Microbial activities in the burrow environment of the potamal mayfly Ephoron virgo

1. The impact of burrowing larvae of Ephoron virgo (Ephemeroptera, Polymitarcidae) on sediment microbiology has not been previously investigated because of difficulties in sampling the sediment of large rivers under in situ conditions. Therefore, we conducted experiments in the on‐ship Ecological Rhine Station of the University of Cologne (Germany), in which ambient conditions of the River Rhine can be closely mimicked. 2. In two consecutive seasons, experimental flow channels were stocked with Ephoron larvae and continuously supplied with water taken directly from the River Rhine. Sediment from the immediate vicinity of Ephoron burrows (i.e. U‐shaped cavities reaching 10–80 mm deep into the sediment) and bulk sediment samples were analysed for (i) particulate organic matter content, (ii) microscale in situ distribution of O₂, NO, and NH, and (iii) potential activities of exoenzymes. 3. Sediment surrounding the Ephoron burrows had markedly higher organic matter contents and exoenzyme activities compared with the bulk sediment. Microsensor measurements demonstrated that local O₂ and NO penetration into the sediment were greatly enhanced by larval ventilation behaviour. Volumetric O₂ and NO turnover rates that were calculated from steady state concentration profiles measured directly in the burrow lining were considerably higher than at the sediment surface. 4. In the sediment of the fast flowing River Rhine Ephoron burrows are preferential sites of organic matter accumulation and dissolved oxidant penetration. Our data suggest that the burrows are surrounded by a highly active microbial community that responds to the inputs from the water column with elevated O₂ and NO turnover, and release of exoenzymes into the sediment pore water. Especially during periods of mass occurrence, the larvae of E. virgo may thus significantly contribute (i) to the ecological connection between the water column and the sediment and (ii) to biogeochemical processing of organic matter in the riverbed.     

Are geothermal streams important sites of nutrient uptake in an agricultural and urbanising landscape (Rotorua,New Zealand)?

1. Lakes in the Rotorua region of New Zealand are affected by eutrophication from urbanisation and agricultural land use. Some lake tributaries contain geothermally influenced waters, and it is currently unknown whether geothermal tributaries are active sites of nutrient cycling or represent point sources of nutrients to the lakes. 2. Using government data sets, we characterised the physicochemical conditions of geothermal and non‐geothermal streams. We then measured ecosystem metabolism and reach‐scale uptake of nitrate (), ammonium () and phosphate () in summer 2010 (n = 8 streams). Finally, we used government data to compare annual nutrient flux from geothermal and non‐geothermal surface water inputs to Lake Rotoiti. 3. As expected, geothermal streams had higher temperature, conductivity and nutrient concentrations and lower pH. However, primary production, community respiration and uptake rates in geothermal streams were not different from those in their non‐geothermal counterparts. Uptake rates of were higher in geothermal streams, and uptake was below detection in geothermal streams, probably due to the saturation by naturally high concentrations. 4. A comparison of Lake Rotoiti inputs suggested that geothermal streams are not significant sources of and , while geothermal inputs of represent an average of 46% of total flux from Lake Rotoiti tributaries. 5. Despite their high temperature and low pH, geothermal streams are active sites of photosynthesis, respiration and and cycling, indicating dynamic biofilm communities. 6. Management options for geothermal streams, if any, should focus on retention (e.g. uptake or coupled nitrification and denitrification) but could prove challenging given the persistent, naturally occurring high flux.     

Submersed macrophyte growth at low pH: carbon source influences response to dissolved inorganic carbon enrichment

1. Twenty‐one submersed macrophyte species were grown in the greenhouse at low and high dissolved inorganic carbon (DIC) concentrations and at pH 6 to test the hypothesis that growth response to DIC enrichment in the water column would be greatest for species able to use both bicarbonate (HCO) and free CO₂ from the water column, intermediate for species restricted to free CO₂ in the water column, and least for those species able to use free CO₂ from both sediment porewater and the water column. 2. Relative growth rates (RGR) increased significantly with DIC enrichment for 12 of the 21 species. Bicarbonate users as a group averaged a 4.6‐fold increase in RGR with DIC enrichment, significantly greater than the 2.5‐ and 1.7‐fold increases of the water column CO₂ users and sediment CO₂ users, respectively. 3. The ability to use HCO, as measured by a pH drift technique, was positively correlated with the ratio of RGR at high DIC to RGR at low DIC for the 21 species. 4. Bicarbonate users also exhibited significantly lower belowground : total biomass (R : T) ratios than the other two groups, and alone showed a significant increase in R : T ratio with DIC enrichment. 5. Our results indicate that DIC availability may influence community structure in acidic and circumneutral lakes.     

