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1.
The magnitude and physiological characteristics of biological nitrogen fixation have been studied in the oligotrophic waters of the North pacific gyre. The filamentous blue-green algae Trichodesmium spp. and Richelia intracellularis were the important nitrogen-fixing phytoplankton. Most of the nitrogen fixation occurs in the upper 40 m of the water column, with detectable fixation as deep as 90 m, which corresponds to about the 1 % light depth. There was no evidence of photoinhibition of nitrogen fixation, although CO2 reduction was depressed slightly at the highest light levels. The rate of nitrogen fixation in the water column varied throughout the day, being highest in mid-morning and in late afternoon. Relatively high fixation rates were also found during periods of darkness. Elevated oxygen concentrations had a marked inhibitory effect on rates of nitrogen fixation, a pO2 of 0.4 atm causing a 75% inhibition. Data from studies of nitrogen fixation and assimilation rates of 15N-labelled nitrate, ammonium, and urea indicate that nitrogen fixation furnished about 3% of the total daily fixed nitrogen requirement for phytoplankton growth. Studies with isolated colonies of Trichodesmium spp. indicated that 100% of their nitrogen requirement was met by nitrogen fixation. Chemical composition of the Trichodesmium colonies showed that the C:N ratio was 4.1 and that their phosphorus content relative to carbon or nitrogen was much lower than that of the total particulate material in the water column. Elevated ratios of carbon: adenosine triphosphate (ATP) also suggest that phosphorus deficiency may be limiting the growth of Trichodesmium. The magnitude of nitrogen fixation in the gyre is seasonally dependent, with high rates in late summer and autumn. At these times the water column is stratified, with phosphate and nitrate barely detectable in the upper 100 m. Our data suggest that during these months of stratification, biological fixation of nitrogen amounts to about 33 g-at N/m2/day. 相似文献
2.
Nutrients were added separately and combined to an initial concentration of 10 μM (ammonium) and/or 2 μM (phosphate) in a series of experiments carried out with the giant clam Tridacna maxima at 12 microatolls in One Tree Island lagoon, Great Barrier Reef, Australia (ENCORE Project). These nutrient concentrations
remained for 2 to 3 h before returning to natural levels. The additions were made every low tide (twice per day) over 13 and
12 mo periods for the first and second phase of the experiment, respectively. The nutrients did not change the wet tissue
weight of the clams, host C:N ratio, protein content of the mantle, calcification rates or growth rates. However, ammonium
(N) enrichment alone significantly increased the total population density of the algal symbiont (Symbiodinium sp.: C = 3.6 · 108 cell clam−1, N = 6.6 · 108 cell clam−1, P = 5.7 · 108 cell clam−1, N + P = 5.7 · 108 cell clam−1; and C = 4.1 · 108 cell clam−1, N = 5.1 · 108 cell clam−1, P = 4.7 · 108 cell clam−1, N + P = 4.5 · 108 cell clam−1, at the end of the first and second phases of the experiment, respectively), although no differences in the mitotic index
of these populations were detected. The total chlorophyll a (chl a) content per clam but not chlorophyll a per cell also increased with ammonium addition (C = 7.0 mg chl a clam−1, N = 13.1 mg chl a clam−1, P = 12.9 mg chl a clam−1, N + P = 11.8 mg chl a clam−1; and C = 8.8 mg chl a clam−1, N = 12.8 mg chl a clam−1; P = 11.2 mg chl a clam−1, N + P = 11.3 mg chl a clam−1, at the end of the first and second phases of the experiment, respectively). The response of clams to nutrient enrichment
was quantitatively small, but indicated that small changes in inorganic nutrient levels affect the clam–zooxanthellae association.
Received: 2 June 1997 / Accepted: 9 June 1997 相似文献
3.
Phytoplankton community structure and primary production in small intertidal estuarine-bay ecosystem (eastern English Channel,France) 总被引:1,自引:0,他引:1
From May 2002 to October 2003, a fortnightly sampling programme was conducted in a restricted macrotidal ecosystem in the
English Channel, the Baie des Veys (France). Three sets of data were obtained: (1) physico-chemical parameters, (2) phytoplankton community structure illustrated
by species composition, biovolume and diversity, and (3) primary production and photosynthetic parameters via P versus E curves. The aim of this study was to investigate the temporal variations of primary production and photosynthetic parameters
in this bay and to highlight the potential links with phytoplankton community structure. The highest level of daily depth-integrated
primary production Pz (0.02–1.43 g C m−2 d−1) and the highest maximum photosynthetic rate P
B
max (0.39–8.48 mg C mg chl a
−1 h−1) and maximum light utilization coefficient αB [0.002–0.119 mg C mg chl a
−1 h−1 (μmol photons m−2 s−1)] were measured from July to September. Species succession was determined based on biomass data obtained from cell density
and biovolume measurements. The bay was dominated by 11 diatoms throughout the year. However, a Phaeocystis globosa bloom (up to 25 mg chl a m−3, 2.5 × 106 cells l−1) was observed each year during the spring diatom bloom, but timing and intensity varied interannually. Annual variation of
primary production was due to nutrient limitation, light climate and water temperature. The seasonal pattern of microalgal
succession, with regular changes in composition, biovolume and diversity, influenced the physico-chemical and biological characteristics
of the environment (especially nutrient stocks in the bay) and thus primary production. Consequently, investigation of phytoplankton
community structure is important for developing the understanding of ecosystem functioning, as it plays a major role in the
dynamics of primary production. 相似文献
4.
