Glutamine synthetase I-deficiency in Mesorhizobium loti differentially affects nodule development and activity in Lotus japonicus.

In this study, we focused on the effect of glutamine synthetase (GSI) activity in Mesorhizobium loti on the symbiosis between the host plant, Lotus japonicus, and the bacteroids. We used a signature-tagged mutant of M. loti (STM30) with a transposon inserted into the GSI (mll0343) gene. The L. japonicus plants inoculated with STM30 had significantly more nodules, and the occurrence of senesced nodules was much higher than in plants inoculated with the wild-type. The acetylene reduction activity (ARA) per nodule inoculated with STM30 was lowered compared to the control. Also, the concentration of chlorophyll, glutamine, and asparagine in leaves of STM30-infected plants was found to be reduced. Taken together, these data demonstrate that a GSI deficiency in M. loti differentially affects legume-rhizobia symbiosis by modifying nodule development and metabolic processes.

EXTREMELY HIGH GROWTH RATE OF THE SMALL DIATOM CHAETOCEROS SALSUGINEUM ISOLATED FROM AN ESTUARY IN THE EASTERN SETO INLAND SEA, JAPAN(1).

Small single-celled Chaetoceros sp. are often widely distributed, but frequently overlooked. An estuarine diatom with an extremely high growth potential under optimal conditions was isolated from the Shinkawa-Kasugagawa estuary in the eastern part of the Seto Inland Sea, western Japan. It was identified as Chaetoceros salsugineum based on morphological observations. This strain had a specific growth rate of 0.54 h(-1) at 30°C under 700 µmol · m(-2) · s(-1) (about 30% of natural maximal summer light) with a 14:10 L:D cycle; there was little growth in the dark. However, under continuous light it grew at only 0.35 h(-1) or a daily specific growth rate of 8.4 d(-1) . In addition, cell density, chlorophyll a, and particulate organic carbon concentrations increased by about 1000 times in 24 h at 30°C under 700 µmol · m(-2) · s(-1) with a 14:10 L:D cycle, showing a growth rate of close to 7 d(-1) . This very rapid growth rate may be the result of adaptation to this estuarine environment with high light and temperature. Thus, C. salsugineum can be an important primary producer in this estuary in summer and also an important organism for further physiological and genetic research.

Chemical composition of Coscinodiscus wailesii and the implication for nutrient ratios in a coastal water, Seto Inland Sea, Japan.

Chemical compositions of Coscinodiscus wailesii were determined for four samples of natural cells. Results revealed that the cellular Si:N ratio of C. wailesii cell was 2.4:1.0-5.2:1.0. The impacts of C. wailesii on surrounding coastal water were evaluated from hydrographic observations, in which C. wailesii cell density, nutrients concentrations and temperature were monitored from November 2001 to February 2005 in Harima-Nada, the Seto Inland Sea, Japan. In low temperature periods, from October to December, two peaks of C. wailesii cell density were observed and nutrient concentrations were depleted. The draw-down ratio of Si(OH)(4) and DIN (Si(OH)(4):DIN ratio) in the water column were similar to the cellular Si:N ratio of C. wailesii cells, which have high Si contents. In addition the effects of different Si(OH)(4):DIN ratio were determined for in situ bottle incubation experiments. In the culture experiments, picoplankton (0.2-2.0 microm) which consisted of small flagellates became dominant under low Si(OH)(4):DIN ratios. These results suggested that the sizes distribution shifted to small size and the phytoplankton community was changed to small flagellates after the C. wailesii bloom. These changes would influence predators at higher trophic levels in its coastal ecosystem.

Is there any seasonal variation in marine nematodes within the sediments of the intertidal zone?

The sediment parameters and nematode assemblages in the intertidal zone of the Hichirippu shallow lagoon, Hokkaido, Japan, were investigated. The objectives of this study were to observe the seasonal variation in the nematodes in the sediment, and to investigate the relationships between the nematodes and environmental factors. Samples were collected bi-monthly from five stations on the tidal flat from April 2003 to February 2004. It was found that the sediment parameters (Chl a concentration, AVS, TOC and TN contents) varied throughout the 10-month study. Fifty-four species of nematodes were found in the study area. The density and biomass of the nematodes varied in accordance with the sediment temperature during the sampling period. In this study, there was a seasonal variation in the nematode assemblage found in the intertidal zone of this shallow lagoon. The important factors affecting this variation were sediment temperature, and food competition among the nematodes themselves. The seasonal variation of the nematode also showed a relationship with the Chl a concentration in the sediment during the sampling period.

The influence of environmental variability on silicate exchange rates between sediment and water in a shallow-water coastal ecosystem, the Seto Inland Sea, Japan.

Silicate regeneration was determined in a shallow-water coastal ecosystem (Shido Bay, the Seto Inland Sea, Japan) during 1999-2000. The present study was carried out directly by core incubation and by determining gradients of dissolved silicate (DSi) in the pore water. Incubated fluxes ranged from 25.5 to 132.6 mgSim(-2)d(-1), and were 1.6-21.6 times greater than diffusive fluxes (5.4-43.3 mgSim(-2)d(-1)). The disparity between fluxes measured by core incubation and modeling pore water indicated that other physical, chemical or biological processes, in addition to diffusion of DSi from below, contribute to DSi fluxes measured during the incubation of undisturbed cores. A regression analysis revealed that water temperature and salinity explained 24% and 23%, respectively, of season to season variability in incubated fluxes. Microphytobenthos was responsible for 37% of the variability in measured rate of DSi fluxes, with greatly reducing DSi release rates due to their own DSi demand. Moreover, the inverse relationship between the DSi fluxes and biogenic silica (Bio-Si) concentrations in the surface sediment, suggested that about 41% of the variability in the DSi fluxes were explained by Bio-Si concentrations in the surface sediment. As a result, Shido Bay showed silicate regeneration of incubated cores to be a consequence of Bio-Si dissolution depending on microphytobenthos, temperature and salinity, while diffusive fluxes appeared to be limited by DSi in the pore water. An annual average of DSi flux (68.7+/-32.9 mgSim(-2)d(-1)) from the sediments to the water column corresponds to 38% of DSi, required for primary production by phytoplankton in Shido Bay.