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1.
This study investigated the variation of bioerosional processes in relation to disturbances of reefal communities due to eutrophication. La Saline fringing reef (Reunion Island) is subjected to nutrient inputs from the adjacent land. Bioerosion by grazers, microborers, and macroborers was measured using experimental substrata exposed for 1 year in three sites characterized by different levels of nutrient input and benthic community response. The relationship between bioerosion and epilithic algal cover of hard substrata and the interactions between the various agents of bioerosion were analyzed with parametric statistics. Significant variations in bioerosion were found among sites, ranging from 1.63 to 3.52 kg CaCO3 m-2 year-1 for grazing rates, from 6.73 to 32.25 g m-2 year-1 for macroboring rates, and from 43.78 to 67.56 g m-2 year-1 for microboring rates. One of the major factors controlling these variations appeared to be changes in the epilithic algal cover on substrata in response to changes in reefal water chemistry. In low nutrient areas, where dead corals were colonized mainly by algal turfs, erosion by microorganisms was low (43.78 g m-2 year-1) due to intense grazing (3.52 kg m-2 year-1). In reef zones receiving high nutrient inputs, the development of encrusting calcareous algae and macroalgae was associated with the lowest grazing (1.63 kg m-2 year-1) and macroboring (6.73 g m-2 year-1) rates recorded among sites. In contrast, high microboring rates (57.54 and 67.56 g m-2 year-1) were found in enriched areas in association with high macroalgal cover.  相似文献   

2.
Phytoplankton photosynthesis was measured during spring-summer 1991-1992 in the inner and outer part of the shallow Potter Cove, King George Island. Strong winds characterise this area. Wind-induced turbulent mixing was quantified by means of the root-mean square expected vertical displacement depth of cells in the water column, Zt. The light attenuation coefficient was used as a measure of the influence of the large amount of terrigenous particles usually present in the water column; 1% light penetration ranged between 30 and 9 m, and between 30 and 15 m for the inner and outer cove, respectively. Obvious differences between photosynthetic capacity [P*max; averages 2.6 and 0.6 µg C (µg chlorophyll-a)-1 h-1] and photosynthetic efficiency {!*; 0.073 and 0.0018 µg C (µg chlorophyll-a)-1 h-1 [(µmol m-2 s-1)-1]} values were obtained for both sites during low mixing conditions (Zt from 10 to 20 m), while no differences were found for high mixing situations (Zt>20 m). This suggests different photoacclimation of phytoplankton responses, induced by modifications of the light field, which in turn are controlled by physical forcing. Our results suggest that although in experimental work P*max can be high, wind-induced mixing and low irradiance will prevent profuse phytoplankton development in the area.  相似文献   

3.
Physiological traits related to water transport were studied in Rhizophora mangle (red mangrove) growing in coastal and estuarine sites in Hawaii. The magnitude of xylem pressure potential (Px), the vulnerability of xylem to cavitation, the frequency of embolized vessels in situ, and the capacity of R. mangle to repair embolized vessels were evaluated with conventional and recently developed techniques. The osmotic potential of the interstitial soil water (?sw) surrounding the roots of R. mangle was c. -2.6LJ.52᎒-3 and -0.4Lj.13᎒-3 MPa in the coastal and estuarine sites, respectively. Midday covered (non-transpiring) leaf water potentials (OL) determined with a pressure chamber were 0.6-0.8 MPa more positive than those of exposed, freely-transpiring leaves, and osmotic potential of the xylem sap (?x) ranged from -0.1 to -0.3 MPa. Consequently, estimated midday values of Px (calculated by subtracting ?x from covered OL) were about 1 MPa more positive than OL determined on freely transpiring leaves. The differences in OL between covered and transpiring leaves were linearly related to the transpiration rates. The slope of this relationship was steeper for the coastal site, suggesting that the hydraulic resistance was larger in leaves of coastal R. mangle plants. This was confirmed by both hydraulic conductivity measurements on stem segments and high-pressure flowmeter studies made on excised leafy twigs. Based on two independent criteria, loss of hydraulic conductivity and proportions of gas- and liquid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to be more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred more readily in intact plants than in excised branches that were air-dried in the laboratory. Cryo-SEM analyses also revealed that, in both sites, the proportion of gas-filled vessels was 20-30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred during the late afternoon when considerable tension was present in neighboring vessels. These results and results from pressure-volume relationships suggest that R. mangle adjusts hydraulic properties of the water-transport system, as well as the leaf osmotic potential, in concert with the environmental growing conditions.  相似文献   

