Physiological acclimation to light in Chara intermedia nodes

In this study we investigated the ability of Chara intermedia to acclimate to different irradiances (i.e. “low-light” (LL): 20–30 μmol photons m⁻² s⁻¹ and “high-light” (HL): 180–200 μmol photons m⁻² s⁻¹) and light qualities (white, yellow and green), using morphological, photosynthesis, chlorophyll fluorescence and pigment analysis.Relative growth rates increased with increasing irradiance from 0.016 ± 0.003 (LL) to 0.024 ± 0.005 (HL) g g⁻¹ d⁻¹ fresh weight and were independent of light quality. A growth-based branch orientation towards high-light functioning as a mechanism to protect the plant from excessive light was confirmed. It was shown that the receptor responsible for the morphological reaction is sensitive to blue-light.C. intermedia showed higher oxygen evolution (up to 10.5 (HL) vs. 4.5 (LL) nmol O₂ mg Chl⁻¹ s⁻¹), photochemical and energy-dependent Chl fluorescence quenching and a lower Fv/Fm after acclimation to HL. With respect to qP, the acclimation of the photosynthetic apparatus depended on light quality and needed the blue part of the spectrum for full development. In addition, pigment composition was influenced by light and the Chl a/Car and Antheraxanthin (A) + Zeaxanthin (Z)/Violaxanthin (V) + A + Z (DES) ratios revealed the expected acclimation behaviour in favour of carotenoid protection under HL (i.e. decrease of Chl a/Car from 3.41 ± 0.48 to 2.30 ± 0.35 and increase of DES from 0.39 ± 0.05 to 0.87 ± 0.03), while the Chl a/Chl b ratios were not significantly affected. Furthermore it was shown that morphological light acclimation mechanisms influence the extent of the physiological modifications.     

Survival and photosynthetic activity of different Dinophysis acuminata populations in the northern Baltic Sea

This study deals with a recently found phenomenon in the northern Baltic Sea: the occurrence of the dinoflagellate Dinophysis acuminata in the deep water below the thermocline. This was first observed in July 2001 at the station BY 15 in the Gotland Deep, where a sharp and intensive chlorophyll fluorescence signal was encountered at 77 m depth. The fluorescence peak was due to a dinoflagellate community dominated by Dinophysis acuminata (approximately 18 000 cells l⁻¹). The survival of this community was followed in laboratory incubations in low light (20 μE m⁻² s⁻¹) and low temperature (+5 °C). After 5 weeks incubation, 67–84% of the initial cell abundance was lost, while few D. acuminata cells survived up to 24 weeks in the original sample. During the incubation, the fluorescence signal of the cells became fainter and the chloroplasts smaller and aggregated. On two occasions a D. acuminata cell was found attached to a smaller cell by a thin cytoplasm strand, possibly indicating mixotrophic behavior. During the following summer (2002), the photosynthetic efficiency of D. acuminata collected from thermocline layers of few stations and from the nitracline (75–80 m) at one station was studied in photosynthesis irradiance (P–E) incubations. Photosynthetic activity occurred in all populations, with differences in their photosynthetic carbon uptake rates. Photosynthesis of D. acuminata populations was saturated between 250 and 500 μE m⁻² s⁻¹; maximum cell-specific carbon uptake rates (P_m) ranged from 160–925 pg C cell⁻¹ h⁻¹. The P_m-rates in populations originating below the thermocline and in an artificially darkened population were markedly lower than in populations from upper water layers. The varying maximum photosynthetic rates of these populations may reflect their history, e.g. time spent in different light environments.     

Optical colorimetric sensor strip for direct readout glucose measurement

A novel direct readout colorimetric optical glucose sensor strip was constructed based on a three-layer film, including a green-emitted CdTe/CdS quantum dots (QDs) layer as a stable color background, a red-fluorescent platinum-porphyrin oxygen-sensing layer and a glucose oxidase layer. The sensor achieved high resolution (up to 0.2 mmol L⁻¹) glucose determination with a detection range from 0 to 3.0 mmol L⁻¹. A “glucose ruler” which acts as a glucose standard colorimetric card was obtained. Glucose concentration could easily be directly readout using the “glucose ruler”, which made the glucose determination rapid, convenient and easy. The effects of pH, salinity and temperature were systematically investigated. The prepared sensor was finally applied for glucose sample analysis, compared with the “glucose ruler”, accurate results could be directly readout.     