Heterotrophic nitrogen-fixation (acetylene reduction) was measured during decomposition (under dark conditions) of Rhizophora mangle L. and Avicennia germinans (L.) Stearn leaf litter. Nitrogen-fixation rates in leaf litter increased following 24 d incubation, then decreased after
≃44 d for both species. Maximum rates of 66.2 and 64.6 nmol C2H4 g−1 dry wt h−1 were reached by R. mangle and A. germinans leaf litter, respectively. Higher fixation rates of leaf litter were associated with an increase in water content and sediment
particles on leaf surfaces of both species. Rates of nitrogen fixation by diazotrophs attached to sediment particles were
not significantly different from zero. With additions of d-glucose, ethylene production rates increased by factors of 625-, 34- and 7-fold for sediment, R. mangle and A.␣germinans leaf litter, respectively, compared to rates prior to enrichment. These organically enhanced rates of nitrogen fixation on
leaves could be accounted for by increased activity associated with attached sediment particles and not the leaf material.
Total phenolics [reported as tannic acid equivalent (TAE) units] decreased nitrogen-fixation rates when added to d-glucose-enriched sediment at >20 mg TAE l−1. Phenolic compounds could explain the initial lag in rates of nitrogen fixation during leaf-litter decomposition of R. mangle (initial content of 110.8 mg TAE g−1 dry wt), but not of A. germinans (initial content of 23.4 mg TAE g−1 dry wt). The higher phenolic content and reportedly lower carbon substrate of R. mangle did not result in species-specific differences in either the magnitude or temporal pattern of nitrogen fixation compared
to A. germinans leaf litter. We conclude that the availability of organic substrates leached from the leaf litter along with colonization
by the heterotrophic diazotrophs (as indicated by sediment accumulation) controls nitrogen-fixation rates in a similar manner
in the leaf litter of both species.
Received: 8 August 1997 / Accepted: 4 December 1997 相似文献
5.
Grazing effects on nitrogen fixation in coral reef algal turfs 总被引:2,自引:0,他引:2
This study addressed whether grazing by the sea urchin Diadema antillarum influenced rates of nitrogen fixation by algal turf communities on Caribbean coral reefs. Because the turfs were nitrogen-limited,
we also assessed whether newly-fixed nitrogen was important for supporting net primary productivity by the turfs. We measured
acetylene reduction in turfs grown in treatments excluding or including D. antillarum in the presence of other herbivores at 3 m water depth on Tague Bay forereef, St. Croix, U.S. Virgin Islands. These were
the first measurements of acetylene reduction on coral reefs under quasi-natural conditions of high water-flow and photosynthetic
oxygen generation. Rates of acetylene reduction under these conditions were as high as any measured previously in coral reef
communities (mean 7.6 nmol C2H4 cm−2 h−1). Algal turfs grazed by D. antillarum and other herbivores had chlorophyll-specific acetylene reduction rates up to three times higher than when D. antillarum was excluded. High rates of nitrogen fixation by the turfs were sufficient to meet <2% of the nitrogen required to support
net chlorophyll-specific primary productivity over 24 h. Grazer-mediated increases in nitrogen fixation do not appear responsible
for a parallel enhancement of net primary productivity. Algal turfs at this site must be dependent primarily on external sources
of nitrogen.
Received: 1 July 1997 / Accepted: 5 September 1997 相似文献
6.