4.
Scots pine (Pinus sylvestris L.) forests of northern Sweden are often considered to be N limited. This limitation may have been exacerbated by the elimination of wildfire as a natural disturbance factor in these boreal forests. Phenolic inhibition of N mineralization and nitrification (due to litter and exudates of ericaceous shrubs) has been proposed as a mechanism for N limitation of these forests, but this hypothesis remains largely untested. N mineralization rates, nitrification rates, and sorption of free phenolic compounds were assessed along a fire-induced chronosequence in northern Sweden. A total of 34 forest stands varying in age since the last fire were identified and characterized. Overstorey and understorey vegetative composition and depth of humus were analysed in replicated plots at all 34 sites. Eight of the forest stands aged 3-352 years since the last fire were selected for intensive investigation in which ten replicate ionic resin capsules (used to assess net N mineralization and nitrification) and non-ionic carbonaceous resin capsules (used to assess free phenolic compounds) were installed at the interface of humus and mineral soil. A highly significant correlation was observed between site age and net sorption of inorganic N to resin capsules. Net accumulation of NH4+ and NO3- on resin capsules followed a linear decrease (R2=0.61, P<0.01) with time perhaps as a result of increased N immobilization with successional C loading. NO3- sorption to resin capsules followed a logarithmic decrease (R2=0.80, P<0.01) that may be related to a logarithmic increase in dwarf shrub cover and decreased soil charcoal sorption potential along this chronosequence. A replicated field study was conducted at one of the late successional field sites to assess the influence of charcoal and an added labile N source on N turnover. Three rates of charcoal (0, 100, and 1,000 g M-2) and two rates of glycine (0 and 50 g N as glycine M-2) were applied in a factorial design to microplots in a randomized complete block pattern. Net ammonification (as assessed by NH4+ sorption to resins) was readily increased by the addition of a labile N source, but this increase in NH4+ did not stimulate nitrification. Nitrification was stimulated slightly by the addition of charcoal resulting in similar levels of resin-sorbed NO3- as those found in early successional sites. Resin-sorbed polyphenol concentrations were decreased with charcoal amendments, but were actually increased with N amendments (likely due to decomposition of polyphenols). Net N mineralization appears to be limited by rapid NH4+ immobilization whereas nitrification is limited by the lack of an appropriate environment or by the presence of inhibitory compounds in late successional forests of northern Sweden.  相似文献   