Dried-up zone as a temporal stock of chironomid larvae: survival periods and density in a reservoir bank

The survival period, community composition and density of chironomid larvae were investigated at the reservoir bank in Yachiyoko Lake (Haji Dam), Hiroshima, Japan to verify their survivability in a dried-up zone. We define “dried-up zone” as the one that had been exposed to drought due to drawdown by evaporation and/or drainage. The larvae were collected from sediments at different altitudes along a transectional line. After larval rearing, 189 indiv. m⁻² of adults emerged even from the sediment of a 7-day drought (17.1% sedimental water content). The proportion of Tanytarsus oyamai Sasa in the community tended to be larger with dry periods, indicating that this species has a higher drought tolerance than other species. The mean density in the dried-up zone was estimated as 886.4 indiv. m⁻². Biomass is generally estimated by using the samples of only underwater sediment. These results show that some species have the potential to survive for at least one week in a dried-up zone and suggest that the biomass is likely to be highly underestimated for the entire ecosystem. A pathway of matter flow from aquatic to terrestrial ecosystems via the larvae in a dried-up zone should be taken into consideration.     

Underwater light penetration,phytoplankton biomass and photosynthetic activity in Lake Xolotlán (Managua)

We measured underwater light penetration, phytoplankton biomass and photosynthetic activity during three years (1987–1990) in Lake Xolotlán (L. Managua), Nicaragua. Phytoplankton biomass governed the light climate of the photic zone, but as biomass also was composed of a varying proportion of dead algae, light availability for the potential biomass of actively photosynthesizing algae (170 mg Chl-a.m^–2) was reduced. The concentration of chlorophyll-a within the photic zone was thus lower and ranged between 58 and 141 mg Chl-a.m^–2. Still, photosynthetic activity was high (2,162 mg 02.m-2.h^–1) due to an extremely high specific rate of photosynthesis; light was the only factor that limited growth. As also other conditions in Lake Xolotlán, beside light limitation, met with the requirements of the models that have been used to analyse production and photosynthetic characteristics in tropical lakes there was a striking agreement between observed and predicted values.     

Response of cylindrospermopsin production and release in Aphanizomenon flos-aquae (Cyanobacteria) to varying light and temperature conditions

The influence of light and temperature on the cylindrospermopsin (CYN) production of two Aphanizomenon flos-aquae strains, isolated from North-eastern German lakes, was investigated with semi-continuously growing cultures. A light gradient from 10 to 60 μE m⁻² s⁻¹ in combination with temperatures of 16, 20, and 25 °C was tested.CYN concentrations varied by a maximum factor of 2.7 in strain 10E9 with a significant decrease with increasing temperature. Strain 22D11 showed less pronounced changes, i.e. by a factor of 1.6, and without clear relationship to temperature.Reaction patterns of CYN production to changing light intensities are different at different temperatures. In both strains CYN concentrations increase significantly at 20 °C between 10 and 60 μE m⁻² s⁻¹, whereas they decrease significantly at 25 °C in the same light gradient. The amount of synthesised CYN is not reflected by growth rates of the strains in a uniform manner. Nonetheless several temperature–light combinations which constitute physiological stress seem to trigger CYN production and particularly CYN release from cells. The lowest growth rate observed at 16 °C and 60 μE m⁻² s⁻¹ of strain 22D11 may reflect photoinhibition due to the lower temperature and related limited CO₂-fixation. Under these conditions, extracellular CYN concentrations increased to 58% of total CYN, while the share of extracellular CYN of all other light and temperature regimes was 11–26%. From the results and the experimental design we conclude an active release of the toxin into medium to be more likely than mere leakage from cells.     

