Distribution, biomass and primary production of an Ahnfeltia tobuchiensis (Ahnfeltiales, Rhodophyta) population in the Bay of Izmena, Kunashir Island

Regularities of distribution and primary production of an Ahnfeltia tobuchiensis (Kanno et Matsubara) Mak. population, an agar-containing red alga, were studied in the Bay ot Izmena. Experiments were conducted in a flow-through system under conditions similar to algal habitats. The population of A. tobuchiensis unattached to the ground may be from a few centimeters to as much as 1 m thick. It has been shown that only the upper part of a stratum 15–20 cm thick receives a sufficient amount ot light to realize its production potential. While 15–20% of photosynthetically active radiation (PAR) of that falling on the water surface reaches the stratum surface, only 0.1% of PAR from that falling on the water surface penetrates through stratum 15 cm thick. It has been shown for A. tobuchiensis that its photosynthetic rate curve during the daytime mainly follows the PAR intensity curve. The highest values of photosynthetic rate have been measured in the afternoon when PAR reaches its maximum. It is noted that a stratum 15–20 cm thick has peak values ot net primary production (NPP) which averages 3.2 g C m^?2 day^?1. The total area of A. tobuchiensis population was 23.4 km², and its biomass was 125 000 tons in this area. On average, the NPP of the A. tobuchiensis population made up in summer and in autumn was 46.8 and 25.0% of its biomass, respectively.     

Photosynthesis and respiration of a deep-water periphyton community (Macclesfield Bank,South China Sea)

The production parameters of the deep-water periphyton were investigated on the Macclesfield Bank (South China Sea) at depths down to 240 m. At 70–90 m depths the substrate is composed of the remains of dead corals and fragments of porous calcareous material overgrown with red coralline and articulated thalline algae Halimeda sp. At 220–240 m depths the substrate consists of rhodolites, which are individual unattached nodules encrusted mainly with red calcareous coralline algae Lithophyllum sp., Porolithon sp. and red lamellar alga Hypoglossum sp. At all depths the substrate is pierced with green filiform algae Ostreobium sp. and covered with benthic diatom algae. The zooperiphyton in the studied area is comprised of bryozoans, sponges, foraminifers and drilling mollusks. Respiration (R) and the gross (Pg) and net (Pn) primary production of the deepwater periphyton community were determined using the oxygen technique (polarographic electrode) at values of photosynthetically active radiation (PAR) close to natural levels. At 70–90 m depths the Pg of the photoperiphyton was 23.3 mg O₂/(m² h), and the Pn of the community had negative values due to respiration of phytoand mainly zooperiphyton and bacteria. At 220–240 m depths, where less than 0.001% of the surface PAR penetrates, the Pg of the phytoperiphyton decreased to 9.6 mg O₂/(m² h), and the R of the community was 25.9 mg O₂/(m² h). A high efficiency of light energy utilization by the phytoperiphyton was found. The apparent quantum yield (Fa) for algae collected at 70–90 m depths was close to the maximum level of 0.097. The values of Fn that we obtained for 220–240 m depths and calculated from identical data [27], were much higher than theoretical values, which cannot be explained based on modern views on the mechanisms of photosynthesis. Possible sources of errors are discussed. It is proposed that we observed additional evolution of oxygen as the result of H₂O₂ degradation.     

Effect of nutrient pulses on photosynthesis of Chaetomorpha linum from a shallow Mediterranean coastal lagoon

The influence of nitrogen and phosphorus pulses on Chaetomorpha linum (Muller) Kutzing growth and photosynthesis was studied in laboratory experiments. Photosynthesis and growth of C. linum from Tancada lagoon seems limited by both nitrogen and phosphorus, as indicated by the high rate (4.7–11.6 mg O₂ g⁻¹ dry weight h⁻¹) of light-saturated photosynthesis (P_m) and growth rates observed under nitrogen plus phosphorus enrichment in relation to enrichment by nitrogen alone (2.9–7.6 mg O₂ g⁻¹ dry weight h⁻¹). Significant increase in nitrogen and phosphorus content as percentage of dry weight was observed in C. linum fertilized with a single nutrient or with nitrogen plus phosphorus. In Tancada lagoon, when availability of nitrogen to primary producers is by pulses, an increase of nitrate concentration in the water column (from 6 to 100 μM) has a greater effect on growth of C. linum (growth rate: 0.13 day⁻¹) than an increase in ammonium concentration (from 20 to 100 μM and growth rate: 0.11 day⁻¹). For a given thallus nitrogen content (0.6–1.4% N), both P_m and the photosynthetic efficiency (α) normalized to dry weight were correlated (r² = 0.73, p < 0.005) indicating that variations in electron transport were coupled to variations in C-fixation capacity. Optimizing both α and P_m may be a general characteristic of thin-structured opportunistic algae in more variable estuarine environments.     