The fate of assimilated nitrogen in streams: an in situ benthic chamber study

1. Nitrogen (N) processing in streams has been investigated using whole‐stream ¹⁵N addition experiments that, in general, have found that a large proportion of added nitrate removed from the water column appears to be assimilated by the stream benthos. The long‐term fate of this retained N is unknown, and of particular interest is the possibility that it becomes denitrified through coupled mineralisation–nitrification–denitrification processes (indirect denitrification). 2. We used in situ chambers to produce highly ¹⁵N‐enriched benthic biofilms and removed the chambers to allow biofilms to interact with ambient stream conditions. Nitrogen assimilation and direct denitrification were estimated from the first chamber deployment. Chambers were periodically reinstalled over 4 weeks to measure tracer ¹⁵N in ammonium (), nitrate () and dinitrogen (N₂), from which we estimated subsequent rates of biotic N transformations, including N mineralisation (ammonification), nitrification and indirect denitrification. We also estimated rates of depuration of ¹⁵N tracer from benthic biomass compartments. 3. Nitrate uptake was roughly equivalent in the sand and cobble habitats that dominated the stream. Direct denitrification (denitrification of from the water column) was an order of magnitude higher in cobble habitats than in sand habitats, accounting for c. 26 and 2% of total nitrate uptake in cobble and sand, respectively. 4. Mean residence times of actively cycling organic N in stream benthos (algae and microbes) were 16 days in cobble habitats and 9 days in sand habitats. The difference between habitat types was driven by the influence of N residence time in epilithic biofilms (18 days) on cobbles. 5. Release of enriched ¹⁵ was the primary flux of remineralised N, while release of enriched ¹⁵ was an order of magnitude less. We detected slight ¹⁵N enrichment in dissolved nitrogen gas (N₂) in post‐enrichment sampling, indicating that indirect denitrification was taking place. However, indirect denitrification accounted for <0.1% of the assimilated N. 6. These experiments agree with results of whole‐stream ¹⁵N additions, in that most added N was assimilated rather than directly denitrified. Assimilation was primarily a short‐term N retention mechanism in this stream, and indirect denitrification of assimilated N accounted for only a minor proportion of the observed ¹⁵N loss over time. 7. Remaining possible fates include export of N as particulate organic matter, which may lead to additional storage of assimilated N in downstream habitats, and consumption by grazers.     

Isolation by distance in a continuous population under stochastic demographic fluctuations

The local density of individuals is seldom uniform in space and time within natural populations. Yet, formal approaches to the process of isolation by distance in continuous populations have encountered analytical difficulties in describing genetic structuring with demographic heterogeneities, usually disregarding local correlations in the movement and reproduction of genes. We formulate exact recursions for probabilities of identity in continuous populations, from which we deduce definitions of effective dispersal () and effective density (D_e) that generalize results relating spatial genetic structure, dispersal and density in lattice models. The latter claim is checked in simulations where estimates of effective parameters obtained from demographic information are compared with estimates derived from spatial genetic patterns in a plant population evolving in a heterogeneous and dynamic habitat. The simulations further suggest that increasing spatio‐temporal correlations in local density reduce and generally decrease the product , with dispersal kurtosis influencing their sensitivity to density fluctuations. As in the lattice model, the expected relationship between the product and the genetic structure statistic a_r holds under fluctuating density, irrespective of dispersal kurtosis. The product D σ² between observed census density and the observed dispersal rate over one generation will generally be an upwardly biased (up to 400% in simulations) estimator of in populations distributed in spatially aggregated habitats.     