The vertical distribution and abundance of <Emphasis Type="Italic">Gammarus lacustris</Emphasis> in the pelagic zone of the meromictic lakes Shira and Shunet (Khakassia,Russia)

The vertical distribution and abundance of Gammarus lacustris in the pelagic zone of two fishless meromictic lakes, L. Shira and L. Shunet, in Southern Siberia (Russia), was studied with the underwater video recording system and using vertical hauls. In both lakes, during summer stratification, Gammarus was distributed non-homogenously, with a stable peak in the metalimnion. The average depth of Gammarus population in the pelagic zone was significantly correlated with the depth of the thermocline. Gammarus abundances obtained using vertical plankton hauls with net were quite comparable with those obtained from video records. The peak abundance of Gammarus in the pelagic zone of the lakes observed with underwater video amounted up to 400 individuals m⁻², while the peak animal densities in the metalimnion reached 50 ind. m⁻³. The data are compared with previously published abundances of Gammarus in the littoral of Lake Shira. Both littoral and pelagic can be equally important habitats for amphipods in meromictic lakes. The absence of fish in the pelagic zone, high oxygen concentration, low water temperature, increased seston concentration, elevated water density in the metalimnion and the anoxic hypolimnion can be the most probable combination of factors that are responsible for the peak of Gammarus in the metalimnion of these lakes.     

Water relations and CO2 exchange of the terrestrial lichen Teloschistes capensis in the Namib fog desert: Measurements during two seasons in the field and under controlled conditions

Although the coastal zone of the Central Namib Desert (Namibia) has negligible rainfall, frequent fog, dew and high air humidity support a luxurious lichen flora. Large areas of soil crust communities are dominated by the multibranched, fruticose Teloschistes capensis interspersed by a (still indeterminable) Ramalina species. In earlier communications, based on field measurements in autumn, we began the analysis of functional mechanisms that allow these lichens to exist under the special conditions of a fog desert. We have extended this work by monitoring lichen CO₂ exchange and water relations in spring and by experiments under controlled conditions.In both seasons, nocturnal hydration, by fog and/or dew, activated dark respiration of the lichens which was followed, after sunrise, by a short period of positive net photosynthesis (NP) that continued until metabolic inactivation occurred from desiccation. Dry thalli of T. capensis were able to reactivate NP through water vapour uptake alone, beginning at an air relative humidity of 82%, i.e. at a water potential of −26.3 MPa; the moisture compensation point during desiccation was at 13% thallus water content (WC, dry weight related). Optimal WC for photosynthesis was around 100%, and both species showed a large and extended suprasaturation depression of CO₂ assimilation. Light response showed “sun-plant” characteristics with saturation >1000 μmol m⁻² s⁻¹ photosynthetically active photon flux density (PPFD). However, due to rapid desiccation, the combination of light saturation with optimal WC very rarely occurred under field conditions. Light compensation point after sunrise was highly dependent on actual WC: at low hydration, it amounted to only ca. 10 μmol m⁻² s⁻¹ PPFD so that even the smallest levels of hydration could be used for carbon gain before desiccation took place again. This phenomenon was probably due to a hydration gradient in the thallus branches during transient moistening so that the outer photobiont layer was favoured in contrast to the internal mycobiont which remained dry longer and did not contribute respiratory CO₂ loss. Fully hydrated thalli had light compensation points around 50 μmol m⁻² s⁻¹ PPFD. Extended desiccation of 1–3 days had no impact on the magnitude and recovery of photosynthesis but, imposed desiccation of 10 days reduced NP in lab and field experiments and caused an extended period of recovery. “Resaturation respiration” was not detected in the field data, although it was present after experimental moistening of dry thalli.In spring, the higher fog frequency and intensity increased maximal nocturnal WC, maximal attained NP as well as integrated daily carbon income (ΣNP) compared to the autumn measurements. NP_max and ΣNP depended on maximal nocturnal WC with a saturation-type response. In terms of carbon gain both species seem to be optimally adapted to nocturnal moistening up to 160% WC and were not able to make use of higher degrees of hydration, a feature that might well influence their habitat selection.Maximal daily carbon-related ΣNP for T. capensis was 4.6 mg_C (g_C)⁻¹ day⁻¹. A rough estimate of the annual (projected) area-related carbon balance (photosynthetic income minus respiratory losses) based on published fog and dew frequencies and personal observations was 15–34 mg_C m⁻² yr⁻¹.     