Many symbioses involve multiple partners in complex, multi-level associations, yet little is known concerning patterns of
nutrient transfer in multi-level marine mutualisms. We used the anemonefish symbiosis as a model system to create a balance
sheet for nitrogen production and transfer within a three-way symbiotic system. We quantified diel patterns in excretion of
ammonia by anemonefish and subsequent absorption by host sea anemones and zooxanthellae under laboratory conditions. Rates
of ammonia excretion by the anemonefish Amphiprion bicinctus varied from a high of 1.84 μmole g−1 h−1 at 2 h after feeding, to a basal rate of 0.50 μmole g−1 h−1 at 24–36 h since the last meal. Conversely, host sea anemones Entacmaea quadricolor absorbed ammonia at a rate of 0.10 μmole g−1 h−1 during the daytime in ammonia-enriched seawater, but during the night reduced their absorption rate to near zero, indicating
that ammonia uptake was driven by zooxanthella photosynthesis. When incubated together, net ammonia excretion was virturally
zero, indicating that host anemones absorbed most of the ammonia produced by resident fish. Adult anemonefish weighed about
11 g under laboratory conditions, but on the coral reef may reach up to 64 g, resulting in a maximal potential ammonia load
of >200 μmole h−1 produced by two adult fish during daylight hours. In contrast, host sea anemones weighed about 47 g in the laboratory, but
under field conditions, large individuals may reach 680 g, so their maximal ammonia clearance rates may reach about 70 μmole h−1 during the daytime. As such, the ammonia load produced by adult anemonefish far exceeds the clearance rate of host anemones
and zooxanthellae. Ammonia transfer likely occurs mainly during the daytime, when anemonefish consume zooplankton and excrete
rapidly, and in turn the zooxanthellae are photosynthetically active and drive rapid ammonia uptake. We conclude that zooplanktivorous
fishes that form mutualisms with coral reef cnidarians may serve as an important link between open water and benthic ecosystems,
through the transfer of large quantities of nutrients to zooxanthellate hosts, thus enhancing coral reef productivity. 相似文献
7.
We investigated heterogeneity of light acclimation of photosynthesis in sun- and shade-adapted coenosarc and polyp tissues of Pocillopora damicornis. The zooxanthellar community within P. damicornis colonies at Heron Island is genetically uniform, yet they showed a large degree of plasticity in their photo-physiological acclimation linked to light microclimates characterised by fibre-optic microprobes. Microscale scalar irradiance measurements showed higher absorption in polyp than coenosarc tissues and higher absorption in the more densely pigmented shade-adapted polyps than in sun-adapted polyps. The combination of an O2 microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabled parallel measurements of O2 concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface under steady-state conditions as a function of increasing irradiances. Lower O2 levels at the tissue surface and higher compensation irradiance indicated a higher respiration activity in sun-adapted polyp tissue as compared to shade-adapted polyps. Shade-adapted coenosarc and polyp tissues exhibited lower maxima of relative electron transport rates (rETRmax) (84±15 and 41±10, respectively) than sun-adapted coenosarc and polyp tissues (136±14 and 77±13, respectively). Shade-adapted tissues showed stronger decrease of rETR at high scalar irradiances as compared to sun-adapted tissues. The relationship between the relative PSII electron transport and the rate of gross photosynthesis, as well as O2 concentration, was non-linear in sun-adapted tissues over the entire irradiance range, whereas for shade-adapted tissues the relationship became non-linear at medium to high scalar irradiances >200 μmol photons m−2 s−1. This suggests that rETR measurements should be used with caution in corals as a proxy for photosynthesis rates. The apparently high rates of photosynthesis (oxygen evolution rates) suggest that there must be a considerable electron transport rate through the photosystems that is not observed by the rETR measurements. This may be accounted for by vertical heterogeneity of zooxanthellae in the tissue and the operation of an alternative electron pathway such as cyclic electron flow around PSII. 相似文献
8.
E. A. Pakhomov C. D. Dubischar V. Strass M. Brichta U. V. Bathmann 《Marine Biology》2006,149(3):609-623
Distribution, density, and feeding dynamics of the pelagic tunicate Salpa thompsoni have been investigated during the expedition ANTARKTIS XVIII/5b to the Eastern Bellingshausen Sea on board RV Polarstern in April 2001. This expedition was the German contribution to the field campaign of the Southern Ocean Global Ocean Ecosystems Dynamics Study (SO-GLOBEC). Salps were found at 31% of all RMT-8 and Bongo stations. Their densities in the RMT-8 samples were low and did not exceed 4.8 ind m−2 and 7.4 mg C m−2. However, maximum salp densities sampled with the Bongo net reached 56 ind m−2 and 341 mg C m−2. A bimodal salp length frequency distribution was recorded over the shelf, and suggested two recent budding events. This was also confirmed by the developmental stage composition of solitary forms. Ingestion rates of aggregate forms increased from 2.8 to 13.9 μg (pig) ind−1 day−1 or from 0.25 to 2.38 mg C ind−1 day−1 in salps from 10 to 40 mm oral-atrial length, accounting for 25–75% of body carbon per day. Faecal pellet production rates were on average 0.08 pellet ind−1 h−1 with a pronounced diel pattern. Daily individual egestion rates in 13 and 30 mm aggregates ranged from 0.6 to 4.8 μg (pig) day−1 or from 164 to 239 μg C day−1. Assimilation efficiency ranged from 73 to 90% and from 65 to 76% in 13 and 30 mm aggregates, respectively. S. thompsoni exhibited similar ingestion and egestion rates previously estimated for low Antarctic (~50°S) habitats. It has been suggested that the salp population was able to develop in the Eastern Bellingshausen Sea due to an intrusion into the area of the warm Upper Circumpolar Deep Water 相似文献
9.