5.
Photosynthesis and related leaf characteristics were measured in canopies of co-occurring Quercus rubra L. (red oak), Quercus prinus L. (chestnut oak) and Acer rubrum L. (red maple) trees. Mature (20+ m tall) trees were investigated at sites of differing soil water availability within a catchment (a drier upper site and a wetter lower site). Leaf photosynthetic characteristics differed significantly between species and in response to site and position in the canopy. Photosynthetic capacity (Amax) was significantly greater at the wetter site in all canopy strata in A. rubrum but not in Q. rubra or Q. prinus. Our findings for A. rubrum are generally consistent with those predicting that species with higher specific leaf area (SLA) will have higher Amax per unit leaf nitrogen (N) and that species with leaves with lower SLA (e.g. Q. rubra and Q. prinus) will have shallower slopes of the Amax-N relationship. Importantly, the relationships between Amax and Narea (and by implication photosynthetic nitrogen-use efficiency, PNUE) differed in A. rubrum between the sites, with PNUE significantly lower at the drier site. The lower photosynthetic capacity and PNUE must substantially reduce carbon acquisition capacity in A. rubrum under these field conditions. Maximum stomatal conductance (gsmax) differed significantly between species, with gsmax greatest in Q. rubra and Q. prinus. In Q. rubra and Q. prinus, gsmax was significantly lower at the upper site than the lower site. There was no significant response of gsmax to site in A. rubrum. These stomatal responses were consistent with the Ci/Ca ratio, which was significantly lower in leaves of Q. rubra and Q. prinus at the upper site, but did not differ between sites in A. rubrum. Leaf '13C was significantly lower in A. rubrum than in either Q. rubra or Q. prinus at both sites. These findings indicate differences in stomatal behaviour in A. rubrum which are likely to contribute to lower water use efficiency at both sites. Our results support the hypothesis that the two Quercus species, in contrast to A. rubrum, maintain photosynthetic capacity at the drier site whilst minimising transpirational water loss. They also suggest, based primarily on physiological evidence, that the ability of A. rubrum to compete with other species of these deciduous forests may be limited, particularly in sites of low moisture availability and during low rainfall years.  相似文献   

6.
Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (Gs) and stand water use for sites covering a latitudinal range of 5° or 700 km. Measurements were made at three locations of increasing distance from the northern coastline and represent high- (1,750 mm), medium- (890 mm) and low- (520 mm) rainfall sites. This rainfall gradient arises from the weakened monsoonal influence with distance inland. Data were coupled to seasonal estimates of leaf area index (LAI) for the tree and understorey strata. All parameters were measured at the seasonal extremes of late wet and dry seasons. During the wet season, daily rates of evapotranspiration were 3.1-3.6 mm day-1 and were similar for all sites along the rainfall gradient and did not reflect site differences in annual rainfall. During the dry season, site differences were very apparent with evapotranspiration 2-18 times lower than wet season rates, the seasonal differences increasing with distance from coast and reduced annual rainfall. Due to low overstorey LAI, more than 80% of water vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area during the dry season. Water use of individual trees did not differ between the wet and dry seasons at any of the sites and stand water use was a simple function of tree density. Gs declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by Gs and its response to atmospheric and soil water content and by seasonal adjustments to canopy leaf area.  相似文献   

7.
Patterns of bioerosion of dead corals and rubbles on the northern Great Barrier Reef were studied by using blocks of the massive coral Porites experimentally exposed at six sites, located on an inshore–offshore profile, for 1 year and 3 years. Rates of microbioerosion by microborers, grazing by fish, and macrobioerosion by filter-feeding organisms were simultaneously evaluated using image analysis. Microbioerosion, grazing, and total bioerosion were lower at reefs near the Queensland coast than at the edge of the continental shelf (1.81 kg m−2 and 6.07 kg m−2 after 3 years of exposure respectively, for total bioerosion). The opposite pattern was observed for macrobioerosion. Bioaccretion was negligible. These patterns were evident after 1 year of exposure, and became enhanced after 3 years. Microborers were established and were the main agent of bioerosion after 1 year of exposure, and as the principal support for grazing, continued to be the main cause of carbonate loss after 3 years. Full grazing activity and establishment of a mature community of macroborers required more than 1 year of exposure. After 1 year, macroborers and grazers were the second most important agents of bioerosion on both inshore and offshore reefs. However, after 3 years, grazers became the main agents at all sites except at the inshore sites, where macroborers were the principal agents. Because the contribution of microborers, grazers, and macroborers to bioerosion varies in space and time, we suggest that the estimation of reef carbonate budgets need to take in account the activities of all bioerosion agents.  相似文献   