Production characteristics of the population of the red algaAhnfeltia tobuchiensis in Baklan Bight (Peter the Great Bay, Sea of Japan)

Spatial distribution and production characteristics of the population of the unattached red algaAhnfeltia tobuchiensis (Kanno and Matsubara) Mak were studied in Baklan Bight (Sea of Japan) in September 1990 and June 1991. The following environmental factors were determined: the level of photosynthetically active radiation (PAR) penetrating the water column; the water temperature; and the content of oxygen, nitrogen, phosphorus, and dissolved organic substance. the main factor limiting the net primary production (NPP) of theA. tobuchiensis population in Baklan Bight appears to be PAR intensity at the surface of the stratum. In June and September, respectively it constituted about 10% and 0.2% of the PAR on the sea surface. By means of regression analysis, the following equations were obtained to describe the relationship between NPP at the surface of the stratum and the environmental factors studied, PAR intensity being the principal variable: NPP=0.02+0.81 PAR (in June) and NPP=0.02+0.23PAR (in September). In June and September, the NPP of theA. tobuchiensis stratum amounted to 17.2 and 1.3 g/(m² day), and the biomass to 21 and 31 thousand tons, respectively. During the eight months, its distribution changed considerably. In June and September, the daily biomass growth equaled 140.7 and 8.5 t, respectively. A 10-cm-thick layer displayed the highest production characteristics.     

Effects of increased UV-B radiation and elevated levels of tropospheric ozone on physiological processes in European beech (Fagus sylvatica)

As a consequence of the ongoing reduction of the stratospheric ozone layer, the vegetation is exposed to increasing levels of UV-B radiation (280–320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study, 5-year-old European beech trees were exposed from 1 July to October 1993 to two levels of UV-B radiation and two levels of ozone, alone and in combination, in open-top chambers equipped with lamps. The simulated UV-B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastern Denmark, resulting in a 23% difference in biologically effective UV-B (UV-B_BE) irradiance. The maximum UV-Bbe given was 8.61 kJ m⁻² day⁻¹. The ozone levels were either the ambient (average 32 nl l⁻¹) or ambient with ozone addition (average resulting concentration 71 nl l⁻¹). Compared to the control treatment (ambient UV-B, ambient O₃) the elevated levels of UV-B and O₃ affected the trees negatively, expressed as declines in net photosynthesis (P_n), stomatal conductance (g_s), chlorophyll fluorescence (F_v/F_m) and acceleration of senescence, measured as yellowing of the leaves. The UV-B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in P_n and F_v/F_m occurred in the order: control 3 3. Compared to the control, the combination treatment with high levels accelerated the visual senescence processes by ca 27 days, while for high UV-B and O₃ alone, there was an acceleration by 14 and 21 days, respectively. UV-B and O₃ in combination enhanced the negative effects compared with UV-B and O₃ alone. The P_n and F_v/F_m results could be related to this acceleration process. The chamber effect was investigated by comparing the control plots with a plot without open-top chamber. The trees in the chambers showed a higher P_n and F_v/F_m and a 14-day delayed senescence compared to the trees outside, probably caused by higher temperatures, a more protected environment and altered conditions inside the chambers.     

The hydrochemical regime of the habitat of the red algaAhnfeltia tobuchiensis population in the Bay of Izmena (Kunashir Island)

In 1989–1990 and 1997, in the Bay of Izmena (Kunashir Island), the spatial distribution ofAhnfeltia tobuchiensis (Kanno et Matsubara) Mak, an unattached agar-containing red alga, was studied, and the availability of biogenic elements for the process ofA. tobuchiensis primary production was assessed. The concentrations of dissolved O₂, ammonium [NH ₄ ⁺ ], and orthophosphate [PO ₄ ³⁻ ], were highest in the central and northeastern areas of the bay, where the bulk of the population is situated. During the day, the mean oxygen concentration above the alga bed increased from 8.2±0.09 to 8.7±0.08 mg/l, showing a high rate of photosynthesis in the population. As a result of diurnal ammonium and orthophosphate assimilation by the alga, their concentration in bottom waters dropped from 5.1 to 3.5 and from 0.45 to 0.35 μM/l, respectively. Proceeding from the mean values of the nitrogen to phosphorus atomic ratio in the bay waters (11.5±3.3 in the morning and 10.6±3.3 in the evening) and in algal thalluses (33 in the case ofA. tobuchiensis), we conclude that the concentration of major biogenic elements is sufficient for the processes of primary production in the Bay of Izmena.     