Land treatment of olive oil mill wastewater

Experiments carried out in lysimeters filled with two calcareous clayey soils (ca 40% CaCO₃; ca 40% clay), showed that a 2m layer of soil almost completely removed the organic and inorganic components of olive oil mill wastewater (OMW) when it was applied in doses of 5000–10000m³ha⁻¹year⁻¹. This efficiency was maintained for at least 2 years. In field experiments, the application of OMW to one of these soils during three successive years at an annual rate of up to 6000m³ha⁻¹ caused changes in some chemical properties of the soil, especially in the upper layer (0–50cm). Concentrations of soil organic matter, Kjeldahl N, soluble NO₃ and available P increased enhancing soil fertility. On the other hand, soil electrical conductivity and sodium adsorption ratio also increased but below the levels representing salinization or sodification hazard for the soil. Furthermore, leaching of Na⁺ and NO₃⁻ below the 1 m layer were detected.     

“Action potentials” in NEUROSPORA CRASSA, a mycelial fungus

Occasional spontaneous “action potentiais” are found in mature hyphae of the fungus Neurospora crassa. They can arise either from low-level sinusoidal oscillations of the membrane potential or from a linear slow depolarization which accelerates into a rapid upstroke at a voltage 5–20 mV depolarized from the normal resting potential (near − 180 mV). The “action potentiais” are long-lasting, 1–2 min and at the peak reach a membrane potential near −40 mV. A 2− to 8−fold increase of membrane conductance accompanies the main depolarization, but a slight decrease of membrane conductance occurs during the slow depolarization. Two plausible mechanisms for the phenomenon are (a) periodic increases of membrane permeability to inorganic ions, particularly H⁺ or Cl^- and (b) periodic decreases in activity of the major electrogenic pump (H⁺) of the Neurospora membrane, coupled with a nonlinear (inverse sigmoid) current-voltage relationship.Identification of action potential-like disturbances in fungi means that such behavior has now been found in all major biologic taxa which have been probed with suitable electrodes. As yet there is no obvious function for the events in fungi.     

Primary Production of Phytoplankton in a Strongly Stratified Temperate Lake

Lake Verevi (12.6 ha, maximum depth 11.0 m, mean depth 3.6 m) is a strongly eutrophic and stratified lake. Planktothrix agardhii is the most characteristic phytoplankton species in summer and autumn, while photosynthesizing sulphur bacteria can occur massively in the metalimnion. Primary production (PP) and chlorophyll a concentration (Chl a) were seasonally studied in 1991, 1993, 2000, and 2001. Vertical distribution of PP was rather complex, having usually two peaks, one at or near the surface (0–1 m), and another deeper (at 3–7 m) in the metalimnion. The values of dark fixation of CO₂ in the metalimnion were in most cases higher than those in the upper water layer. Considering the average daily PP 896 mg C m⁻² and yearly PP 162 mg C m⁻², Secchi depth 2.34 m, and epilimnetic concentrations of chlorophyll a (19.6 mg m⁻³), total nitrogen and total phosphorus (TP, 52 mg m⁻³) in 2000, L. Verevi is a eutrophic lake of a ‘good’ status. Considering the total amounts of nutrients stored in the hypolimnion, the average potential concentrations in the whole water column could achieve 1885 mg m⁻³ of TN and 170 mg m⁻³ of TP reflecting hypertrophic conditions and a ‚bad’ status. Improvement of the epilimnetic water quality from the 1990s to the 2000s may have resulted from incomplete spring mixing and might not reflect the real improvement. A decreased nutrient concentration in the epilimnion has supported the establishment of a ‘clear epilimnion state’ allowing light to penetrate into the nutrient-rich metalimnion and sustaining a high production of cyanobacteria and phototrophic sulphur bacteria.     

