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Domoic acid (DA), a neuroexcitatory amino acid, was detected in batch culture of the newly recognized species Nitzschia navis‐varingica Lundholm et Moestrup. The production of DA by this diatom was confirmed by electrospray ionization mass spectrometry. The diatom was collected from a shrimp‐culture pond in Do Son, Vietnam. The production of DA (1.7 pg·cell−1) is within the levels reported for Pseudo‐nitzschia multiseries (Hasle) Hasle. The DA production started during the late exponential growth phase and reached a maximum during the early stationary growth phase. Maximum DA levels in the axenic culture decreased to about half that of the nonaxenic culture (0.9 pg·cell−1 vs. 1.7 pg·cell−1), suggesting that DA production by the new species is influenced by bacteria.

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In the Bach Dang-Cam Estuary, northern Vietnam, mechanisms governing cohesive sediment aggregation were investigated in situ in [2008][2009]. As part of the Red River delta, this estuary exhibits a marked contrast in hydrological conditions between the monsoon and dry seasons. The impact on flocculation processes was assessed by means of surveys of water discharge, suspended particulate matter concentration and floc size distributions (FSDs) conducted during a tidal cycle at three selected sites along the estuary. A method was developed for calculating the relative volume concentration for the modes of various size classes from FSDs provided by the LISST 100X (Sequoia Scientific Inc.). It was found that all FSDs comprised four modes identified as particles/flocculi, fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic energy, their proportions varied but without significant modification of their median diameters. In particular, when the turbulence level corresponded to a Kolmogorov microscale of less than ∼235 μm, a major breakup of flocs resulted in the formation of particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater discharge and by the tidal cycle. During the wet season, strong freshwater input induced a high turbulent energy level that tended to generate sediment transfer from the coarser size classes (macroflocs, coarse microflocs) to finer ones (particles/flocculi and fine microflocs), and to promote a transport of sediment seawards. During the dry season, the influence of tides predominated. The turbulent energy level was then only episodically sufficiently high to generate transfer of sediment between floc size classes. At low turbulent energy, modifications in the proportions of floc size classes were due to differential settling. Tidal pumping produced a net upstream transport of sediment. Associated with the settling of sediment trapped in a near-bed layer at low turbulent energy, this causes the silting up of the waterways leading to the harbour of Haiphong.

Mangrove forests are common in subtropical regions, and have received considerable attention as vegetative buffers against anthropogenic N-loading. In this study, we investigated anaerobic ammonium oxidation (anammox) as one of potentially important microbial N-removing pathways in mangrove and shrimp pond sediment in Haiphong, Vietnam. Measurements with 15 N-labeled compounds demonstrated the occurrence of anammox in sediment of mangrove forest and a water channel connecting shrimp ponds to the sea in both 2005 and 2007, and of a semi-intensive shrimp pond in 2005. The rate of potential anammox activity reached to 0.7 nmol-N2 cm −3 h −1 , although the contribution of anammox was less significant than denitrification. Anammox-type 16S rRNA gene fragments phylogenetically related to 'Scalindua' species were predominantly recovered from mangrove forest and water channel sediment in a PCRclone library analysis targeting anammox bacteria. 'Kuenenia'-like gene fragments were also recovered from shrimp pond sediment as the major component. We demonstrated the occurrence of potential anammox activity, and suggested the possibility that diverse species of uncultured anammox bacteria contribute to the nitrogen cycle in subtropical mangrove-aquaculture ecosystems. Furthermore, this study provides new insight into the biogeography of anammox bacteria: 'Scalindua' and 'Kuenenia'-like species coexisted in the blackish sediment as in some temperate estuarine sediment.

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Increasing human activity has raised concerns about the impact of deposition of anthropogenic combustion aerosols (i.e., black carbon; BC) on marine processes. The sea surface microlayer (SML) is a key gate for the introduction of atmospheric BC into the ocean; however, relatively little is known of the effects of BC on bacteria-virus interactions, which can strongly influence microbially mediated processes. To study the impact of BC on bacteria-virus interactions, field investigations involving collection from the SML and underlying water were carried out in Halong Bay (Vietnam). Most inorganic nutrient concentrations, as well as dissolved organic carbon, were modestly but significantly higher (p = 0.02-0.05) in the SML than in underlying water. The concentrations of particulate organic carbon (though not chlorophyll a) and of total particulate carbon, which was composed largely of particulate BC (mean = 1.7 ± 6.4 mmol L -1 ), were highly enriched in the SML, and showed high variability among stations. On average, microbial abundances (both bacteria and viruses) and bacterial production were 2-and 5fold higher, respectively, in the SML than in underlying water. Significantly lower bacterial production (p < 0.01) was observed in the particulate fraction (>3 µm) compared to the bulk sample, but our data overall suggest that bacterial production in the SML was stimulated by particulate BC. Higher bacterial production in the SML than in underlying water supported high viral lytic infection rates (from 5.3 to 30.1%) which predominated over percent lysogeny (from undetected to 1.4%). The sorption of dissolved organic carbon by black carbon, accompanied by the high lytic infection rate in the black carbon-enriched SML, may modify microbially mediated processes and shift the net ecosystem metabolism (ratio of production and respiration) to net heterotrophy and CO 2 production in this critical layer between ocean and atmosphere.