8.
This study assessed the effects of salinity changes over space and time upon leaf gland secretion in Avicennia germinans trees growing naturally in an area featuring markedly seasonal rainfall. Soil ? (, soil N MPa) during the wet season was -0.95ǂ.05 and -2.12ǂ.08 at low and high salinity sites, respectively. During the dry season, these values decreased to -3.24ǂ.09 at low salinity and to -5.75ǂ.06 at high salinity. Consequently, predawn and midday plant water potential were lowered during drought at both sites. The rates of secretion (mmol m-2 h-1 ) increased during drought from 0.91ǂ.12 during the wet season to 1.93ǂ.12 at low salinity, and from 1.69ǂ.12 during the wet season to 2.81ǂ.15 at high salinity. Conversely, stomatal conductance (gs) was lowered by both salinity and drought. As xylem osmolality increased during drought, secretion tended to rise exponentially, and gs decayed hyperbolically. Thus, a trade-off is obtained between enhancement in salt secretion and control of water loss suggested by gs.  相似文献   

9.
Surface waters in forested watersheds in the Adirondack Mountains and northern New York State are susceptible to nitrogen (N) saturation. Atmospheric deposition of N to watersheds in this region has been measured but the extent of internal N inputs from symbiotic N2 fixation in alder-dominated wetlands is not known. We estimated N2 fixation by speckled alder in these wetlands by the 15N natural abundance method and by acetylene reduction using a flow-through system. Foliar N derived from fixation (%Ndfa) was estimated for five wetlands. The '15N of speckled alder foliage from four of the five sites did not differ significantly (PА.05) from that of nodulated speckled alders grown in N-free water culture (-1.2ǂ.1‰). Estimates from the 15N natural abundance method indicated that alders at these sites derive 85-100% of their foliar N from N2 fixation. At one of the sites, we also measured biomass and N content and estimated that the alder foliage contained 43 kg N ha-1 of fixed N in 1997. This estimate was based on a foliar N content of 55.4lj kg N ha-1 (mean-SE), 86dž%Ndfa, and an assumption that 10% of foliar N was derived from reserves in woody tissues. At this site, we further estimated via acetylene reduction that 37ᆞ kg N ha-1 was fixed by speckled alders in 1998. This estimate used the theoretical 4:1 C2H2 reduction to N2 fixation ratio and assumed no night-time fixation late in the season. Nitrogen inputs in wet and dry deposition at this site are approximately 8 kg N ha-1 year-1. We conclude that speckled alder in wetlands of northern New York State relies heavily on N2 fixation to meet N demands, and symbiotic N2 fixation in speckled alders adds substantial amounts of N to alder-dominated wetlands in the Adirondack Mountains. These additions may be important for watershed N budgets, where alder-dominated wetlands occupy a large proportion of watershed area.  相似文献   

10.
Variation in the carbon isotopic composition of ecosystem respiration ('13CR) was studied for 3 years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipitation across these sites ranged from 227 to 2,760 mm. Overall '13CR varied from -23.1 to -33.1‰, and within a single forest, it varied in magnitude by 3.5-8.5‰. Mean annual '13CR differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between '13CR and the vapor saturation deficit of air (vpd) 5-10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the '13CR versus vpd relationship, resulting in higher than expected '13CR values.  相似文献   

11.
Synechococcus was more abundant and had a greater biomass thanProchlorococcus at most inshore and mid-shelf sites in the central(17°S) Great Barrier Reef (GBR), and at all shelf sitesin the southern (20°S) GBR. Significant Prochlorococcuspopulations were confined to mid- and outer-shelf sites withmixed or partially stratified water columns of greater oceaniccharacter in the central GBR, where depth-weighted average Synechococcusand Prochlorococcus abundances were better correlated with salinity,shelf depth and chlorophyll a concentration, than with concentrationsof NH4+, NOx (i.e. NO2 + NO3), or PO43–.Vertical gradients of normalized mean cellular red and orangefluorescenceof Synechococcus and Prochlorococcus populations imply thatvertical mixing rates were sufficiently low to allow these populationsto photoacclimate at depth at shelf locations in thecentralGBR, but too great for substantial photoacclimation to be observedat sites in the southern GBR. The presence of Prochlorococcuspopulations at inshore sites in the central GBR in the absenceof extensive intrusion events suggests that Prochlorococcuspopulations were actively growing.  相似文献   