The effect of environmental factors on the growth rate of Karenia brevis (Davis) G. Hansen and Moestrup

The red tide dinoflagellate Karenia brevis (Davis) G. Hansen and Moestrup is noted for causing mass mortalities of marine organisms in the Gulf of Mexico. Most research has focused on culture isolates from the eastern Gulf of Mexico. In this investigation, we examine the effects of light, temperature and salinity on the growth rate of K. brevis from the western Gulf of Mexico. Growth rates of K. brevis were determined under various combinations of irradiance (19, 31, 52, 67, and 123 μmol m⁻² s⁻¹), salinity (25, 30, 35, 40 and 45), and temperature (15, 20, 25, and 30 °C). Maximum growth rates varied from 0.17 to 0.36 div day⁻¹ with exponential growth rates increasing with increasing irradiance. Little or no growth was supported at 19 μmol photons m⁻² s⁻¹ for any experiment. Maximum growth rates at 15 °C were much lower than at other temperatures. Maximum growth rates of the Texas clone (SP3) fell within the range of Florida clones reported in the literature (0.17–0.36 div day⁻¹ versus 0.2–1.0 div day⁻¹). The Texas clone SP3 had a very similar light saturation point compared to that of a Florida isolate (Wilson's clone) (67 μmol m⁻² s⁻¹ versus 65 μmol m⁻² s⁻¹), and light compensation (20–30 μmol m⁻² s⁻¹1). The upper and lower salinity tolerance of the Texas clone was similar than that of some Florida clones (45 versus 46 and 25 versus 22.5, respectively). In our study, the Texas clone had the same temperature tolerance reported for Florida clones (15–30 °C). While individual clones can vary considerably in maximum growth rates, our results indicate only minor differences exist between the Texas and Florida strains of K. brevis in their temperature and salinity tolerance for growth. While the literature notes lower salinity occurrences of K. brevis in nearby Louisiana, our isolate from the southern Texas coast has the higher salinity requirements typical of K. brevis in the eastern Gulf of Mexico.     

Facteurs contrôlant la production primaire dens deux rivières soumises a une forte réduction de débit

The effects of river diversion on phytoplankton primary production and biomass in the downstream part of two rivers were studied in relation to physical and chemical variables. These rivers, situated north of the 52nd parallel, are characteristic of oligotrophic systems with phytoplankton primary production less than 10.76 mg C m^–2 h^–1, chlorophyll -a lower than 3.0 mg m^–3 and biomass between 118–1007 mg m^–3. The decrease in flow favored the establishment of an algal biomass approximately two times greater then that present before diversion. This increase in biomass was associated in one river with an increase of 2.5 times of the mean primary production. In the other river the primary production per unit of surface area remained stable but increased when expressed by unit volume, due to a great decrease in underwater light penetration, consequence of inorganic particular matter increase.

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Facteurs contrôlant la production primaire dens deux rivières soumises a une forte réduction de débit

     

The evolution and phytoplankton composition of mucilaginous aggregates in the northern Adriatic Sea

Recurrent occurrences of visible mucilage “clouds” that cover areas up to several hundred kilometres with vertical dimensions of 20–30 m have been recorded in the stratified water column in the northern Adriatic. In the past this was described as “mare sporco” phenomenon. Past studies of the phenomenon indicated that phytoplankton is an important component of mucilage. Our research revealed the composition of phytoplankton assemblages in different types of mucilaginous aggregates collected during the summers of 1997 and 2000 using pigment biomarkers (HPLC). Phytoplankton biomass in the mucilage samples was very high, ranging from 7.9 μg g⁻¹ to 390.8 μg g⁻¹ of chlorophyll a per unit of dry mass of mucilage. The phytoplankton community in the early, loose stage of mucilaginous aggregates was heterogeneous, as indicated by the diversity of detected pigments. The number of phytoplankton groups decreased as the aggregates aged and diatoms increased in relative biomass (up to 92.7%). Phytoplankton biomass in seawater was similar in years with and without mucilage; however, significantly higher contributions to the total biomass of 19′-hexanoyloxyfucoxanthin-containing phytoplankton (prymnesiophytes) were found in the upper 10 m in spring of the “mucilaginous years” (1997 and 2000) followed by prevalence of diatoms in summer. The F_p pigment index used to assess seawater trophic conditions reached lower values in April–May in mucilaginous years (1997 and 2000) compared to non-mucilaginous years (1998 and 1999). We conclude that the role of prymnesiophytes and other small flagellates is crucial for the initial phases of mucilage appearance. Aggregates represent a favourable environment for the secondary development of opportunistic diatoms that foster mucilage formation.     

