Cuong Tu Ho

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

Abstract 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 × 1013 ± 4.0 × 1013 MgC.yr−1 (range 27.9–35.8 × 1013 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. Copyright

Inhibition effect of engineered silver nanoparticles to bloom forming cyanobacteria

Silver nanoparticle (AgNP) has a wide range antibacterial effect and is extensively used in different aspects of medicine, food storage, household products, disinfectants, biomonitoring and environmental remediation etc. In the present study, we examined the growth inhibition effect of engineered silver nanoparticles against bloom forming cyanobacterial M. aeruginosa strain. AgNPs were synthesized by a chemical reduction method at room temperature and UV–Vis spectroscopy, scanning electron microscopy (SEM), transmission electron microscope (TEM) showed that they presented a maximum absorption at 410 nm and size range between 10 and 18 nm. M. aeruginosa cells exposed during 10 d to AgNPs to a range of concentrations from 0 to 1 mg l−1. The changes in cell density and morphology were used to measure the responses of the M. aeruginosa to AgNPs. The control and treatment units had a significant difference in terms of cell density and growth inhibition (p < 0.05). Increasing the concentration of AgNPs, a reduction of the cell growths in all treatment was observed. The inhibition efficiency was reached 98.7% at higher concentration of AgNPs nanoparticles. The term half maximal effective concentration (EC50) based on the cell growth measured by absorbance at 680 nm (A680) was 0.0075 mg l−1. The inhibition efficiency was 98.7% at high concentration of AgNPs (1 mg l−1). Image of SEM and TEM reflected a shrunk and damaged cell wall indicating toxicity of silver nanoparticles toward M. aeruginosa.

Biologically based method for the synthesis of Hg–Se nanostructures by Shewanella spp.

Living organisms, especially microorganisms, have the potential to offer cheap and benign synthetic routes for the production of nanomaterials. In this study, we demonstrated the formation of various Hg–Se nanostructures by a combinatory method with the major participation of Shewanella bacteria. The quantum dots of Hg–Se were produced and located on the membrane surface of Shewanella cells by incubating the Shewanella putrefaciens 200 with Hg(II) and Se(IV) under anaerobic conditions. The selenium nanowires and stellated polyhedral structures, successfully synthesized by Shewanella-mediated formation in 80 and 90% of DMSO, were used as templates for the formation of Hg–Se nanowires and stellated polyhedral structures. The liquid mercury was directly reacted with Se nanowires or stellated polyhedral structures in water and coated the Se nanostructures to form the core–shell structures of Hg–Se after 12 hours incubation at ambient temperature. SEM, TEM, XRD and EDX were used to characterize and confirm the Hg–Se nanoparticles and nanowires or stellated polyhedral structures.

CO 2 partial pressure and CO 2 emissions from the lower Red River (Vietnam)

The Red River (Vietnam) is a good example of a South-East Asian river system, strongly 17 affected by climate and human activities. This study aims to quantify the spatial and seasonal 18 variability of carbon dynamic and CO2 outgassing at the water-air interface of the lower Red River 19 system. The monitoring of water quality and CO2 emission were carried out for 24h cyclings at the five 20 stations during the dry and monsoon seasons. The riverine water pCO2 was supersaturated with CO2 in 21 contrast to the atmospheric equilibrium (400 ppm), averaging about 1588.6 ± 884.6 ppm, thus resulting 22 in a water–air CO2 flux of 26.9 ± 18.4 mmol m -2 day. The CO2 outgassing rate was characterized by 23 significant spatial variations, highest at Hoa Binh station (Da River) due to the dam impoundment and 24 the highest river flow. Surprisingly, CO2 outgassing was higher in the day time (30.4 ± 21.2 mmol m 25 2 day) than in the night time (23.3 ± 15.4 mmol m day), probably as a result of the combined effect 26 of higher wind speed and water temperature in the day time. Seasonal differences were also observed, 27 higher in the wet season (30.7 ± 23.1 mmol m day) than in the dry season (23.0 ± 12.2 mmol m 28 2 day), due to higher river discharges and higher external inputs of organic matters from watersheds. 29 Conversely during dry season, temperature was among the main factors influencing C dynamic, with 30 higher pCO2 and fluxes, probably as a result of increased metabolic rates. 31

Responses of Aquatic Bacteria to Terrestrial Runoff: Effects on Community Structure and Key Taxonomic Groups.

Organic fertilizer application is often touted as an economical and effective method to increase soil fertility. However, this amendment may increase dissolved organic carbon (DOC) runoff into downstream aquatic ecosystems and may consequently alter aquatic microbial community. We focused on understanding the effects of DOC runoff from soils amended with compost, vermicompost, or biochar on the aquatic microbial community of a tropical reservoir. Runoff collected from a series of rainfall simulations on soils amended with different organic fertilizers was incubated for 16 days in a series of 200 L mesocosms filled with water from a downstream reservoir. We applied 454 high throughput pyrosequencing for bacterial 16S rRNA genes to analyze microbial communities. After 16 days of incubation, the richness and evenness of the microbial communities present decreased in the mesocosms amended with any organic fertilizers, except for the evenness in the mesocosms amended with compost runoff. In contrast, they increased in the reservoir water control and soil-only amended mesocosms. Community structure was mainly affected by pH and DOC concentration. Compared to the autochthonous organic carbon produced during primary production, the addition of allochthonous DOC from these organic amendments seemed to exert a stronger effect on the communities over the period of incubation. While the Proteobacteria and Actinobacteria classes were positively associated with higher DOC concentration, the number of sequences representing key bacterial groups differed between mesocosms particularly between the biochar runoff addition and the compost or vermi-compost runoff additions. The genera of Propionibacterium spp. and Methylobacterium spp. were highly abundant in the compost runoff additions suggesting that they may represent sentinel species of complex organic carbon inputs. Overall, this work further underlines the importance of studying the off-site impacts of organic fertilizers as their impact on downstream aquatic systems is not negligible.