Akihiko Hattori

Natural abundance of 15N in particulate organic matter in the North Pacific Ocean

Abstract The abundance of 15N in particulate organic matter in the euphotic layer of the North Pacific Ocean was investigated. δ15N values ranged from −1.7 to +9.7% relative to atmospheric nitrogen. 15N contents in plankton samples collected in the central and northwestern North Pacific were inversely correlated with concentrations of NO−3. The 15N contents of Trichodesmium sp. (−1.7 to +0.5%) and associated Zooplankton (ca. + 2%) were low, suggesting the significance of nitrogen supply via molecular nitrogen fixation which is assumed to involve little isotope fractionation. The variation of 15N in particulate organic nitrogen in the euphotic layer of the ocean can be explained by biochemical isotope fractionation in the assimilation of nitrate and fixation of molecular nitrogen.

Geographical variation of the water column distrubution of suspended particulate organic nitrogen and its 15N natural abundance in the Pacific and its marginal seas

The water column distribution of suspended particulate organic nitrogen (PON) and its natural abundance ratio of 15N: 14N were investigated to a depth of ∼4000 m at 13 stations in the North Pacific, and the South China, Philippine and Bering seas. At two stations in the northern North Pacific, sediment trap experiments were also carried out. The δ15N of PON ranged from −1.5 to 23.3 per mil. The 15N natural abundance of PON increased with depth between 0 and 200 m, while the PON concentration decreased sharply in the same depth range. In the vertical profiles, the PON in the deep water was, on an average, enriched with 15N by approximately 6 per mil as compared with that in the euphotic zone. These findings imply that the vertical transport of organic matter is mediated primarily by rapidly sinking particles, and that most of the decomposition of organic matter takes place in the shallow layer beneath the bottom of the euphotic zone (<200 m) in a similar manner at all locations. The average 15N abundance of PON in the water column was higher in the eastern tropical and central gyre portions of the Pacific than in the western Pacific, the South China Sea, the Philippine Sea, and the Bering Sea. Year-round stratification, the influence of 15N enriched nitrate produced during denitrification and the lack of significant nitrogen fixation in the surface layer probably caused the 15N enrichment in the eastern tropical Pacific.

Growth and organic production of eelgrass (Zostera marina L.) in temperate waters of the Pacific coast of Japan. III. The kinetics of nitrogen uptake

Kinetics of ammonium and nitrate uptake by eelgrass, Zostera marina L., were investigated by a 15N tracer technique. The rate of ammonium uptake by leaves was linear to an ammonium concentration at least up to 20 μg atoms N l−1. The uptake of nitrate by leaves was of equal magnitude but was half-saturated at a nitrate concentration of 23 μg atoms N l−1. Ammonium uptake by roots seemed to saturate at an ammonium concentration of ca. 100 μg atoms N l−1, but, it increased when the ammonium concentration was raised to 500 μg atoms N l−1. Nitrate uptake by roots was repressed by high ammonium concentration. The overall process of nitrate uptake by roots appeared to b by roots. There was no diurnal variation in uptake of ammonium or nitrate by leaves and roots. Total nitrogen uptake by the plant agreed with the specific growth rate measured by a marking method. The translocation of nitrogen from the root—rhizome tissues to leaf tissues was more active during day than during night, suggesting involvement of a process(es) which is photosynthetically dependent. Nitrogen was translocated from other parts of the plant to the most actively growing young leaves and the leaves with flowering organs.

Nitrate and nitrite in interstitial waters of eelgrass beds in relation to the rhizosphere

The distribution of nitrate and nitrite in the interstitial water of the sediment of eelgrass (Zostera marina) bed of Izembek Lagoon, Alaska, were investigated. Their concentrations were relatively high (0 to 9.8 μg-at.N·1−1, average 4.8 for nitrate; 0 to 4.0 μ-at.N·1−1, average 1.9 for nitrite) although the sediments were anoxic and contained hydrogen sulphide. The rates of bacterial denitrification measured by 15N tracer technique ranged from 0.49×10−10 to 1.2 × 10−9 g-atN·g−1·h−1. When a steady state is maintained, the loss of nitrate and nitrite must be balanced by their production by bacterial nitrification. Experimentally determined rate of nitrification in the sediment was of the same order. A model experiment demonstrated that oxygen is transported from leaves to rhizomes and roots of eelgrass and released into the sediment. The oxygen is used for nitrification in the rhizosphere in anoxic sediments.