Nitrogen and phosphorus removal by wetland mesocosms subjected to different hydroperiods

The effect of hydroperiod on nutrient removal efficiency from simulated wastewater was investigated in replicate wetland mesocosms (area, 2 m², planted with Scirpus californicus). Alternate draining and flooding of sediments (pulsed discharge) increased nutrient removal efficiency compared to the continuous-flow “control”. Average PO₄³⁻ removal efficiency was 20–30% higher in wetland mesocosms that drained twice daily compared to the control. Inorganic N removal efficiency was less affected than phosphate removal by hydroperiod variation. At the higher NH₄⁺ loading rate (1.83 g N m⁻² day⁻¹), inorganic N removal efficiency was consistently 5–20% higher in pulsed-discharge wetland mesocosms than in the control. At the lower NH₄⁺ loading rate (0.9 g N m ⁻² day ⁻¹), pulsed-discharge hydrology had no effect on inorganic N removal efficiency. Twice-daily drainage exhibited average inorganic N removal efficiencies of 96% (lower N loading rate) and 87% (higher N loading) and average phosphate removal efficiencies of 81% (lower P loading) and 90% (higher P loading). Mass balance data from the continuous-flow treatment revealed that the aquatic macrophyte Scirpus californicus was the most important nutrient sink, assimilating 50% of the NH₄⁺ and PO₄³⁻ supply. The high plant productivity in the mesocosms (15.6 kg m⁻² year⁻¹) occurred under conditions of high light (high edge per mesocosm area) and high root contact with nutrient-rich influent (shallow, sandy substrate) and may overestimate plant uptake in larger wetlands. The addition of a nitrification-inhibitor (N-Serve) indicated that 34% of the NH₄⁺ supply was transformed to NO₃⁻ by nitrifying bacteria.     

Photosynthetic characteristics of the economic brown seaweed <Emphasis Type="BoldItalic">Hizikia fusiforme</Emphasis> (Sargassaceae,Phaeophyta), with special reference to its “leaf” and receptacle

Photosynthetic responses to irradiance and temperature of “leaves” and receptacles were compared in February (vegetative stage) and May (reproductive stage) in the seaweed, Hizikia fusiforme (Harvey) Okamura (Sargassaceae, Phaeophyta) from Nanao Island, Shantou, China. Irradiance-saturated photosynthesis (P_max) was significantly higher in receptacles than in “leaves” on a fresh weight basis, and that of “leaves” was greater in May than in February at ambient seawater temperatures. The optimum temperature for P_max was 30^∘C for both “leaves” and receptacles, being 5–10^∘C higher than the ambient seawater temperature. The apparent photosynthetic efficiencies were greater in receptacles than in “leaves” within the tested temperature range of 10–40^∘C. The irradiance for saturating photosynthesis for both “leaves” and receptacles was temperature-dependent, with the highest values (about 200μmolphotonsm⁻²s⁻¹) at 30^∘C.     

A mesocosm study of Phaeocystis globosa population dynamics: I. Regulatory role of viruses in bloom control

The regulatory role of viruses on population dynamics of the prymnesiophyte Phaeocystis globosa was studied during a mesocosm experiment in relation to growth and loss by microzooplankton grazing and cell lysis. The mesocosms were conducted under varying light conditions (20 and 150 μmol photons m⁻² s⁻¹) and nutrient regime (inorganic nitrogen to phosphorus ratios of 4, 16 and 44). Overall, viruses infecting P. globosa (PgV) were found to be an important cause of cell lysis (30–100% of total lysis) and a significant loss factor (7–67% of total loss). We demonstrate that the morphology of P. globosa cells (solitary versus colonial) differently regulated viral control of P. globosa bloom formation. Reduced irradiance (20 μmol photons m⁻² s⁻¹) was provided for 11 days to select for the solitary cell morphotype. Viruses were able to restrict P. globosa bloom formation even after irradiance became saturating again (150 μmol photons m⁻² s⁻¹). Saturating light conditions from the start of the experiment allowed colony formation and because the colony-morphotype acted as a mechanism reducing viral infection bloom formation succeeded. Nutrient depletion, however, affected specifically the colonies that disintegrated while releasing single cells. Virus infection of these solitary cells resulted in the termination of the bloom. The nature of phytoplankton growth-limiting nutrient (nitrate and/or orthophosphate) did not seem to noticeably affect the level of viral control.     