12.
Inshore reefs of the Great Barrier Reef (GBR) are subject to episodic nutrient supply, mainly by flood events, whereas midshelf reefs have a more consistent low nutrient availability. Alkaline phosphatase activity (APA) enables macroalgae to increase their phosphorus (P) supply by using organic P. APA was high (~4.0 to 15.5 µmol PO43- g DW-1 h-1) in species colonising predominantly inshore reefs and low (<2 µmol PO43- g DW-1 h-1) in species with a cross-shelf distribution. However, APA values of GBR algae in this study were much lower than data reported from other coral reef systems. In experiments with two Sargassum species tissue P levels were correlated negatively, and N:P ratios were positively correlated with APA. High APA can compensate for a relative P-limitation of macroalgae in coral reef systems that are subject to significant N-inputs, such as the GBR inshore reefs. APA and other mechanisms to acquire a range of nutrient species allow inshore species to thrive in habitats with episodic nutrient supply. These species also are likely to benefit from an increased nutrient supply caused by human activity, which currently is a global problem.  相似文献   

13.
Specific conductivity (ks, m2s-1MPa-1) describes the permeability of xylem and is determined by all aspects of xylem anatomy that create resistance to the flow of water. Here we test the hypothesis that ks is a function of radial and vertical position within the stem, rather than solely a function of cambial age (ring number from the pith), by measuring ks on samples excised from 35-year-old Douglas-fir [Pseudotsuga menziesii var. menziesii (Mirb.) Franco] trees at six heights and two or three radial positions. Sapwood ks decreased from the cambium to the heartwood boundary, and the difference between outer and inner sapwood increased with height in the tree. Beneath the live crown, inner sapwood had 80-90% the ks of outer sapwood, but only 55% just 10 m higher in the stem (about 10 nodes down from the tree top). Outer sapwood ks peaked near the base of the crown and declined toward both the base and top of the stem. These patterns can be explained by two superimposed effects: the effect of cambial age on the dimensions of tracheids as they are produced, and the effect of xylem aging, which may include accumulation of emboli and aspiration of bordered pits. Tracheid lumen diameter and earlywood and latewood density and width, all factors known to vary with cambial age, were measured on different trees of the same age and from the same stand. Lumen diameter increased with cambial age, whereas the proportion of latewood and growth ring density increased after an initial decrease in the first 5 years. Our results suggest that the effect of cambial age on xylem anatomy is not sufficient to explain variation in ks. Instead, physical position (both vertical and radial) in the stem and cambial age must be considered as determinants of conductivity.  相似文献   

14.
In-situ estimates of fast-ice algal productivity at Cape Evans, McMurdo Sound, in 1999 were lower than at the same site in previous years. Under-ice irradiance was between 0 and 8 µmol photons m-2 s-1; the ice was between 1.9 and 2.0 m thick and the algal biomass averaged 150 mg chl a m-2, although values as high as 378 mg chl a m-2 were recorded. Production on 11 and 12 November was between 0.053 and 1.474 mg C m-2 h-1. When the data from 11 November were fitted to a hyperbolic tangent function, a multilinear regression gave estimates for Pmax of 0.571 nmol O2 cm-2 s-1, an ! of 0.167 nmol O2 cm-2 s-1 µmol-1 photons m-2 s-1 and an Ek of 3.419 µmol photons m-2 s-1. A Pmax of 2.674 nmol O2 cm-2 s-1, an ! of 0.275 nmol O2 cm-2 s-1 µmol-1 photons m-2 s-1, r of 0.305 nmol O2 cm-2 s-1 and an Ek of 9.724 µmol-1 photons m-2 s-1 were estimated from the 12 November data. The sea-ice algal community was principally comprised of Nitzschia stellata, Entomoneis kjellmanii and Berkeleya adeliensis. Other taxa present included N. lecointei, Fragilariopsis spp., Navicula glaciei, Pleurosigma spp. and Amphora spp. Variations in the method for estimating the thickness of the diffusive boundary layer were not found to significantly affect the measurements of oxygen flux. However, the inability to accurately measure fine-scale variations in biomass is thought to contribute to the scatter of the P versus E data.  相似文献   