Effects of different Nitrogen forms and osmotic stress on water use efficiency of rice (Oryza sativa)

A hydroponic experiment with simulated water stress induced by polyethylene glycol (PEG) was conducted in greenhouse to study the effects of different nitrogen (N) forms (; and the mixture of and ) on water stress tolerance and water use efficiency (WUE and WUE_T) of different rice cultivars. Two rice cultivars (cv. ‘Shanyou 63’ hybrid indica and ‘Yangdao 6’ indica, China) were grown under non‐water‐ or water‐stressed condition [10% (w/v) PEG, molecular weight 6000] with different N forms for 3 weeks. Under non‐water stress, the biomass of Shanyou 63 was 50.0% and 64.3% and of Yangdao 6 was 6.9% and 87.8% higher under the supply of mixture of and than either under the sole supply of or , respectively; under water stress, the biomass of both rice cultivars decreased in all three nitrogen forms compared with non‐water stress; however, the inhibitory effect of water stress on biomass varied between and nutrition; the reduction of dry matter was significantly higher in than in nutrition. Compared with non‐water stress, under water stressed condition, WUE of both two rice cultivars significantly decreased in supply; WUE did not vary in and the mixture supply. It is concluded that (a) the resistance of water stress of rice seedlings is related to nitrogen form; (b) under water stress, could maintain a higher WUE compared with ; (c) hybrid indica rice seedlings have a higher water stress tolerance than indica rice seedlings.     

Relative influence of bioturbation and predation on organic matter processing in river sediments: a microcosm experiment

1. Our objective was to measure the effects of bioturbation and predation on the physical characteristics and biogeochemical processes in river sediments. 2. We investigated the impacts of tubificid worms tested separately and together with an omnivore (Gammarus pulex), which does feed on tubificids, on sediment distribution, water flux, sediment organic carbon, biofilm biomass and microbial activities, and the concentrations of dissolved oxygen, dissolved organic carbon, PO, NO, NO and NH in slow filtration sand–gravel columns. We hypothesised that gammarids, which exploit the top 2–3 cm of the sediment, would modify the impact of worms at the sediment surface. 3. In experiments both with and without gammarids, bioturbation by the tubificids modified both the distribution of surface particles in the sediment column and water flux. In addition, microbial aerobic (oxygen consumption) and anaerobic (denitrification and fermentative decomposition of organic matter) processes in the sediment were stimulated in the presence of tubificid worms. However, G. pulex did not affect either the density or bioturbation activity of the tubificid worms. 4. Bioturbation by the benthos can be a major process in river habitats, contributing to the retention of organic matter in sediment dynamics. The presence of at least one predator had no effect on bioturbation in sediments. In such systems, physical heterogeneity may be sufficient for tubificids to escape from generalist predators, though more specialised ones might have more effect.     

PHYCOBILIPROTEIN COMPONENTS AND CHARACTERISTICS OF THE PHYCOBILISOME FROM A THERMOPHILIC CYANOBACTERIUM MYXOSARCINA CONCINNA1

A phycocyanin (PC) and three allophycocyanin (AP) components (designated PC, AP1, AP2, and AP3) were prepared from Myxosarcina concinna Printz phycobilisomes by the native gradient PAGE performed in a neutral buffer system combined with the ion exchange column chromatography on DEAE‐DE52 cellulose. PC contained one β subunit () and two α ones ( and ), and it carried two rod linkers ( and ) and one rod‐core linker (). AP1 and AP3 were characterized as peripheral core APs, whereas AP2 was an inner‐core one. AP2 and AP3 were demonstrated to function as the terminal emitters. Each of the three APs contained two β subunits ( and ), two α subunits ( and ) and an inner‐core linker (). AP2 and AP3 had another subunit of the allophycocyanin B (AP‐B) type () belonging to the β subunit group, and AP1 and AP3 carried their individual specific core linkers ( and ), respectively. No AP component was shown to associate with the core‐membrane linker L_CM. The functions of the linker polypeptides in the phycobilisome (PBS) construction are discussed.     

Regulation of nitrous oxide emission associated with benthic invertebrates

1. A number of freshwater invertebrate species emit N₂O, a greenhouse gas that is produced in their gut by denitrifying bacteria (direct N₂O emission). Additionally, benthic invertebrate species may contribute to N₂O emission from sediments by stimulating denitrification because of their bioirrigation behaviour (indirect N₂O emission). 2. Two benthic invertebrate species were studied to determine (i) the dependence of direct N₂O emission on the preferred diet of the animals, (ii) the regulation of direct N₂O emission by seasonally changing factors, such as body size, temperature and availability and (iii) the quantitative relationship between direct and indirect N₂O emission. 3. Larvae of the mayfly Ephemera danica, which prefer a bacteria‐rich detritus diet, emitted N₂O at rates of up to 90 pmol Ind.^?1 h^?1 under in situ conditions and 550 pmol Ind.^?1 h^?1 under laboratory conditions. In contrast, larvae of the alderfly Sialis lutaria, which prefer a bacteria‐poor carnivorous diet, emitted N₂O at invariably low rates of 0–20 pmol Ind.^?1 h^?1. The N₂O emission rate of E. danica larvae was positively correlated with seasonally changing factors (body size, temperature and availability). Direct N₂O emission by E. danica larvae was limited by low temperature in winter, larval development in spring and low availability in summer. 4. Both E. danica and the non‐emitting S. lutaria increased the total N₂O and N₂ emission from sediment in a density‐dependent manner. While N₂O directly emitted by benthic invertebrates can be partially consumed in the sediment (E. danica), non‐emitting species can still indirectly contribute to total N₂O emission from sediment (S. lutaria).     