Chromatic photoacclimation, photosynthetic electron transport and oxygen evolution in the Chlorophyll d-containing oxyphotobacterium Acaryochloris marina

Changes in photosynthetic pigment ratios showed that the Chlorophyll d-dominated oxyphotobacterium Acaryochloris marina was able to photoacclimate to different light regimes. Chl d per cell were higher in cultures grown under low irradiance and red or green light compared to those found when grown under high white light, but phycocyanin/Chl d and carotenoid/Chl d indices under the corresponding conditions were lower. Chl a, considered an accessory pigment in this organism, decreased respective to Chl d in low irradiance and low intensity non-white light sources. Blue diode PAM (Pulse Amplitude Modulation) fluorometry was able to be used to measure photosynthesis in Acaryochloris. Light response curves for Acaryochloris were created using both PAM and O₂ electrode. A linear relationship was found between electron transport rate (ETR), measured using a PAM fluorometer, and oxygen evolution (net and gross photosynthesis). Gross photosynthesis and ETR were directly proportional to one another. The optimum light for white light (quartz halogen) was about 206 ± 51 μmol m^− 2 s^− 1 (PAR) (Photosynthetically Active Radiation), whereas for red light (red diodes) the optimum light was lower (109 ± 27 μmol m^− 2 s^− 1 (PAR)). The maximum mean gross photosynthetic rate of Acaryochloris was 73 ± 7 μmol mg Chl d^− 1 h^− 1. The gross photosynthesis/respiration ratio (P_g/R) of Acaryochloris under optimum conditions was about 4.02 ± 1.69. The implications of our findings will be discussed in relation to how photosynthesis is regulated in Acaryochloris.     

不同秧苗素质和移栽密度条件下臭氧胁迫对水稻光合作用、物质生产和产量的影响

近地层臭氧浓度升高使水稻生长受抑进而使产量下降,但这种影响是否因不同栽培条件而异尚不清楚。2011年依托先进的稻田臭氧FACE(Free Air gas Concentration Enrichment)技术平台,以汕优63为供试材料,臭氧设置大气臭氧浓度(Ambient)和高臭氧浓度(比Ambient高50%),秧苗素质设置弱苗(移栽时无分蘖)和壮苗(移栽时带两个分蘖),移栽密度设置低密度(16穴/m2)、中密度(24穴/m2)和高密度(32穴/m2),研究不同秧苗素质和移栽密度条件下臭氧胁迫对水稻生长和产量的影响。结果表明:高浓度臭氧使水稻结实期叶片SPAD值、净光合速率、气孔导度和蒸腾速率明显下降,但胞间CO2浓度和叶温无显著变化。高浓度臭氧对水稻拔节前物质生产量没有影响,但使拔节至抽穗期、抽穗至成熟期物质生产量平均分别降低13%和29%,进而使成熟期生物产量和籽粒产量均显著下降。方差分析表明,臭氧与秧苗素质间没有互作效应,但臭氧与移栽密度的互作对最终产量的影响达显著水平。以上结果表明,臭氧胁迫使水稻生长后期光合受阻,导致物质生产和产量显著下降;适当增加移栽密度可能会减少臭氧胁迫下水稻产量的损失。     

The effect of photosynthetically active radiation and temperature on the photosynthesis of two Vietnamese species of Sargassum,S. mcclurei and S. oligocystum,based on the field and laboratory measurements