In the Black Sea, during summer stratification, Calanus euxinus (Hulsemann) undertakes diel vertical migrations with an amplitude of about 117 m from oxygenated, warm (18 °C) surface layers
to hypoxic (∼0.8 mg O2 l−1) zones with lower temperature (7.9 °C). When such changes in temperature and oxygen concentration are reproduced in the laboratory,
total metabolism, basal metabolism and scope of activity of copepods decrease 7.2, 7.8 and 6.7 times, respectively, while
the frequency of locomotory acts and mechanical power decline 3.4- and 9.5-fold, respectively. These changes allowed the copepods
to conserve a significant portion of food consumed near the surface for transformation to lipid reserves. Diel respiratory
oxygen consumption of migrating individuals, calculated so as to include actual duration of residence in layers with different
temperature and oxygen concentrations, is estimated at 17.87 μg O2 ind−1. The net energy cost of vertical migration made up only 11.6% of the total. Copepods expend 78.6% of diel energy losses during
approximately 10 h in the surface layers, while about 5.4% is required during about 9 h at depth. Hypoxia is shown to have
a significant metabolic advantage during diel vertical migrations of C. euxinus in the Black Sea.
Received: 1 October 1999 / Accepted: 11 July 2000 相似文献
10.
Nitrogen fixation (acetylene reduction) at rates of up to 1.2 g N2 g dry wt-1 h-1 was measured for the siphonous green seaweed Codium decorticatum. No nitrogenase activity was detected in C. isthmocladum. The nitrogenase activity was light sensitive and was inhibited by the addition of DCMU and triphenyl tetrazolium chloride. Additions of glucose did not stimulate nitrogen fixation. Blue-green algae (Calothrix sp., Anabaena sp., and Phormidium sp.) were implicated as the organisms responsible for the nitrogenase activity. They occurred in a reduced microzone within the C. decorticatum thallus where nitrogen fixation was optimized. Nitrogen fixation did not affect the kinetic constants for ammonium uptake in C. decorticatum (Ks=12.0 M, Vmax=13.4 mol NH3 g dry wt-1 h-1) determined using the perturbation method. Nevertheless, C. decorticatum thalli which fixed nitrogen had internal dissolved nitrogen concentrations which were over 1.4 times higher than in non-fixing thalli. This suggests that if C. decorticatum does derive part of its nitrogen requirement from the blue-green algae which it harbors, the transfer does not involve competition between this process and the uptake of ambient ammonium. 相似文献
11.
The relative contribution of dissolved nitrogen (ammonium and dissolved free amino acids DFAAs) to the nitrogen budget of
the reef-building coral Pocillopora damicornis was assessed for colonies growing on control and ammonium-enriched reefs at One Tree Island (southern Great Barrier Reef)
during the ENCORE (Enrichment of Nutrient on Coral Reef; 1993 to 1996) project. P. damicornis acquired ammonium at rates of between 5.1 and 91.8 nmol N cm−2 h−1 which were not affected by nutrient treatment except in the case of one morph. In this case, uptake rates decreased from
80.5 to 42.8 nmol cm−2 h−1 (P < 0.05) on exposure to elevated ammonium over 12 mo. The presence or absence of light during measurement did not influence
the uptake of ammonium ions. Nitrogen budgets revealed that the uptake of ammonium from concentrations of 0.11 to 0.13 μM could completely satisfy the demand of growing P. damicornis for new nitrogen. P. damicornis also took up DFAAs at rates ranging from 4.9 to 9.8 nmol N cm−2 h−1. These rates were higher in the dark than in the light (9.0 vs 5.1 nmol m−2 h−1, P < 0.001). Uptake rates were highest for the amino acids serine, arginine and alanine, and lowest for tyrosine. DFAA concentrations
within the ENCORE microatolls that received ammonium were undetectable, whereas they ranged up to 100 nM within the control microatolls. The contribution of DFAAs to the nitrogen budget of P. damicornis constituted only a small fraction of the nitrogen potentially contributed by ammonium under field conditions. Even at the
highest field concentrations measured during this study, DFAAs could contribute only ≃11.3% of the nitrogen demand of P.␣damicornis. This contribution, however, may be an important source of nitrogen when other sources such as ammonium are scarce or during
periods when high concentrations of DFAAs become sporadically available (e.g. cell breakage during fish-grazing).