Cover Caption: Plant Vascular System

This issue focuses on the plant vascular system, with a comprehensive review article written by Lucas et al. (pp. 294–388). The cover drawing illustrates the phosphate‐stress signaling and response network (pp. 347–351). A Pi deficiency signal is generated in roots and transported to shoots via the xylem (blue lines). This signal is recognized by source leaves to activate the Pi stress response pathway and then to load the subsequent signals into the phloem (red lines). Phloemmobile RNAs move to roots to increase Pi uptake and alter root architecture . Different phloem‐mobile RNAs are also delivered from source leaves to developing leaves and the shoot apex where they regulate development under Pi‐stress conditions.     

Release rates and potential fates of nitrogen and phosphorus from sediments in a eutrophic reservoir

1. Nutrients released from lake sediments can influence water column nutrient concentrations and planktonic productivity. We examined sediment nutrient release [soluble reactive phosphorus (SRP) and ammonia (NH)] at two sites in a eutrophic reservoir (Acton Lake, OH, U.S.A.) that differed in physical mixing conditions (a thermally stratified and an unstratified site). 2. Sediment nutrient release rates were estimated with three methods: sediment core incubations, seasonal in situ hypolimnetic accumulation and a published regression model that predicted sediment phosphorous (P) release rate from sediment P concentration. All three methods were applied to the deeper stratified site in the reservoir; however, we used only sediment core incubations to estimate SRP and NH release rates at the shallow unstratified site because of the lack of thermal stratification. We also compared the total P concentration (TP_S) of sediments and the concentration of P in various sediment fractions at both sites. 3. Anoxic sediments at the stratified site released SRP at rates more than an order of magnitude greater than oxic sediments at the shallow unstratified site. However, P accumulated in the hypolimnion at much lower rates than predicted by sediment core incubations. In contrast, NH was released at similar rates at both sites and accumulated in the hypolimnion at close to the expected rate, indicating that P was ‘lost’ from the hypolimnion through biogeochemical pathways for P, such as precipitation with inorganic material or biological uptake and sedimentation. 4. TP_S was significantly greater at the deeper stratified site and organically bound P accounted for >50% of TP_S at both sites. 5. We examined the magnitude of SRP fluxes into the study reservoir in 1996 by comparing the mean summer daily SRP fluxes from anaerobic sediments, aerobic sediments, stream inflows and gizzard shad excretion. While the SRP release from anaerobic sediments was high, we hypothesise that little of this SRP gained access to the epilimnion in mid‐summer. SRP flux to the reservoir from aerobic sediments was less than from gizzard shad excretion and streams. Large interannual variability in thermocline stability, gizzard shad biomass and stream discharge volumes, will affect SRP loading rates from different sources in different years. Therefore, construction of P budgets for different years should account for interannual variation in these parameters.     

Nitrogen cycling and dynamics in a macrophyte‐rich stream as determined by a release