SUMMARY The effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of two Vietnamese brown algae, Sargassum mcclurei and S. oligocystum (Fucales), were determined by field and laboratory measurements. Dissolved oxygen sensors and pulse‐amplitude modulated (PAM) fluorometry were used for the measurements of photosynthetic efficiency. A Diving‐PAM revealed that underwater measurements of the effective quantum yield (Φ _PSII ) of both species declined with increasing incident PAR, with minimum Φ _PSII occurring during noon to early afternoon. Φ _PSII recovered in the evening, indicating photo‐adaptation to excessive PAR. In laboratory experiments, Φ _PSII also decreased under continuous exposure to 1000 μmol photons m^?2 s^?1; and full recovery occurred after 12 h of dark acclimatization. The net photosynthesis – PAR experiments of S. mcclurei and S. oligocystum conducted at 28°C revealed that the net photosynthetic rate quickly increased at PAR below the saturation irradiance of 361 and 301 μmol photons m^?2 s^?1 and nearly saturated to maximum net photosynthetic rates of 385 and 292 μg O₂ g_ww ^{? 1} min^?1 without photoinhibition, respectively. Gross photosynthesis and dark respiration experiments determined over a range of temperatures (12–40°C), revealed that the maximum gross photosynthetic rates of 201 and 147 μg O₂ g_ww ^{? 1} min^?1 occurred at 32.9 and 30.7°C for S. mcclurei and S. oligocystum, respectively. The dark respiration rates increased exponentially over the temperature ranges examined. The estimated maximum value of the maximum quantum yield occurred at 19.3 and 20.0°C and was 0.76 and 0.74, respectively. Similar to the natural habitat of the study site, these two species tolerated the relatively high temperatures and broad range of PAR. The ability of these species to recover from exposure to high PAR is one of the mechanisms that allow them to flourish in the shallow water environment.     

Impacts of Sediments on Coral Energetics: Partitioning the Effects of Turbidity and Settling Particles

Sediment loads have long been known to be deleterious to corals, but the effects of turbidity and settling particles have not previously been partitioned. This study provides a novel approach using inert silicon carbide powder to partition and quantify the mechanical effects of sediment settling versus reduced light under a chronically high sedimentary regime on two turbid water corals commonly found in Singapore (Galaxea fascicularis and Goniopora somaliensis). Coral fragments were evenly distributed among three treatments: an open control (30% ambient PAR), a shaded control (15% ambient PAR) and sediment treatment (15% ambient PAR; 26.4 mg cm⁻² day⁻¹). The rate of photosynthesis and respiration, and the dark-adapted quantum yield were measured once a week for four weeks. By week four, the photosynthesis to respiration ratio (P/R ratio) and the photosynthetic yield (F_v/F_m) had fallen by 14% and 3–17% respectively in the shaded control, contrasting with corals exposed to sediments whose P/R ratio and yield had declined by 21% and 18–34% respectively. The differences in rates between the shaded control and the sediment treatment were attributed to the mechanical effects of sediment deposition. The physiological response to sediment stress differed between species with G. fascicularis experiencing a greater decline in the net photosynthetic yield (13%) than G. somaliensis (9.5%), but a smaller increase in the respiration rates (G. fascicularis = 9.9%, G. somaliensis = 14.2%). These different physiological responses were attributed, in part, to coral morphology and highlighted key physiological processes that drive species distribution along high to low turbidity and depositional gradients.     

Influences of PAR, temperature and water vapor concentration on gas exchange by ferns

The effects of photosynthetically active radiation (PAR), leaf temperature and the leaf-to-air water vapor concentration drop on net CO₂ uptake and water vapor conductance were surveyed for 14 species of ferns. Most previous studies indicated that ferns have extremely low maximal rates of net CO₂ uptake, below 2 umol m^?2 s^?1, whereas the average maximal rate observed here at 25⁰ C was 7 umol m^?2 s^?1. Net CO₂ uptake reached 90% of saturation at an average PAR (400 to 700 nm) of only 240 umol m^?2 s^?1, consistent with the typically shaded habitats of most ferns. Maximal CO₂ uptake rates were positively correlated with the PAR for 90% saturation (r²=0.59), the chlorophyII per unit leaf area (r²=0.30), the water vapor conductance (r²=0.65), and the CO₂ residual conductance (r²=0.69). A higher water vapor conductance (gwv) was correlated with a greater fractional change in gwv as the leaf-to-air water vapor concentration drop (Δc_wv) was raised from 5to20 g m^?3 (r²=0.90). Specifically, for species with low g_wv of about I mm s^?1 the ratio of g_wv at Δc_wv= 5 g m^?3 to that at Δc_wv= 20 g m^?3 was near 1, but it was near 2 for species with g_wv of about 4 mm s^?1. Such a relationship, which can prevent excessive transpiration, has apparently not previously been pointed out in surveys of other plant groups.     