Received: 22 April 1998 / Accepted: 3 November 1998 相似文献
12.
In situ seagrass photosynthesis measured using a submersible, pulse-amplitude modulated fluorometer 总被引:8,自引:0,他引:8
Assessments of photosynthetic activity in marine plants can now be made in situ using a newly developed, submersible, pulse-amplitude
modulated (PAM) fluorometer: Diving-PAM. PAM fluorometry provides a measure of chlorophyll a fluorescence using rapid-light curves in which the electron-transport rate can be determined for plants exposed to ambient
light conditions. This technique was used to compare the photosynthetic responses of seagrasses near Rottnest Island, Western
Australia. Several fluorescence parameters were measured as a function of time of day and water depth; electron-transport
rate (ETR), quantum yield, photochemical quenching and non-photochemical quenching and Photosystem II (PSII) photochemical
efficiency (F
v
:F
m
ratio) were measured. Results indicate that recent light-history plays a crucial role in seagrass photosynthetic responses.
Maximum ETR of Posidonia australis, Amphibolis antarctica and Halophila ovalis is influenced by the irradiance during the diurnal cycle, with low rates at dawn and dusk (<10 μmol electron m−2 s−1), highest rates in late morning (40 to 60 μmol electron m−2 s−1) and a mid-day depression. Maximum ETR and PSII photochemical efficiency varied widely between seagrass species and were
not correlated. A comparison of photochemical to non-photochemical quenching indicated that seagrasses in shallow water receiving
high light have a high capacity for non-photochemical quenching (e.g. light protection) compared to seagrasses in deep water.
These results indicate that in situ measurements of photosynthesis will provide new insights into the mechanisms and adaptive
responses of marine plants.
Received: 26 May 1997 / Accepted: 27 May 1998 相似文献
13.
A. Cornils S. B. Schnack-Schiel M. Böer M. Graeve U. Struck T. Al-Najjar C. Richter 《Marine Biology》2007,151(4):1261-1274
A total of 12 feeding experiments were conducted in the northern Gulf of Aqaba during spring (March/April) and autumn (September/October)
2002 at the Marine Science Station (MSS) in Aqaba. Females of three species of clausocalanids were selected: Clausocalanus
farrani, C. furcatus and Ctenocalanus vanus. Natural occurring particle (NOP) larger than 5 μm were investigated as food source. The ambient chlorophyll a concentration at sampling depth (∼70 m) ranged between 0.15 and 1.00 μg chl a l−1 and NOP concentrations ranged between 1.78 and 14.0 × 103 cells l−1 during the sampling periods. The division of particles into five size classes (5–10, 10–20, 20–50, 50–100 and >100 μm) revealed
that most of the particles were found in the size classes below 50 μm (81–98%), while most of the natural occurring carbon
(NOC) was concentrated in the size classes larger than 20 μm (70–95%). Ingestion rates were food density dependent rather
than size dependent ranging between 0.02 and 1.65 × 103 NOP ind−1 day−1 and 0.01 and 0.41 μg NOC ind−1 day−1, respectively, equivalent to a body carbon (BC) uptake between 0.4 and 51.8% BC day−1. The share of the size classes to the total ingestion resembled in most cases the size class composition of the natural particle
community. 相似文献
14.
Carbon consumption and nitrogen requirements were estimated for populations of the sandy beach bivalve Donax serra on nine beaches of the west coast of South Africa. Subtidal populations composed mainly of adult clams were responsible for
the bulk of standing stock (3538 g C m−1), annual carbon consumption (13 444 g C m−1 yr−1), faeces production (6478 g C m−1 yr−1 ) and nitrogen regeneration (2525 g N m−1 yr−1). Kelp detritus, bacteria and kelp consumers' faeces available in the water column surpass several times the carbon and nitrogen
requirements of intertidal and subtidal clam populations. Individual Donax serra pop ulations, in turn, may regenerate up to 3.2% of the total nitrogen requirements of all primary producers from kelp beds
and 14% of the requirements of phytoplankton. These high standing stocks of clams are presumably supported mainly by organic
matter originating from kelp which, in contrast to phytoplankton, is in constant supply and comprises the largest proportion
of the annual production of particulate organic matter on this coast. Wide and shallow continental shelves with gentle slopes
probably limit the penetration of upwelled waters to the nearshore waters, decreasing the influence of external inputs and
increasing the importance of internal flows of nutrients and carbon within the nearshore zone. In this context, sandy beaches,
rocky shores and kelp beds may be more closely interlinked compartments of a larger ecosystem encompassing the whole nearshore
than traditionally thought.
