Impact of iron limitation on primary production (dissolved and particulate) and secondary production in cultured Trichodesmium sp.

International audienceDiazotrophic cyanobacteria play an important role in biogeochemical cycles of carbon and nitrogen and, hence, in oceanic productivity in the tropical and subtropical regions of the ocean. Although many studies have examined the impact of iron (Fe) limitation on particulate primary production and dinitrogen (N2) fixation in the colonial cyanobacterium Trichodesmium, none have looked at the impact of Fe limitation on the percentage extracellular release (PER) and secondary production (SP) in Fe-limited cultures of this cyanobacterium. Here, we present the results of a series of culture experiments during which we examined the impact of 3 concentrations of dissolved iron (DFe) on total primary production (TPP = dissolved + particulate primary production, i.e. DPP + PPP), PER and on SP. Under severe Fe limitation (5 nM DFe), biomass, growth rates, TPP and N2 fixation were strongly reduced, while PER increased relative to the rates ob served at the highest Fe concentration. Moreover, reducing Fe concentration induced an increase in the percentage of photosynthetically fixed C used for algal growth, while the percentage of C used to support algal respiration decreased. Reduced Fe concentrations also induced a decrease in SP and in the SP:DPP ratio, indicating that the efficiency of transfer of fixed carbon from autotrophic to heterotrophic processes is reduced. This suggests that Fe, either directly through influencing cellular processes or indirectly through influencing organic matter structure or nitrogen availability, is controlling SP and, thus, microbial carbon utilization. These results suggest that the amount of carbon entering into the microbial loop may be reduced under Fe limitation, thus leading to an accumulation of dissolved organic carbon with potentially important impacts on microbial carbon cycling and, ultimately, on the biological carbon pum

Seasonal variability of faecal indicator bacteria numbers and die-off rates in the Red River basin, North Viet Nam

International audienceThe Red River is the second largest river in Viet Nam and constitutes the main water source for a large percentage of the population of North Viet Nam. Here we present the results of an annual survey of Escherichia coli (EC) and Total Coliforms (TC) in the Red River basin, North Viet Nam. The objective of this work was to obtain information on faecal indicator bacteria (FIB) numbers over an annual cycle and, secondly, to determine the die-off rates of these bacterial indicators. Monthly observations at 10 stations from July 2013–June 2014 showed that TC and EC reached as high as 39100 cfu (colony forming units) 100 ml−1 and 15300 colonies 100 ml−1, respectively. We observed a significant seasonal difference for TC (p < 0.05) with numbers being higher during the wet season. In contrast, no significant seasonal difference was found for EC. The FIB die-off rates ranged from 0.01 d−1 to a maximum of 1.13 d−1 for EC and from 0.17 d−1 to 1.33 d−1 for TC. Die-off rates were significantly higher for free bacteria than for total (free + particle attached) bacteria, suggesting that particle attachment provided a certain level of protection to FIB in this system

02 A 817 Rochelle-Newall BN

ABSTRACT: The functional response of a seawater bacterial community transplanted into freshwater dissolved organic matter (DOM) was investigated together with the response of natural populations of bacteria to size-fractioned natural source water. Seawater bacteria were incubated over a period of 8 d in size-fractionated, freshwater DOM collected from Randers Fjord, Denmark, during spring (April) and summer (August) of 2001. Three fractions were used: 0.2 µm filtered (&lt; 0.2 µm-DOM), &gt;1 kDa (high molecular weight, HMW-DOM) and &lt;1 kDa (low molecular weight, LMW-DOM). The results were compared with parallel control incubations of freshwater bacteria in size-fractionated freshwater DOM and seawater bacteria in size-fractionated seawater DOM. There were few differences in bacterial abundance (BA) and production (BP) within each incubation type in spring, but this was not the case in summer. While the seawater bacteria transplanted into freshwater HMW-DOM performed similarly to those in seawater HMW-DOM, freshwater bacteria in freshwater HMW-DOM exhibited higher BA, BP and bacterial respiration (BR), indicating a difference in the physiological abilities of the seawater bacterial assemblage compared to that of the natural freshwater assemblage. Bacterial growth efficiency (BGE) varied between 11 and 41% and the highest values were generally in the HMW-DOM size fraction. Comparison of the &apos;bioavailability&apos; of the DOM predicted from BGE, the amino acid degradation index (DI), dissolved organic carbon degradation rates and the bioavailability index showed that net change in DI of dissolved combined amino acids over the course of an incubation can be a good predictor of most of the other indices. The separation of DOM into molecular weight size fractions resulted in different estimates of bioavailability than would have been predicted from the rates observed in the &lt; 0.2 µm-DOM fraction. These results further demonstrate the flexibility of bacteria in their ability to utilize different sources of DOM, and highlight the variability that can be observed when different indices are used to determine the bioavailability of organic matter to heterotrophic bacteria