Regulation of photosynthetic rates of submerged rooted macrophytes

Summary Fourteen temperate, submerged macrophytes were cultivated in the laboratory at high DIC levels (3.3–3.8 mM), 10.4–14.4 mol photons (PAR) m^-2 d^-1 and 15°C. Photosynthesis at photosaturation ranged between 0.59 and 17.98 mg O₂ g^-1 DW h^-1 at ambient pH (8.3) and were markedly higher between 1.76 and 47.11 mg O₂ g^-1 DW h^-1 at pH 6.5 under elevated CO₂ concentrations. Photosynthetic rates were significantly related to both the relative surface area and the chlorophyll content of the leaves. Consequently, the photosynthetic rate was much less variable among the species when expressed per surface area and chlorophyll content instead of dry mass. The chlorophyll content was probably a main predictor of photosynthesis of submerged leaves because of the direct relationship of chlorophyll to the light harvesting capacity and/or a coupling to the capacity for photosynthetic electron transport and carboxylation. A comparison with terrestrial leaves characterized the submerged leaves by their low chlorophyll concentrations and low photosynthetic rates per surface area due to the thin leaves. Photosynthetic rates per chlorophyll content in submerged leaves at CO₂ saturation, however, were at the same level as photosynthesis in terrestrial leaves measured at ambient CO₂ when appropriate corrections were made for differences in incubation temperature.     

Limited effect of ozone reductions on the 20‐year photosynthesis trend at Harvard forest

Ozone (O₃) damage to leaves can reduce plant photosynthesis, which suggests that declines in ambient O₃ concentrations ([O₃]) in the United States may have helped increase gross primary production (GPP) in recent decades. Here, we assess the effect of long‐term changes in ambient [O₃] using 20 years of observations at Harvard forest. Using artificial neural networks, we found that the effect of the inclusion of [O₃] as a predictor was slight, and independent of O₃ concentrations, which suggests limited high‐frequency O₃ inhibition of GPP at this site. Simulations with a terrestrial biosphere model, however, suggest an average long‐term O₃ inhibition of 10.4% for 1992–2011. A decline of [O₃] over the measurement period resulted in moderate predicted GPP trends of 0.02–0.04 μmol C m^?2 s^?1 yr^?1, which is negligible relative to the total observed GPP trend of 0.41 μmol C m^?2 s^?1 yr^?1. A similar conclusion is achieved with the widely used AOT40 metric. Combined, our results suggest that ozone reductions at Harvard forest are unlikely to have had a large impact on the photosynthesis trend over the past 20 years. Such limited effects are mainly related to the slow responses of photosynthesis to changes in [O₃]. Furthermore, we estimate that 40% of photosynthesis happens in the shade, where stomatal conductance and thus [O₃] deposition is lower than for sunlit leaves. This portion of GPP remains unaffected by [O₃], thus helping to buffer the changes of total photosynthesis due to varied [O₃]. Our analyses suggest that current ozone reductions, although significant, cannot substantially alleviate the damages to forest ecosystems.     

Photosynthetic refixing of CO2 is responsible for the apparent disparity between mitochondrial respiration in the light and in the dark

The net photosynthetic rate (P _N), the sample room CO₂ concentration (CO₂S) and the intercellular CO₂ concentration (C _i) in response to PAR, of C₃ (wheat and bean) and C₄ (maize and three-colored amaranth) plants were measured. Results showed that photorespiration (R _p) of wheat and bean could not occur at 2 % O₂. At 2 % O₂ and 0 μmol mol^?1 CO₂, P _N can be used to estimate the rate of mitochondrial respiration in the light (R _d). The R _d decreased with increasing PAR, and ranged between 3.20 and 2.09 μmol CO₂ m^?2 s^?1 in wheat. The trend was similar for bean (between 2.95 and 1.70 μmol CO₂ m^?2 s^?1), maize (between 2.27 and 0.62 μmol CO₂ m^?2 s^?1) and three-colored amaranth (between 1.37 and 0.49 μmol CO₂ m^?2 s^?1). The widely observed phenomenon of R _d being lower than R _n can be attributed to refixation, rather than light inhibition. For all plants tested, CO₂ recovery rates increased with increasing light intensity from 32 to 55 % (wheat), 29 to 59 % (bean), 54 to 87 % (maize) and 72 to 90 % (three-colored amaranth) at 50 and 2,000 μmol m^?2 s^?1, respectively.     