1. A tracer release study was conducted in a macrophyte‐rich stream, the River Lilleaa in Denmark. The objectives of the study were to compare uptake rates per unit area of by primary producers and consumers in macrophyte and non‐macrophyte habitats, estimate whole‐stream uptake rates of and compare this to other stream types, and identify the pathways and estimate the rate at which enters the food web in macrophyte and non‐macrophyte habitats. 2. Macrophyte habitats had four times higher primary uptake rates and an equal uptake rate by primary consumers per unit habitat area as compared to non‐macrophyte habitats. These rates represent the lower limit of potential macrophyte effects because the rates will be highly dependent on macrophyte bed height and mean bed height in the River Lilleaa was low compared to typical bed heights in many lowland streams. Epiphytes accounted for 30% of primary uptake in macrophyte habitats, illustrating a strong indirect effect of macrophytes as habitat for epiphytes. N flux per unit habitat area from primary uptake compartments to primary consumers was four times lower in macrophyte habitats compared to non‐macrophyte habitats, reflecting much greater biomass accrual in macrophyte habitats. Thus, we did not find higher N flux from macrophyte habitats to primary consumers compared to non‐macrophyte habitats. 3. Whole‐stream uptake rate was 447 mgN m^?2 day^?1. On a habitat‐weighted basis, fine benthic organic matter (FBOM) accounted for 72% of the whole‐stream uptake rate, and macrophytes and epiphytes accounted for 19 and 8%, respectively. 4. We had expected a priori relatively high whole‐stream N uptake in our study stream compared to other stream types mainly due to generally high biomass and the macrophyte’s role as habitat for autotrophic and heterotrophic organisms, but our results did not confirm this. In comparison with other release study streams, we conclude that nutrient concentration is the overall controlling factor for N uptake rates across streams, mostly as a result of high biomass of primary uptake compartments in streams with high nutrient concentrations in general and not in macrophyte streams in particular. 5. Our results indicate that macrophytes play an important role in the longer‐term retention of N and thus a decrease in net downstream transport during the growing season compared to streams without macrophytes, through direct and indirect effects on the stream reach. Direct effects are high uptake efficiency, low turnover rate (partly due to no direct feeding on macrophytes) and high longevity. An indirect effect is increased sedimentation of FBOM in macrophytes compared to non‐macrophyte habitats and streams which possibly also increase denitrification. Increased retention with macrophyte presence would decrease downstream transport during the growing season and thus the N loading on downstream ecosystems.     

Functional linkage between N acquisition strategies and aeration capacities of hydrophytes for efficient oxygen consumption in roots

We evaluated the specific strategies of hydrophytes for root O₂ consumption in relation to N acquisition and investigated whether the strategies varied depending on the aeration capacity. Aeration capacity of roots is an important factor for determining hypoxia tolerance in plants. However, some hydrophytes possessing quite different aeration capacities often co‐occur in wetlands, suggesting that root O₂ consumption also strongly affects hypoxia tolerance. We cultivated Phragmites australis with high aeration capacity and Zizania latifolia with low aeration capacity in hypoxic conditions with NH or NO treatment and compared the growth, N uptake, N assimilation and root respiration between the two species. In Z. latifolia grown with NH treatment, high N uptake activity and restrained root growth led to sufficient N acquisition and decrease in whole‐root respiration rate. These characteristics consequently compensated for the low aeration capacity. In contrast, in P. australis, low N uptake activity was compensated by active root growth, but the whole‐root respiration rate was high. This high root respiration rate was allowed by the high aeration capacity. The O₂ consumption‐related traits of hydrophyte roots were closely correlated with N acquisition strategies, which consequently led to a compensational relationship with the root aeration capacity. It is likely that this functional linkage plays an important role as a core mechanism in the adaptation of plants to hypoxic soils.     

Effects of moderate ammonium enrichment on three submersed macrophytes under contrasting light availability

1. Increased ammonium concentrations and decreased light availability in a water column have been reported to adversely affect submersed vegetation in eutrophic waters worldwide. 2. We studied the chronic effects of moderate enrichment (NH₄–N: 0.16–0.25 mg L^?1) on the growth and carbon and nitrogen metabolism of three macrophytes (Ceratophyllum demersum, Myriophyllum spicatum and Vallisneria natans) under contrasting light availability in a 2‐month experiment. 3. The enrichment greatly increased the contents of free amino acids and nitrogen in the shoot / leaf of the macrophytes. This indicates that was the dominant N source for the macrophytes. 4. Soluble carbohydrate contents remained relatively stable in the shoot / leaf of the macrophytes irrespective of the treatments. Under ambient light, the starch contents in the shoot / leaf of C. demersum and M. spicatum increased with enrichment, whereas V. natans did not exhibit any change. The starch contents decreased in C. demersum, increased in M. spicatum and remained unchanged in V. natans after the combined treatment of enrichment and reduced light. 5. The enrichment did not affect the growth of the three macrophytes under the ambient light. However, it did suppress the growth of C. demersum and M. spicatum under the reduced light. The results indicate that a moderate enrichment was not directly toxic to the macrophytes although it might change their viability in eutrophic lakes in terms of the carbon and nitrogen metabolism.     