Regulation of nitrogenase activity by light in the azolla-anabaena symbiosis

Nitrogenase activity and the rate of photosynthesis were measured simultaneously in Azolla by a continuous gas flow system. The mode of interaction between light, photosynthesis and nitrogenase activity was analysed.Nitrogenase activity dropped off when either Azolla plants or the cyanobiont Anabaena were transferred from light to dark. This decline was immediate and was independent of length or intensity of the prior light phase. Reillumination restored nitrogenase activity.Nitrogenase activity did not depend on the rate of photosynthesis at light intensities below 10 μE m⁻² s⁻¹. Its activity was saturated at 200 μE m⁻² s⁻¹ while CO₂ fixation was saturated at a light intensity of 850 μE m⁻² s⁻¹. Azolla photosynthetic activity followed the absorption spectrum of chlorophyll a, while nitrogenase activity markedly increased between 690 and 710 nm. Inhibition of photosynthesis by DCMU was accompanied by an increase in nitrogenase activity. These results suggest direct light regulation of nitrogenase activity in Azolla independent of CO₂ fixation, and a possible inhibition of nitrogenase activity by the oxygen produced in photosynthesis.     

A comparison of soil carbon pools and profiles in wetlands in Costa Rica and Ohio

Wetlands are large carbon pools and play important roles in global carbon cycles as natural carbon sinks. This study analyzes the variation of total soil carbon with depth in two temperate (Ohio) and three tropical (humid and dry) wetlands in Costa Rica and compares their total soil C pool to determine C accumulation in wetland soils. The temperate wetlands had significantly greater (P < 0.01) C pools (17.6 kg C m⁻²) than did the wetlands located in tropical climates (9.7 kg C m⁻²) in the top 24 cm of soil. Carbon profiles showed a rapid decrease of concentrations with soil depth in the tropical sites, whereas in the temperate wetlands they tended to increase with depth, up to a maximum at 18–24 cm, after which they started decreasing. The two wetlands in Ohio had about ten times the mean total C concentration of adjacent upland soils (e.g., 161 g C kg⁻¹ were measured in a central Ohio isolated forested wetland, and 17 g C kg⁻¹ in an adjacent upland site), and their soil C pools were significantly higher (P < 0.01). Among the five wetland study sites, three main wetland types were identified – isolated forested, riverine flow-through, and slow-flow slough. In the top 24 cm of soil, isolated forested wetlands had the greatest pool (10.8 kg C m⁻²), significantly higher (P < 0.05) than the other two types (7.9 kg C m⁻² in the riverine flow-though wetlands and 8.0 kg C m⁻² in a slowly flowing slough), indicating that the type of organic matter entering into the system and the type of wetland may be key factors in defining its soil C pool. A riverine flow-through wetland in Ohio showed a significantly higher C pool (P < 0.05) in the permanently flooded location (18.5 kg C m⁻²) than in the edge location with fluctuating hydrology, where the soil is intermittently flooded (14.6 kg C m⁻²).     

Leaf traits and photosynthetic parameters of saplings and adult trees of co-existing species in a temperate broad-leaved forest

