DSi anomaly and implications for the transfer of Si from land to ocean

11 pages, 6 figures, 3 tables.-- Article in press.To elucidate the origin of the silicic acid (DSi) anomaly observed along the 4000 isobath on the Congo margin, we have established a benthic Si mass balance and performed direct measurements of biogenic silica (bSiO2) dissolution in the deep waters and in the sediments. Results strongly suggest that the anomaly originates from the sediments; the intensity of DSi recycling is consistent with the degradation of organic matter, as observed from Si:O2 ratios in the benthic fluxes compared to that ratio observed in the anomalies. Strong imbalances, observed in both the Si and C mass balances, suggest that the biogenic matter that degrades and dissolves in these sediments near 4000 m does not come from pelagic sedimentation. It is probably not coming also from the deep channel, because observations were similar in the deep channel vicinity (site D) and further south, far from its influence (site C). The composition of the sediments, with an Si:C ratio close to that observed on continental shelves, suggests that this matter is coming from downslope transport. A first estimate of the magnitude of this flux at global scale, close to 12 T mol Si yr−1, suggests that it may be an important path for transferring Si from land to ocean.This study has been partly funded through the Si-WEBS RTN (HPRN-CT-2002-00218) and the European Commission is greatly acknowledged.Peer reviewe

Microbial mediation of benthic biogenic silica dissolution

Pore water profiles from 24 stations in the South Atlantic (located in the Guinea, Angola, Cape, Guyana, and Argentine basins) show good correlations of oxygen and silicon, suggesting microbially mediated dissolution of biogenic silica. We used simple analytical transport and reaction models to show the tight coupling of the reconstructed process kinetics of aerobic respiration and silicon regeneration. A generic transport and reaction model successfully reproduced the majority of Si pore water profiles from aerobic respiration rates, confirming that the dissolution of biogenic silica (BSi) occurs proportionally to O 2 consumption. Possibly limited to well-oxygenated sediments poor in BSi, benthic Si fluxes can be inferred from O 2 uptake with satisfactory accuracy. Compared to aerobic respiration kinetics, the solubility of BSi emerged as a less influential parameter for silicon regeneration. Understanding the role of bacteria for silicon regeneration requires further investigations, some of which are outlined. The proposed aerobic respiration control of benthic silicon cycling is suitable for benthic–pelagic models. The empirical relation of BSi dissolution to aerobic respiration can be used for regionalization assessments and estimates of the silicon budget to increase the understanding of global primary and export production patterns

Penacée: a neural net system for recognizing on-line handwriting

We report on progress in handwriting recognition and signature verification. Our system, which uses pen-trajectory information, is suitable for use in pen-based computers. It has a multi-modular architecture whose central trainable module is a time-delay neural network. Results comparing our system and a commercial recognizer are presented. Our best recognizer makes three times less errors on hand-printed word recognition than the commercial one

A structure-guided approach to an orthogonal estrogen-receptor-based gene switch activated by ligands suitable for in vivo studies

A strategy to obtain a fully orthogonal estrogen-receptor-based gene switch responsive to molecules with acceptable pharmacological properties is presented. From a series of tetrahydrofluorenones active on the wild-type estrogen receptor (ER) an inactive analogue is chosen as a new lead compound. Coevolution of receptor mutants and ligands leads to an ER-based gene switch suitable for studies in animal models

A structure-guided approach to an orthogonal estrogen-receptor-based gene switch activated by ligands suitable for in vivo studies

A strategy to obtain a fully orthogonal estrogen-receptor-based gene switch responsive to molecules with acceptable pharmacological properties is presented. From a series of tetrahydrofluorenones active on the wild-type estrogen receptor (ER) an inactive analogue is chosen as a new lead compound. Coevolution of receptor mutants and ligands leads to an ER-based gene switch suitable for studies in animal models