Water chemistry in the microenvironment of rainbow trout Oncorhynchus mykiss embryos is affected by development,the egg capsule and crowding

The hypothesis tested was that embryonic metabolism affects the water chemistry in the boundary layer. In addition, embryo crowding would further compound the metabolic effect on the water chemistry in the boundary layer. As development progressed, the magnitude of the boundary layer gradients for O₂ and pH, but not for NH, increased. The presence of the egg capsule hindered the diffusion of O₂ into and H⁺ and NH out of the embryo. The magnitude of the O₂, pH and NH boundary layer gradient was significantly increased when embryos were surrounded by either sham embryos or live embryos. The majority of this crowding effect on embryo boundary layers was due to changes in water flow rather than due to metabolism directly. These results clearly show that the microenvironment adjacent to the developing rainbow trout Oncorhynchus mykiss embryo becomes more stagnant as development progresses in the presence of the egg capsule and is further intensified with embryo crowding.     

Plant–microbial competition for nitrogen uncoupled from soil C:N ratios

A green house experiment was designed to test the idea that competition for inorganic nitrogen (N) between plants and heterotrophic microorganisms occurs in soils with high C:N ratios, qualifying for N limited microbial activity, but not at low C:N ratios. The short‐term (24 h) ¹⁵N uptake by the grass Festuca gigantea and microorganisms in planted and unplanted soils was determined, and the bacterial activity was measured by the ³H‐thymidine incorporation technique. Two deciduous forest soils, with C:N‐ratios of 20 and 31, and the 20 soil amended with litter to a C:N ratio of 34, were used. A novel and important part of the experimental design was the preparation of the unplanted reference soil with plants present until the competition assay started by the addition of ¹⁵N labelled ammonium ( ) or nitrate ). The results suggested that plants and soil microorganisms competed for mineral N but under influence of other factors than the soil C:N ratio. The plants reduced the microbial uptake of and in the soil with low C:N ratio, which also had the lowest bacterial activity. The plants had a larger N uptake than microorganisms in the two natural soils but smaller in the litter‐amended, and their presence enhanced the bacterial activity, especially in the latter soil. The litter‐amended soil with its high C:N ratio and easily decomposable C was the soil that best fulfilled the criteria for competition, including a net consumption of mineral N during the assay, the lowest plant uptake of mineral N due to the high N immobilization by microorganisms, and a reduced microbial ¹⁵N uptake‐to‐bacterial activity in the presence of plants. Thus, other factors, such as the decomposability of the soil C and the bacterial activity, were more important than the soil C:N ratio to the outcome of plant–microbial competition for N.     

Microbial thiosulphate reaction arrays: the interactive roles of Fe(III), O2 and microbial strain on disproportionation and oxidation pathways

In this work, we experimentally evaluate pH and  dynamics associated with abiotic and microbial  oxidation under varying [O₂], [Fe(III)] and microbial strain/consortia (two pure strains, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, their consortia, and two enrichments from an acidic environmental system, Moose Lake 2002 and Moose Lake 2003). Results of the batch experiments demonstrate highly active microbial processing of  while abiotic controls under identical experimental conditions remain static with no pH decrease. When abiotic controls were manually titrated with acid to achieve similar pH decreases to those occurring in the microbial treatments, different S pathways were involved. In particular, disproportionation is a substantial component of initial microbial  processing, and is accelerated by the presence of Fe(III), indicating that recycling of S through intermediate oxidation states is likely to be widespread in acidic mine environments where high [Fe(III)] is common. Furthermore, the microbially mediated S reaction pathways were dependent on both environmental conditions and microbial strain/consortia, indicating that microbial community structure also plays a key role. Collectively, these results highlight the importance of microbial activity, their poor representation by abiotic S models, the likelihood that Fe(III), rather than O₂, is a key control on microbial S processing in acid environments and the need to identify the microbial community/strain involved.     

Relative contribution of AtHAK5 and AtAKT1 to K+ uptake in the high‐affinity range of concentrations

The relative contribution of the high‐affinity K⁺ transporter AtHAK5 and the inward rectifier K⁺ channel AtAKT1 to K⁺ uptake in the high‐affinity range of concentrations was studied in Arabidopsis thaliana ecotype Columbia (Col‐0). The results obtained with wild‐type lines, with T‐DNA insertion in both genes and specific uptake inhibitors, show that AtHAK5 and AtAKT1 mediate the ‐sensitive and the Ba²⁺‐sensitive components of uptake, respectively, and that they are the two major contributors to uptake in the high‐affinity range of Rb⁺ concentrations. Using Rb⁺ as a K⁺ analogue, it was shown that AtHAK5 mediates absorption at lower Rb⁺ concentrations than AtAKT1 and depletes external Rb⁺ to values around 1 μM. Factors such as the presence of K⁺ or during plant growth determine the relative contribution of each system. The presence of in the growth solution inhibits the induction of AtHAK5 by K⁺ starvation. In K⁺‐starved plants grown without , both systems are operative, but when is present in the growth solution, AtAKT1 is probably the only system mediating Rb⁺ absorption, and the capacity of the roots to deplete Rb⁺ is reduced.     