施氮肥缓解臭氧对小麦光合作用和产量的影响

以小麦(Triticum aestivum)品种‘扬麦16’为试材, 利用开放式空气臭氧(O₃)浓度升高平台, 研究了增施氮(N)肥对O₃对小麦光合作用和产量影响的缓解作用。结果表明, O₃胁迫下灌浆期小麦的净光合速率(P_n)、气孔导度(G_s)、蒸腾速率(T_r)、叶绿素a (Chl a)、叶绿素b (Chl b)、类胡萝卜素(Car)、总叶绿素含量(Chl t)和可溶性蛋白的含量显著降低, 降幅分别为28.95%、31.79%、23.17%、58.89%、68.64%、22.89%、60.31%和32.00%; 胞间CO₂浓度(C_i)变化很小; 成熟期生物量和收获时产量也明显下降, 降幅分别为12.23%和12.63%; 而增施N肥可以增加小麦灌浆期的P_n、Chl a、Chl b、可溶性蛋白的含量, 进而增加小麦生物量和产量, 增幅分别为25.66%、83.05%、121.57%、30.33%、14.94%和10.67%, 而对C_i、G_s、T_r、Car含量无明显影响。O₃和N肥对小麦叶片的P_n、Chl t及可溶性蛋白含量有明显的交互作用。因此, 在大气O₃浓度升高条件下增施N肥对小麦O₃损伤有一定的缓解作用。     

Oxygen inhibition of photosynthesis and stimulation of photorespiration in soybean leaf cells

The occurrence of photorespiration in soybean (Glycine max [L.] Merr.) leaf cells was demonstrated by the presence of an O₂-dependent CO₂ compensation concentration, a nonlinear time course for photosynthetic ¹⁴CO₂ uptake at low CO₂ and high O₂ concentrations, and an O₂ stimulation of glycine and serine synthesis which was reversed by high CO₂ concentration. The compensation concentration was a linear function of O₂ concentration and increased as temperature increased. At atmospheric CO₂ concentration, 21% O₂ inhibited photosynthesis at 25 C by 27%. Oxygen inhibition of photosynthesis was competitive with respect to CO₂ and increased with increasing temperature. The Km (CO₂) of photosynthesis was also temperature-dependent, increasing from 12 μm CO₂ at 15 C to 38 μm at 35 C. In contrast, the Ki (O₂) was similar at all temperatures. Oxygen inhibition of photosynthesis was independent of irradiance except at 10 mm bicarbonate and 100% O₂, where inhibition decreased with increasing irradiance up to the point of light saturation of photosynthesis. Concomitant with increasing O₂ inhibition of photosynthesis was an increased incorporation of carbon into glycine and serine, intermediates of the photorespiratory pathway, and a decreased incorporation into starch. The effects of CO₂ and O₂ concentration and temperature on soybean cell photosynthesis and photorespiration provide further evidence that these processes are regulated by the kinetic properties of ribulose-1,5-diphosphate carboxylase with respect to CO₂ and O₂.     

Photosynthetic activity and respiration in an equatorial African soda lake

SUMMARY. Photosynthetic activity and respiration in Lake Sonachi (Kenya), a meromictic soda lake lying in a volcanic crater, were measured through diel cycles during a 15-month period. A pattern of thermal stratification in the morning and mixing in the afternoon and night occurred in the mixolimnion. Diel variations in dissolved oxygen at 50 cm were 2.2–7.5 mgO₂ 1^-11% of the incident photosynthetically available irradiance (PAR) reached a depth of 1.3–2.4 m and, as a consequence, the steepest thermal gradients and highest oxygen concentrations occurred in the top 1–2 m. Vertical profiles of dissolved oxygen were used in three ways to estimate photosynthetic and respiration rates. Changes in dissolved oxygen at the depth of maximal photosynthesis (c. 50 cm) during mid-morning were corrected for vertical diffusion to determine net free water oxygen increases of 70-1800 mg O₂ m^-3 h^-1 Variations in areal oxygen content at successive intervals throughout the day and night were corrected for air-water oxygen exchange to calculate net free water oxygen change per h. Maximal rates of increase (550–4850 mg O₂ m^-2 h^-1) usually occurred in late morning or early afternoon; maximal rates of decrease (440–2600 mg O₂ m^-2 h^-1) were common at sunset. The correction for air-water exchange was usually small because of the low wind speeds and the nearness to saturation of the surface water. Summation of daytime and night-time rates of oxygen change provide estimates of net (-3.4–12 gO₂ m ^-2) and gross (-0.7-18.7 g O₂ m^-2) daily photosynthesis and respiration (0.8-7.2 gO₂ m^-2). Photosynthetic rates of bottled samples ranged from 150 to 870 mgO₂m ^-2h ^-1 and 1.4 to 6.8 g O₂, m^-2 day ^-1The efficiency of utilization of PAR incident on the lake surface varied from 1.0 to 7.2 mmol O₂E^-1 periods with higher irradiance typically had lower efficiencies. Free water estimates of photosynthesis usually exceeded the rates measured in bottles. For example, net, free water changes per hour were 1.2–10 times higher than gross areal rates per hour in bottles. Photosynthetic activity in Lake Sonachi in 1973 and 1974 was modest when compared to other tropical African soda lakes.     