Received: 28 August 1996 / Accepted: 7 October 1996 相似文献
15.
Both climate change and the adverse effects of chemical use on human and environmental health are recognized as serious issues
of global concern. Nowhere is this more apparent than in the agricultural sector where release of greenhouse gases such as
carbon dioxide, nitrous oxide and methane continues to be problematic and where use of nitrogen fertilizer is responsible
for negative impacts on both human populations and ecosystems. The manipulation of biological nitrogen fixation (BNF) could
help alleviate part of the difficulty by decreasing the need for nitrogen fertilizers, which require huge quantities of fossil
fuel to produce and contribute to the release of nitrous oxide from soil as well as being responsible for the contamination
of drinking water systems and natural habitats. BNF is performed by a variety of microorganisms. One of the most studied examples
is the BNF carried out by rhizobial bacteria in symbiosis with their plant hosts such as pea and soybean. Hydrogen gas is
an energy-rich, obligate by-product of BNF. Legume symbioses with rhizobia lacking hydrogenase enzymes (which can recycle
hydrogen) have traditionally been viewed as energetically inefficient. However, recent studies suggest hydrogen release to
soil may be beneficial, increasing soil carbon sequestration and promoting growth of hydrogen-oxidizing bacteria beneficial
to plant growth; the alleged superiority of symbiotic performance in rhizobia possessing functional hydrogenases (HUP+) over those rhizobia without functional hydrogenases (HUP−) has also not been conclusively shown. The structure of the iron-molybdenum cofactor or FeMo-co of nitrogenase (the active
site of the enzyme) has been elucidated through X-ray crystallography but the mechanism of nitrogen fixation remains unknown.
However, studies of effects of hydrogen production on BNF have revealed potential candidate intermediates involved in the
nitrogenase reaction pathway and have also shown the role of hydrogen as a competitive inhibitor of N2, with hydrogen now considered to be the primary regulator of the nitrogenase electron allocation coefficient. The regulation
of oxygen levels within legume root nodules is also being investigated; nitrogen fixation is energetically expensive, requiring
a plentiful oxygen supply but too high an oxygen concentration can irreversibly damage nitrogenase, so some regulation is
needed. There is evidence from gas diffusion studies suggesting the presence of a diffusion barrier in nodules; leghaemoglobin
is another potential O2 regulator. Possible functions of hydrogenases include hydrogen recycling, protection of nitrogenase from damaging O2 levels and prevention of inhibitory H2 accumulation; there is evidence for H2 recycling only in studies where H2 uptake has been strongly coupled to ATP production and where this is not the case, it is believed that the hydrogenase acts
as an O2 scavenger, lowering O2 concentrations. The distribution of hydrogenases in temperate legumes has been found to be narrow and root and shoot grafting
experiments suggest the host plant may exert some influence on the expression of hydrogenase (HUP) genes in rhizobia that
possess them. Many still believe that HUP+ rhizobia are superior in performance to HUP− species; to this end, many attempts to increase the relative efficiency of nitrogenase through the introduction of HUP genes
into the plasmids or chromosomes of HUP− rhizobia have been carried out and some have met with success but many other studies have not revealed an increase in symbiotic
performance after successful insertion of HUP genes so the role of HUP in increasing parameters such as N2 fixation and plant yield is still unclear. One advantage of the hydrogen production innate to BNF is that the H2 evolved can be used to measure N2 fixation using new open-flow gas chamber techniques seen as superior to the traditional acetylene reduction assay (ARA) conducted
in closed chambers, although H2 cannot be used for field studies yet as the ARA can. However, the ARA is now believed to be unreliable in field studies and
it is recommended that other measures such as dry weight, yield and total nitrogen content are more accurate, especially in
determining real food production, particularly in the developing nations. Another potential benefit of H2 release from root nodules is that it stays in the soil and has been found to be consumed by H2-oxidizing bacteria, many of which show plant growth–promoting properties such as the inhibition of ethylene biosynthesis
in the host plant, leading to root elongation and increased plant growth; they may well be promising as biofertilizers if
they can be successfully developed into seed inoculants for non-leguminous crop species, decreasing the need for chemical
fertilizers. It has been suggested that rhizobia can produce nitrous oxide through denitrification but this has never been
shown; it is possible that hydrogen release may provide more ideal conditions for denitrifying, free-living bacteria and so
increase production of nitrous oxide that way and this issue will require more study. However, it seems unlikely that a natural
system would release nitrous oxide to the same degree that chemical fertilizers have been shown to do. 相似文献
16.