Age,growth and reproduction of cutlassfishes Trichiurus spp. in the southern East China Sea

The cutlassfish is one of the most commercially important continental fishes in the East China Sea. However, the current classification of cutlassfishes is suboptimal and complicates fisheries management. This study investigated the growth and reproduction of three cutlassfish species in the southern East China Sea. Between August 2002 and September 2003, a total of 2717 specimens of Trichiurus japonicus, T. sp.2 (likely synonymous for T. nanhaiensis) and T. lepturus were collected monthly from Daxi, Anping, Nanfangao, Donggang and Xingang fishing ports. The von Bertalanffy growth model in preanal length (PL) resulted in PL (mm) for T. japonicus, PL(mm) for T. sp.2 and PL (mm) for T. lepturus. Their growths were significantly different between sexes for T. japonicus and T. sp.2 and among the three species. From macroscopic examination of ovaries, histological inspection of oocytes, and analyses of gonosomatic and hepatosomatic indices, T. japonicus was likely to spawn all year round, with two peaks in February–July and November–December. T. sp.2 spawned relatively later, between April and August. The length at sexual maturity of T. japonicus females was 264 mm PL or 746 mm total length. The sex ratios of T. japonicus and T. sp.2 were significantly different from 1 : 1 whereas T. lepturus showed no significant difference. The three species of trichiurids have different geographical distributions in this region.     

Effects of irradiance on diel and seasonal patterns of nutrient uptake by stream periphyton

1. Irradiance strongly affects the abundance of stream periphyton communities that in turn influence patterns of instream nutrient uptake. We examined relationships between irradiance and periphyton nutrient uptake taking into account diel and seasonal variation in ambient irradiance. 2. Uptake of dissolved N, P and C by periphyton as areal uptake (U) and demand (V_f) was determined under 11 irradiance levels (0–100% of ambient conditions) using shallow stream‐side experimental channels. Experiments were conducted once per season over one annual cycle with both day and night uptake rates assessed, together with periphyton biomass and autotrophic production rates. 3. No consistent diel variation in areal uptake or demand was detected for the predominant inorganic or total dissolved nutrients even at the highest irradiances. Lack of variation may indicate nutrient limitation, with photosynthetic sequestration and storage of C during the day for subsequent utilisation at night. Alternatively, oxygen consumption by photoautotrophs at night may stimulate compensatory heterotrophic uptake (e.g. denitrification). 4. In all seasons, release of dissolved organic N was detected during the day but to a lesser extent at night. This was not directly related to irradiance levels, indicating that heterotrophic metabolism (e.g. microbial decomposition) contributes to this phenomenon. 5. Areal uptake and demand for the predominant inorganic and total dissolved nutrients increased in response to increasing irradiance in some or all seasons, but rates were typically higher during the spring and summer. Saturation of areal uptake and demand at elevated irradiances was evident during the spring. demand was also saturated at higher irradiances in the summer and autumn. Maximum demand was comparable during spring and summer, but saturation occurred at lower irradiance in summer (24 h average 135–145 μmol m^?2 s^?1) relative to spring (312–424 μmol m^?2 s^?1), indicating more efficient nutrient uptake in summer. Higher total periphyton biomass in summer, but comparable autotrophic biomass (chlorophyll a), implies that heterotrophic metabolism may contribute to this greater efficiency. In spring, autotrophic biomass peaked at an irradiance level of 225 μmol m^?2 s^?1, also suggesting a role for heterotrophic metabolism in demand at higher irradiances. 6. The results of this study show that irradiance levels exert a strong influence on the nature and quantity of instream nutrient uptake with N demand saturated at elevated irradiance levels during the spring, summer and autumn. Our results also suggest that heterotrophic metabolism makes a measurable contribution to instream nutrient uptake even under higher irradiances that favour autotrophic activity.