15.
Leaves from several desert and woodland species, including gymnosperms and angiosperms with both C3 and C4 physiology, were analyzed to detect trends in '13Cleaf with elevation and slope aspect along two transects in southeastern Utah and south-central New Mexico, USA. The main difference between the two transects is the steeper elevational gradient for mean annual and summer precipitation in the southern transect. For any given species, we found that isotopic differences between individual plants growing at the same site commonly equal differences measured for plants along the entire altitudinal gradient. In C3 plants, '13Cleaf values become slightly enriched at the lowest elevations, the opposite of trends identified in more humid regions. Apparently, increasing water-use efficiency with drought stress offsets the influence of other biotic and abiotic factors that operate to decrease isotopic discrimination with elevation. For some species shared by the two transects (e.g., Pinus edulis and Cercocarpus montanus), '13Cleaf values are dramatically depleted at sites that receive more than 550 mm mean annual precipitation, roughly the boundary (pedalfer-pedocal) at which soils commonly fill to field capacity in summer and carbonates are leached. We hypothesize that, in summer-wet areas, this may represent the boundary at which drought stress overtakes other factors in determining the sign of '13Cleaf with elevation. The opposition of isotopic trends with elevation in arid versus humid regions cautions against standard correction for elevation in comparative studies of '13Cleaf.  相似文献   

16.
Photosynthetic gas exchange, chlorophyll fluorescence, nitrogen use efficiency, and related leaf traits of native Hawaiian tree ferns in the genus Cibotium were compared with those of the invasive Australian tree fern Sphaeropteris cooperi in an attempt to explain the higher growth rates of S. cooperi in Hawaii. Comparisons were made between mature sporophytes growing in the sun (gap or forest edge) and in shady understories at four sites at three different elevations. The invasive tree fern had 12-13 cm greater height increase per year and approximately 5 times larger total leaf surface area per plant compared to the native tree ferns. The maximum rates of photosynthesis of S. cooperi in the sun and shade were significantly higher than those of the native Cibotium spp (for example, 11.2 and 7.1 µmol m-2 s-1, and 5.8 and 3.6 µmol m-2 s-1 respectively for the invasive and natives at low elevation). The instantaneous photosynthetic nitrogen use efficiency of the invasive tree fern was significantly higher than that of the native tree ferns, but when integrated over the life span of the frond the differences were not significant. The fronds of the invasive tree fern species had a significantly shorter life span than the native tree ferns (approximately 6 months and 12 months, respectively), and significantly higher nitrogen content per unit leaf mass. The native tree ferns growing in both sun and shade exhibited greater photoinhibition than the invasive tree fern after being experimentally subjected to high light levels. The native tree ferns recovered only 78% of their dark-acclimated quantum yield (Fv/Fm), while the invasive tree fern recovered 90% and 86% of its dark-acclimated Fv/Fm when growing in sun and shade, respectively. Overall, the invasive tree fern appears to be more efficient at capturing and utilizing light than the native Cibotium species, particularly in high-light environments such as those associated with high levels of disturbance.  相似文献   

17.
The consumption and assimilation rates of the woodlouse Armadillidium vulgare were measured on leaf litters from five herb species grown and naturally senesced at 350 and 700 µl l-1 CO2. Each type of litter was tested separately after 12, 30 and 45 days of decomposition at 18°C. The effects of elevated CO2 differed depending on the plant species. In Medicago minima (Fabaceae), the CO2 treatment had no significant effect on consumption and assimilation. In Tyrimnus leucographus (Asteraceae), the CO2 treatment had no significant effect on consumption, but the elevated CO2 litter was assimilated at a lower rate than the ambient CO2 litter after 30 days of decomposition. In the three other species, Galactites tomentosa (Asteraceae), Trifolium angustifolium (Fabaceae) and Lolium rigidum (Poaceae), the elevated CO2 litter was consumed and/or assimilated at a higher rate than the ambient CO2 litter. Examination of the nitrogen contents in these three species of litter did not support the hypothesis of compensatory feeding, i.e. an increase in woodlouse consumption to compensate for low nitrogen content of the food. Rather, the results suggest that in herbs that were unpalatable at the start of the experiment (Galactites, Trifolium and Lolium), more of the the litter produced at 700 µl l-1 CO2 was consumed than of that produced at 350 µl l-1 because inhibitory factors were eliminated faster during decomposition.  相似文献   