Dinitrogen fixation associated with bacteria in the gastrointestinal tract of sea urchins appears to be a widespread phenomenon: sea urchins from the tropics (Diadema antillarum, Echinometra lacunter, Tripneustes ventricosus), the temperature zone (Strongylocentrotus droebachiensis) and the arctic (S. droebachiensis) exhibited nitrogenase activity (C2H2 reduction). Pronounced seasonal variation was found in nitrogenase activity of temperate sea urchins feeding on kelp (Laminaria spp.) and eelgrass (Zostera marina). The mean monthly nitrogenase activity was inversely correlated with the nitrogen content of the sea urchin's food, which varied up to fivefold over the course of a year. The highest rate of nitrogenase activity recorded for a temperate sea urchin during the 14 month sampling period was 11.6g N fixed g wet wt-1 d-1, with a yearly mean activity of 1.36 g N fixed g wet wt-1 d-1. Studies with 15N confirmed the C2H2 reduction results and showed incorporation of microbially-fixed nitrogen into S. droebachiensis demonstrating that N2 fixation can be a source of N for the sea urchin. Laboratory experiments indicated that part of the sea urchin's (S. droebachiensis) normal gastrointestinal microflora is responsible for the observed nitrogenase activity. 相似文献
17.
J. W. Fourqurean N. Marbà C. M. Duarte E. Diaz-Almela S. Ruiz-Halpern 《Marine Biology》2007,151(1):219-232
Morphology, elemental content and isotopic composition of leaves of the seagrasses Posidonia oceanica and Cymodocea nodosa were highly variable across the Illes Balears, a Spanish archipelago in the western Mediterranean, and varied seasonally
at one site in the study area. The data presented in this paper generally expand the reported ranges of nitrogen, phosphorus,
iron and arsenic content and δ13C and δ15N for these species. Nitrogen and phosphorus content of P. oceanica leaves also showed significant seasonal variability; on an annual basis, P. oceanica leaves averaged 1.55% N and 0.14% P at this monitoring site. Both N and P were more concentrated in the leaves in winter
than in summer, with winter maxima of 1.76% N and 0.17% P and summer minima of 1.34% N and 0.11% P. There was no significant
annual pattern observed in the δ13C of P. oceanica leaves, but there was a repeated 0.6‰ seasonal fluctuation in δ15N. Mean annual δ15N was 4.0‰; δ15N was lowest in May and it increased through the summer and autumn to a maximum in November. Over the geographic range of
our study area, there were interspecific differences in the carbon, nitrogen and phosphorus content of the two species. Posidonia oceanica N:P ratios were distributed around the critical value of 30:1 while the ratios for C. nodosa were lower than this value, suggesting P. oceanica we collected was not consistently limited by N or P while C. nodosa tended toward nitrogen limitation. Nutrient content was significantly correlated to morphological indicators of plant vigor.
Fe content of P. oceanica leaves varied by a factor of 5×, with a minimum of 31.1 μg g−1 and a maximum of 167.7 μg g−1. Arsenic was present in much lower tissue concentrations than Fe, but the As concentrations were more variable; the maximum
concentration of 1.60 μg g−1 was eight times as high as the minimum of 0.20 μg g−1. There were interspecific differences in δ13C of the two species; C. nodosa was consistently more enriched (δ13C = −7.8 ± 1.7‰) than P. oceanica (−13.2 ± 1.2‰). The δ13C of both species decreased significantly with increasing water depth. Depth related and regional variability in the δ13C and δ15N of both species were marked, suggesting that caution needs to be exercised when applying stable isotopes in food web analyses. 相似文献
18.