Rôle des particules exoplymériques transparentes dans le transfert des contaminants métalliques vers la phase particulaire

International audienc

Impact of a river flash flood on microbial carbon and nitrogen production in a Mediterranean Lagoon (Thau Lagoon, France)

Over half of the total nitrogen, phosphorus, silicate and dissolved organic carbon (DOC) loading was discharged from the Wile River into the Thau Lagoon waters within the first five days of what was considered to be the autumn flash flood period. Such loads represented about 8% and 3% of the yearly averaged total nitrogen and phosphorus load in Thau Lagoon, respectively. Although this event affected >20% of the total lagoon volume, river trace metal loads contributed apparently only weakly to the increase in labile trace metal concentrations in the lagoon surface waters. Differences between theoretical dilution values and observed values were also noticed for phosphate, silicate and dissolved organic carbon (DOC) concentrations. DOC losses (10-50%) mainly through flocculation, together with the substantial increases in some metallic contaminants such as Zn (from 6 to >30 mu g L-1) observed during the flash flood in saline lagoon waters, may have limited the carbon production of bacterial communities. The potential osmotic shock and the increase in turbidity may mainly explain the low phytoplankton C turnover rates (average of 0.02 h(-1)) measured in brackish waters (<30) during periods of heavy flood discharge. The dissolved inorganic nitrogen (DIN: NO3 + NH4) enrichment measured 12 days after the flash flood event in saline lagoon surface waters (from 22 to 143 mu g N L-1) led to a substantial increase in phytoplanktonic C production and associated DIN uptake rates (from 2.6 to 7.0 mu g C L-1 h(-1) and from 0.5 to 1.1 mu g N L-1 h(-1), respectively). Subsequent accumulation in particulate organic carbon and nitrogen was not observed in the area studied during and after the flash flood period, averaging 549 +/- 50 mu g C L-1 and 168 +/- 9 mu g N L-1, respectively. This suggests that most of locally produced microbial production was rapidly filtered by oysters contributing to about 13% of the yearly exploited production in the lagoon and/or exported through sedimentation

Artificial neural network analysis of factors controlling ecosystem metabolism in coastal systems

Knowing the metabolic balance of an ecosystem is of utmost importance in determining whether the system is a net source or net sink of carbon dioxide to the atmosphere. However, obtaining these estimates often demands significant amounts of time and manpower. Here we present a simplified way to obtain an estimation of ecosystem metabolism. We used artificial neural networks (ANNs) to develop a mathematical model of the gross primary production to community respiration ratio (GPP:CR) based on input variables derived from three widely contrasting European coastal ecosystems (Scheldt Estuary, Randers Fjord, and Bay of Palma). Although very large gradients of nutrient concentration, light penetration, and organic-matter concentration exist across the sites, the factors that best predict the GPP:CR ratio are sampling depth, dissolved organic carbon (DOC) concentration, and temperature. We propose that, at least in coastal ecosystems, metabolic balance can be predicted relatively easily from these three predictive factors. An important conclusion of this work is that ANNs can provide a robust tool for the determination of ecosystem metabolism in coastal ecosystems.