Diel variation in nitrogen fixation by a marine blue-green alga, Trichodesmium thiebautii

Abstract A diel variation was demonstrated with respect to nitrogen fixing capacity of a marine blue-green alga, Trichodesmium thiebautii, collected from subtropical waters of the western North Pacific. A 200-fold difference was observed between day and night activities when measured at a light intensity of 12,000 lx. The pattern and extent of variation were different from diel variations reported for photosynthesis, nutrient uptake, and nitrogen fixation of natural populations of marine phytoplankton.

Ammonium regeneration and assimilation in eelgrass zostera marina beds

Regeneration and assimilation of ammonium in the water column and in sediments of eelgrass (Zostera marina L.) beds of Izembek Lagoon and Crane Cove, Alaska, USA and Mangoku-Ura, northeastern Japan, were investigated by using a 15N isotope dilution technique. In the water column of Mangoku-Ura, ammonium was regenerated at a rate of 12 nmol l-1 h-1 and assimilated at a rate of 74 nmol l-1 h-1. The ammonium regeneration rate in sediments ranged from 2 to 150 nmol g-1 h-1, and with one exception, exceeded ammonium assimilation in sediments (0.3 to 77 nmol g-1 h-1). The ammonium regeneration in the water column was of little significance for the nitrogen supply to the eelgrass bed ecosystem. Net ammonium production (regeneration minus assimilation) in the sediment of Izembek Laggon met nitrogen demand for eelgrass growth, suggesting that ammonium regeneration in the sediments was very important for the nitrogen cycle in the eelgrass bed ecosystem.

Hydrographic features of the deep water of the Bering Sea—the sea of Silica

Abstract The deep water of the Bering Sea contains concentrations of dissolved silicate up to 240 μg at. Sil−1. Nitrate concentrations are less than in the North Pacific at the depths with the same oxygen contents. The rates of chemical and biochemical reactions occurring in the deep water (below 2km) were estimated from hydrographic data by applying a modified one-dimensional model. Oxidation of organic matter in the oxygenated water column of the Bering Sea was twice that of the North Pacific. Silicate regeneration, or dissolution of biogenic opal and denitrification, or bacterial nitrate reduction to gaseous nitrogen, on and in the bottom sediments of the deep Bering Sea basin were calculated to be 212 and 20 mg at.m−2 yr−1, respectively. These values are consistent with the ones estimated from vertical profiles of dissolved silicate and nitrate in the interstitial water of the sediments. The chemical anomaly observed in deep water of the Bering Sea can be produced by these reactions in the bottom sediments. The decomposition of organic matter in anoxic sediments accounts for about 8% of the total organic matter decomposing in the water column below 2km and in the sediments.

Estimates of denitrification in sediments of the Bering Sea shelf

Denitrification, i.e. anaerobic reduction of nitrate or nitrite to gaseous nitrogen, in the surface sediments of the Bering Sea was estimated using a 15N-tracer method. N2 production is apparently controlled by the supply of nitrate and nitrite to the sediments. The average rate of N2 production in three locations was 1.2 ng-atoms N (g dry weight of sediment)−1 h−1. The rate of nitrate reduction estimated from the vertical distribution of nitrate in the sediments using a one-dimensional diffusion model agreed well with observed rates of N2 production. The annual loss of combined nitrogen by denitrification in the Bering Sea shelf was estimated to be 5 × 1011 g.

Aerobic nitrogen fixation by the marine non-heterocystous cyanobacterium Trichodesmium (Oscillatoria) spp.: Its protective mechanism against oxygen

Nitrogen fixation (acetylene reduction) by the marine non-heterocystous cyanobacteria, Trichodesmium thiebautii and T. erythraeum, is sensitive to oxygen. Its sensitivity to oxygen was intensified when the colonies of T. thiebautii were disintegrated, but the separate trichomes yielded still retained the capacity for light dependent acetylene reduction. Trichodesmium colonies evolved hydrogen under argon in the light. The addition of carbon monoxide with DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] enhanced the rate of hydrogen evolution to approximately the same level as that of the maximum acetylene reduction on an electron basis. This probably results from the inhibition of the uptake hydrogenase. We propose that the uptake hydrogenase functions to protect nitrogenase from damage by oxygen.

Transient change in the ATP Pool ofAnabaena cylindrica associated with ammonia assimilation

When N2-grown cells ofAnabaena cylindrica were exposed to ammonia (50 μM to 5 mM) in the dark, the size of the ATP pool was reduced by 40% within 1 min, but restored after 5 or 6 min. The decrease in ATP was accompanied by increases in ADP and AMP, while the total adenylate content remained unaltered. The ammonia-induced change in the ATP pool was completely eliminated when algal cells were treated withl-methionine-dl-sulfoximine, an inhibitor of glutamine synthesis. These results suggest that ammonia is rapidly assimilated through the pathway mediated by glutamine synthetase accompanied by reduction of the ATP pool.