In Central European forestry the establishment of broad-leaved mixed forests is attaining increasing importance, but little information exists about gas exchange characteristics of some of the tree species involved, which are less abundant today. In an old-growth forest in Central Germany (Hainich, Thuringia), (i) I compared morphological and chemical leaf traits that are indicative of leaf gas exchange characteristics among eight co-existing species, and (ii) analysed photosynthetic parameters of saplings and adult trees (lower and upper canopy level) in four of these species (Acer pseudoplatanus L., Carpinus betulus L., Fraxinus excelsior L. and Tilia platyphyllos Scop.).Leaves from the upper canopy in the eight species studied varied significantly in their specific leaf area (12.9–19.4 m² kg⁻¹), stomatal density (125–313 stomata mm⁻²), leaf nitrogen concentration (95–157 mmol N m⁻²) and δ¹³C content (–27.81 to –25.85‰). F. excelsior and C. betulus were largely contrasting species, which suggests that the species, which were studied in more detail, include the widest difference in leaf gas exchange among the co-existing species. The saplings of the four selected species exhibited shade acclimated leaves with net photosynthesis rates at saturating irradiance (A_max) between 5.0 and 6.4 μmol m⁻² s⁻¹. In adult trees A_max of fully sunlit leaves was more variable and ranged from 10.5 (C. betulus) to 16.3 μmol m⁻² s⁻¹ (F. excelsior). However, less negative δ¹³C values in F. excelsior sun leaves point to a strong limitation in gas exchange. In the lower canopy of adult trees A_max of F. excelsior (12.0 μmol m⁻² s⁻¹) was also greater than that of A. pseudoplatanus, C. betulus and T. platyphyllos (5.0–5.6 μmol m⁻² s⁻¹). This can be explained by the small leaf area and the absence of shade leaves in mature F. excelsior trees. Thus, a considerable variation in leaf traits and gas exchange was found among the co-existing tree species. The results suggest that species-specific characteristics increase the spatial heterogeneity of canopy gas exchange and should be taken into account in the interpretation and prediction of gas flux from mixed stands.In der Forstwirtschaft Mitteleuropas gewinnt die Begründung von Laubmischwäldern zunehmend an Bedeutung, aber über Eigenschaften im Gasaustausch einiger beteiligter Baumarten, die heute nicht so häufig sind, ist wenig bekannt. In einem Altbestand in Mitteldeutschland (Hainich, Thüringen) habe ich (i) morphologische und chemische Eigenschaften von Sonnenblättern, die Hinweise auf Charakteristika im Blattgaswechsel geben, an acht koexistierenden Baumarten untersucht, und (ii) Photosyntheseparameter von juvenilen und adulten Bäumen (unteres und oberes Kronenniveau) von vier dieser Arten (Acer pseudoplatanus L., Carpinus betulus L., Fraxinus excelsior L. and Tilia platyphyllos Scop.) erhoben.Blätter aus dem oberen Kronenraum der acht untersuchten Arten variierten signifikant in der spezifischen Blattfläche (12.9–19.4 m² kg⁻¹), der Stomatadichte (125–313 Stomata mm⁻²), dem Blattstickstoffgehalt (95–157 mmol N m⁻²) und den δ¹³C-Werten (–27.81 bis –25.85‰). In diesem Kollektiv zeigten F. excelsior und C. betulus groβe Unterschiede, was darauf hindeutet, dass die Arten, die genauer untersucht wurden, die Spannweite an Gaswechseleigenschaften unter den koexistierenden Baumarten umfassen. Die Jungpflanzen der vier ausgewählten Arten besaßen Schattenblätter, deren Netto-Photosyntheserate bei hoher Lichtintensität (A_max) zwischen 5.0 and 6.4 μmol m⁻² s⁻¹ variierte. An Sonnenblättern von Altbäumen war A_max variabler und lag zwischen 10.5 (C. betulus) und 16.3 μmol m⁻² s⁻¹ (F. excelsior). Allerdings weisen hohe δ¹³C-Werte in Sonnenblättern von F. excelsior auf eine starke Limitierung des Gasaustauschs hin. Auch in der unteren Krone der Altbäume war A_max von F. excelsior (12.0 μmol m⁻² s⁻¹) höher als A_max von A. pseudoplatanus, C. betulus und T. platyphyllos (5.0–5.6 μmol m⁻² s⁻¹). Dies kann durch die geringe Blattfläche und die Abwesenheit von Schattenblättern in der Krone adulter Bäume von F. excelsior erklärt werden. Zwischen den koexistierenden Baumarten wurde somit in Bezug auf Blatteigenschaften und Photosyntheseparameter eine erhebliche Variation festgestellt. Die Ergebnisse legen nahe, dass artspezifische Eigenschaften die räumliche Heterogenität des Gaswechsels im Kronenraum erhöhen und bei der Interpretation und Vorhersage von Gasflüssen über Mischbeständen berücksichtigt werden sollten.