Strong variability in bacterioplankton abundance and production in central and western Bay of Bengal
Veronica Fernandes Nagappa Ramaiah Jane T. Paul Sugandha Sardessai R. Jyoti Babu Mangesh Gauns 《Marine Biology》2008,153(5):975-985
With large influx of freshwater that decreases sea-surface salinities, weak wind forcing of <10 m s−1 and almost always warm (>28°C) sea-surface temperature that stratifies and shallows the mixed layer leading to low or no
nutrient injections into the surface, primary production in Bay of Bengal is reportedly low. As a consequence, the Bay of
Bengal is considered as a region of low biological productivity. Along with many biological parameters, bacterioplankton abundance
and production were measured in the Bay of Bengal during post monsoon (September–October 2002) along an open ocean transect,
in the central Bay (CB, 88°E) and the other transect in the western Bay (WB). The latter representing the coastal influenced
shelf/slope waters. Bacterioplankton abundances (<2 × 109cells l−1) were similar to those reported from the HNLC equatorial Pacific and the highly productive northern Arabian Sea. Yet, the
thymidine uptake rates along CB (average of 1.46 pM h−1) and WB (average of 1.40 pM h−1) were less than those from the northwestern Indian Ocean. These abundances and uptake rates were higher than those in the
oligotrophic northwestern Sargasso Sea (<7 × 108 cells l−1; av 1.0 pM h−1). Concentrations of chlorophyll a (chl a), primary production rates and total organic carbon (TOC) were also measured for a comparison of heterotrophic and autotrophic
production. In the WB, bacterioplankton carbon biomass equaled ∼ 95% of chl a carbon than just 31% in the CB. Average bacterial:primary production (BP:PP) ratios accounted for 29% in the CB and 31% in
the WB. This is mainly due to lower primary productivity (PP) in the WB (281 mg C m−2 d−1) than in the CB (306 mg C m−2 day−1). This study indicates that bacteria–phytoplankton relationship differs in the open (CB) and coastal waters (WB). Higher
abundance and contrastingly low bacterial production (BP) in WB may be because of the riverine bacteria, brought in through
discharges, becoming dormant and unable to reproduce in salinities of 28 or more psu. Heterotrophic bacteria appear to utilize
in situ DOC rather rapidly and their carbon demand is ∼50% of daily primary production. It is also apparent that allochthonous
organic matter, in particular in the western Bay, is important for meeting their carbon demand. 相似文献
19.
Hydrodynamic and chemical conditions during onset of a red-tide assemblage in an estuarine upwelling ecosystem 总被引:1,自引:0,他引:1
The hydrodynamics and nitrogen/silicon biogeochemistry accompanying the development of a red-tide assemblage were examined
in the Ría de Vigo (northwest Spain), a coastal embayment affected by upwelling, during an in situ diel experiment in September
1991. Despite a low N:Si molar ratio (0.5) of nutrients entering the surface layer, which was favourable for diatom growth,
the diatom population began to decline. Limited N-nutrient input, arising from moderate coastal upwelling in a stratified
water column, restricted net community production (NCP = 630 mg C m−2 d−1). In addition, light-limitation of gross primary production (GPP = 1525 mg C m−2 d−1) was observed. The relatively high f-ratio (= NCP:GPP) recorded (0.41, characteristic of intense upwelling conditions) would have been as low as 0.15 had not GPP
been limited by light intensity. Temporal separation of carbohydrate synthesis during the photoperiod from protein synthesis
in the dark could be inferred from the time-course of the C:N ratio of particulate organic matter. Severe light-limitation
would lead to diatom collapse were the diatoms not able to meet all their energy requirements during the hours of darkness.
Under the hydrodynamic, nutrient and light conditions of the experiment, an assemblage of red-tide-forming species began to
develop, aided by their ability to migrate vertically and to synthesize carbohydrates during the light in surface waters and
protein during the dark at the 4 m-deep pycnocline. Thermal stratification, reduced turbulence, intense nutrient mineralization,
and the limited nitrogen input through moderate upwelling were all favourable to the onset of a red-tide assemblage.
Received: 15 February 1997 / Accepted: 26 September 1997 相似文献
20.
Organic nitrogen uptake and growth by the chrysophyte Aureococcus anophagefferens during a brown tide event 总被引:5,自引:0,他引:5
The quantitative importance of light-mediated, dissolved organic nitrogen (DON) utilization in relation to overall nitrogen-assimilation
in Aureococcusanophagefferens Hargraves et Sieburth was assessed during a brown tide event in Shinnecock Bay, Long Island, 24 through 26 July 1995. The
growth response of A. anophagefferens was maximal in organic-rich Bay water and decreased proportional to the organic:inorganic nutrient ratio of the water. Short-term
uptake measurements with six nitrogenous substrates revealed that reduced nitrogen could potentially represent 95% of overall
nitrogen uptake of which 70% was due to organic nitrogen alone. Potential uptake of urea by the A. anophagefferens-dominated bloom was substan tially greater than uptake of the other substrates tested during the study, contributing the
largest percentage of total nitrogen uptake (58 to 64%; ρ′
max(urea) 4.4 μg atom N l−1 h−1), followed by NH4
+ (18 to 26%; ρ′max(NH4+) 2 μg atom N l−1 h−1). The combined rates of uptake of algal extract, lysine and glutamic acid contributed between 11 and 16% of total uptake,
whereas NO3
−contributed 5 to 8%. Based on the kinetic determinations from this study we suggest an ecological framework for the events
leading to the dominance and abundance of A. anophagefferens in coastal bays.
Received: 29 March 1997 / Accepted: 24 April 1997 相似文献