18.
Ten-year-old trees from four Italian populations of Pinus leucodermis (populations A, B, C and D), which were collected from different sites at different altitudes, were grown near Florence, Italy. Needle CO2 gas exchange and chlorophyll fluorescence response to increasing light intensities were evaluated; gas exchange and chlorophyll fluorescence variation between April and July were also monitored. Populations A, B and C showed a similar photosynthetic response to increasing photosynthetic photon flux density (PPFD) intensities, while at various light intensities population D, which originated from the highest altitude, showed the highest photosynthetic rates. In this population photosynthesis was saturated at PPFDs higher than 900 µmol m-2s-1 and a slow decrease of effective photosystem II quantum yield and F'V/F'M in response to increasing PPFDs were found. The same trees also showed a faster recovery in photosynthesis from limitations induced by winter temperatures than the other three populations. This work showed that photosynthetic response to light in population D was different from the other populations; trees from this population were probably naturally selected to prevent photoinhibition due to excess light.  相似文献   

19.
The objective of this study was to evaluate the potential contribution of the soil microbial community in the vicinity of two plant covers, Sanionia uncinata and Deschampsia antarctica, at Machu Picchu Station, King George Island, Antarctica. Soil samples were collected at the study site during the southern (pole) summer period from 0-5 cm and 5-10 cm depths, for chemical and biological analyses. Soil microbial biomass reached a maximal value of 144 µg g-1 in soil samples taken from under the S. uncinata upper layer plant. qCO2 ranged from 167 to 239 µg CO2.mgCmic.h-1 at the 0-5 and 5-10 cm depths, respectively. CO2 evolution showed values of 54.3 mg.m-2 h-1 beneath plant cover and 55.9 mg.m-2 h-1 in the open space. CO2 evolved by substrate induced respiration in the soil samples taken under the plant cover in the summer period, oscillated between 0.25 and 4.78 µg CO2 g-1 h-1. The data obtained from this short study may provide evidence that both activity and the composition and substrate utilization of the microbial community appear to change substantially across the moisture level and sample location.  相似文献   

20.
As part of our studies to examine the molecular basis of cold-adaptation, we have determined the kinetic properties, thermal stability and deduced amino acid sequence of the enzyme lactate dehydrogenase (LDH) from an Antarctic zoarcid fish, Lycodichthys dearborni. Unlike Antarctic notothenioid fish which are endemic to the Southern Ocean, zoarcid fish are cosmopolitan and have a substantially longer evolutionary history as a sub-order. The A4-LDH isoform was isolated and purified from the white muscle of L. dearborni. The kinetic parameters KmPYR and kcat were determined at temperatures from 0 to 25°C. KmPYR was substantially higher at low temperatures than those from Antarctic and temperate notothenioid fish, whereas kcat at these temperatures was essentially the same as those of the other fish LDH in this study. The sequence of L. dearborni A4-LDH was determined from cDNA derived from white muscle RNA and found to be similar to, but distinct from, the A4-LDH sequences of Antarctic notothenioid fish. Molecular modelling based on the structure of the A4-LDH from Pagothenia borchgrevinki suggested that three conservative amino acid changes within the core of the protein that are not directly part of the active site but which might nonetheless influence the active site, may be important in cold-adaptation in L. dearborni A4-LDH, and that several other changes on the surface of the protein might also play a role in cold-adaptation.  相似文献   

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