Hitoshi Iizumi

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.

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.

Wind-dependent formation of phytoplankton spring bloom in Otsuchi Bay, a ria in Sanriku, Japan

Spring blooms of phytoplankton composed of centric diatoms developed in late February, March, and April in Otsuchi Bay on Sanriku ria coast, Japan. During this period, associated with prolonged seasonal west wind (>1 day), intense exchange of waters occurred between inside and outside the bay: outflow of nearsurface brackish water over inflow of oceanic water at depth. This circulation interrupted formation of the blooms, and transported phytoplankton populations seaward. By such water movements, a significant amount of nutrients in the bay was carried out, otherwise replenished into the bay, depending on water masses located outside the bay. Owing to irregular features of wind events, a bloom lasted from several days to a week. From February to April, supply of nutrients seemed to be replete except for the latter half of the bloom period, and estimates of the critical depth exceeded the depth of the bottom consistently. Thus, net growth of phytoplankton was expected throughout the observation period, and potentially blooms could be formed. However, the blooms were only formed under calm weather. We hypothesize that the exchange of waters dilutes populations in the bay, and that formation of the bloom, that is, accumulation of biomass depends on a balance between the growth of phytoplankton and the dilution of bay water.

Fruit anatomy, seed germination and seedling development in the Japanese seagrass Phyllospadix (Zosteraceae)

Abstract Phyllospadix iwatensis Makino and Phyllospadix japonicus Makino have similar fruit morphology and anatomy. The rhomboid fruit of Japanese Phyllospadix is dark brown in colour and is characterized by two arms bearing stiff inflected bristles which can act as an anchoring system. The fruit covering consists of a thin cuticular seed coat and pericarp remains mainly fibrous endocarp. In the groove region of the fruit, the cuticular seed coat and endocarp are replaced by nucellus cells with wall in-growths and crushed pigment strands with lignified walls. These tissues appear to control the transfer of nutrients to developing seed. The seed is oval with a small embryo and a large hypocotyl. The embryo is straight and simple, with the plumule containing three leaf primordia and a pair of root primordia surrounded by a cotyledon. The hypocotyl has a large ventral lobe containing central provascular tissue and two small dorsal lobes. The hypocotyl contains starch, lipid and protein, and acts as a nutrient store. The seed of P. iwatensis has a dormancy period of ∼6 weeks and germination eventually reaches ∼65%, but is not synchronized. During germination the leaves emerge first, and then after at least three young leaves have formed and abscised, the roots emerge, usually >6 months after the commencement of germination. Utilization of the nutrient reserves is initially from the periphery of the hypocotyl and then progressively towards its centre.

Comparative leaf structure and its functional significance in Phyllospadix iwatensis Makino and Phyllospadix japonicus Makino (Zosteraceae).

Abstract The leaf anatomy and ultrastructure of the Japanese Phyllospadix species have been studied. Subcuticular cavities, which sometimes contain fibrillar substances, are present in the leaf blades but are absent in the leaf sheaths. Blade and sheath epidermal cells have many chloroplasts, mitochondria, lipid droplets and much endoplasmic reticulum but no starch grains. Wall ingrowths are pronounced in the inner tangential wall of epidermal cells and are closely associated with mitochondria. Plasmodesmata are present between adjacent epidermal cells and between epidermal and mesophyll cells, suggesting both symplastic and apoplastic pathways for solute transport. Vascular bundles in both blade and sheath are represented by a single xylem lacuna and phloem tissue containing several nacreous-walled sieve elements. Fibre bundles are associated with the epidermis, vascular bundles and the leaf margin. The walls of fibre cells are thickened, but not lignified, suggesting they provide mechanical support and flexibility for leaves against wave action. The two Japanese Phyllospadix species can be distinguished by anatomical features including the leaf surface, shape of the epidermal cells, and distribution and size of fibre bundles.

Notes on Archaeozostera in relation to the Zosteraceae

Abstract A literature review indicates that Archeozostera is a synonym of Archaeozostera. The morphology and anatomy of fossil specimens of Archaeozostera are compared with those of modern seagrasses, particulary the Zosteraceae, and it is concluded that Archaeozostera is most unlikely to be related to the Zosteraceae, being neither a protozosteroid nor the ancestor of the modern Zosteraceae. It probably was not a marine angiosperm. The reasons for these conclusions are presented and discussed.

An improved procedure for 15N determination by emission spectrometry

Abstract An improved method for emission spectrometric determination of 15N content with a small amount of nitrogen (2 μg, minimum) is described. Ammonium nitrogen and organic nitrogen are converted to N2 gas by the method of Rittenberg and by the method of Dumas, respectively. The N2 gas is purified, introduced into a MacLeod vacuum gauge for measuring the total quantity, and then an appropriate amount of the N2 gas is collected in a discharge tube containing a molecular sieve 5A so as to give a gas pressure of about 5 Torr. Emission is stable and reproducible, and 15N abundance can be determined with an error less than 0.01 and 0.10 atom% at a natural abundance level of 0.366 and at 9 atom%, respectively. Predetermination of N content of samples is not required.

Acetylene Reducing Activity at a Tropical Seagrass Bed in Papua New Guinea

Acetylene reduction activity at a tropical seagrass bed in Papua New Guinea was studied. Blades of every species of seagrass studied (Enhalus acoroides, Thalassia hemprichii, Halodule uninervis, Syringodium isoetifolium) at the bed showed active acetylene reduction in the rage of 0.025–0.050 μmol cm−2d−1. It is suggested that epiphytic blue-green algae would be responsible for active acetylene reduction. Acetylene reducing activity was observed also at rhizosphere of seagrasses, microbial communities on the detritus and periphyton on snail shells. It is estimated that total nitrogen fixed at the seagrass bed will be equivalent to 6.7% of that required for the growth of seagrasses.