Appendix B. A table of values of the weights from the input layer to the hidden layer and from the hidden layer to the output layer of the model of the ratio of gross primary production to community respiration (GPP:CR) developed in this study based on an artificial neural network (ANN).

A table of values of the weights from the input layer to the hidden layer and from the hidden layer to the output layer of the model of the ratio of gross primary production to community respiration (GPP:CR) developed in this study based on an artificial neural network (ANN)

Total organic carbon fluxes of the Red River system (Vietnam)

Riverine transport of organic carbon from terrestrial ecosystems to the oceans plays an important role in the global carbon cycle. The Red River is located in Southeast Asia where river discharge, sediment loads and fluxes of elements (carbon, nitrogen and phosphorus) associated with suspended solids have been dramatically altered over past decades as a result of reservoir impoundment and land use, population, and climate change. Dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations were measured monthly at four stations of the Red River system from January 2008 to December 2010. The results reveal that POC changed synchronically with total suspended solids (TSS) concentration and with the river discharge, whereas no clear trend was observed for DOC concentration. The mean value of total organic carbon (TOC = DOC + POC) flux in the delta of the Red River was 31.5 x 10(13) +/- 4.0 x 10(13) MgC.yr (1) (range 27.9-35.8 x 10(13) MgC.yr (1) which leads to a specific TOC flux of 2012 +/- 255 kgC.km(-2).yr(-1) during this 2008-2010 period. About 80% of the TOC flux was transferred to the estuary during the rainy season as a consequence of the higher river water discharge. The high mean value of the POC:Chl-a ratio (1585 +/- 870 mgC.mgChl-a(-1)) and the moderate C:N ratio (7.3 +/- 0.1) in the water column system suggest that organic carbon in the Red River system is mainly derived from erosion and soil leaching in the basin. The effect of two new dam impoundments in the Red River was also observable with lower TOC fluxes in 2010 compared with 2008

Distribution of Burkholderia pseudomallei and soil properties within a 300-cm deep profile

This dataset has been collected thanks to the long-term partnership with Department of Agricultural Land Management (DALaM), Lao PDR, which granted the permission for field access, and to the financial, scientific, technical, and/or logistical support of the Multiscale TROPIcal CatchmentS (SNO M-TROPICS; https://mtropics.obs-mip.fr/) critical zone observatory (previously MSEC). This .csv dataset includes Burkholderia pseudomallei concentrations (CFU g-1) within a 300-cm soil profile, along with physico-chemical parameters: soil texture, i.e. clay < 2 μm, silt 2–50 μm, sand 50–2000 μm (%); pH (-); organic matter content (%); bulk density (1000 kg m-3); total porosity (100(1-BD/2.65)) considering a real density value of 2.65 (%); groundwater persistence rate (%); saturated hydraulic conductivity (mm h-1); soil water content (m3 m-3); exchangeable cations (Ca2+, Mg2+, K+, and Na+; meq 100 g-1); and soil elemental concentrations (Fe, Mn, Mg, Ca, K, Na, Al, Si, Ti, and S; ppm). The authors acknowledge financial support from French national grant CNRS/INSU EC2CO-Biohefect/Ecodyn//Dril/MicrobiEen(BurLuMic) and the IRD International Joint Laboratory LUSES. Essential support with microbiological analyses was provided by the Lao-Oxford-Mahosot-Wellcome Trust-Research Unit (LOMWRU), Vientiane, Lao PDR, funded by the Wellcome Trust [Grant number 106698/Z/14/Z]. The Department of Agricultural Land Management (DALaM) of the Ministry of Agriculture and Forestry of Lao PDR facilitated access to the field and provided invaluable support with soil analyses. IRD staff in Laos would like to sincerely thank Mr Outhong Sengouthai, owner of the paddy field as well as village authorities to have permitted us to make field measurements in Ban Nabone. KP would like to acknowledge financial support for her PhD from Campus France (BGF) and expresses her gratitude to IRD GET and iEES-Paris staff in France and Lao PDR for assistance and guidance with her work