Measurement of CO(2) and H(2)O Vapor Exchange in Spinach Leaf Discs : Effects of Orthophosphate

A leaf chamber has been designed which allows the measurement of both CO₂ and water vapor exchange in Spinacia oleracea leaf discs. The center of the disc lies within a cylindrical gas chamber and its margins are enclosed within a cavity through which water or various metabolites can be pumped. In saturating light and normal atmospheres, the leaf discs have a relatively low resistance to H₂O vapor transfer (r_w = 1.87 seconds per centimeter) and can support high rates of photosynthesis for several hours. The abaxial surface of a disc had a higher resistance to water vapor transfer (r_w = 3.22 seconds per centimeter) than the adaxial (r_w = 2.45 seconds per centimeter) despite having a higher stomatal frequency (abaxial, 105/square millimeter; adaxial, 58/square millimeter). In 2% O₂, the discs required an internal concentration of CO₂ of 115 microliters per liter to support one-half of the maximal velocity of apparent photosynthesis (average value, 66 milligrams CO₂ per square decimeter per hour). In 20% O₂, the comparable values are 156 microliters per liter and 56 milligrams CO₂ per square decimeter per hour. In air, apparent photosynthesis saturated at intensities (750 microeinsteins per square meter per second) well below that of daylight but, when the internal CO₂ was raised to 700 to 900 microliters per liter, photosynthesis was not saturated even at daylight intensities (2025 microeinsteins per square meter per second). The distribution of Prussian blue crystals, formed after ferrocyanide feeding, showed that water entered the disc via the vasculature. When 25-minute pulses of orthophosphate were provided in the feeding solution, there were concentration-dependent increases in both r_w and r_m leading to inhibition of photosynthesis. The orthophosphate-dependent inhibitions were reversible.     

Moisture content and CO2 exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance

Summary Thalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 E m^-2 s^-1 light intensity (PAR) were obtained during drying of the thalli. At ambient CO₂ concentrations from 200 to 1,000 ppm, CO₂ uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO₂ concentration, and it was fully absent at 1,600 ppm ambient CO₂. Under these conditions of CO₂ saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO₂ uptake of the water saturated lichen at lower (e.g. natural) ambient CO₂ must be due exclusively to increased resistance to CO₂ diffusion from the external air to the sites of carboxylation.     

CO2 environment,microclimate and photosynthetic characteristics of the moss Hylocomium splendens in a subarctic habitat

Summary In order to document the natural CO₂ environment of the moss Hylocomium splendens, and ascertain whether or not the moss was adapted to this, and its interactions with other microenvironmental factors, two studies were carried out. Firstly, the seasonal variations of CO₂ concentration, photosynthetically active radiation (PAR), tissue water content and temperature were measured in the natural microenvironment of H. splendens in a subarctic forest during the summer period (July–September). Secondly, the photosynthetic responses of the species to controlled CO₂ concentrations, PAR, temperature, and hydration were measured in the laboratory. CO₂ concentrations around the upper parts of the plant, when PAR was above the compensation point (30 mol m^–2 s^–1), were mostly between 400 and 450 ppm. They occasionally increased up to 1143 ppm for short periods. PAR flux densities below saturating light levels for photosynthesis (100 mol m^–2 s^–1), occurred during 65% (July), 76% (August) and 96% (September) of the hours of the summer period. The temperature optimum of photosynthesis was 20° C: this temperature coincided with PAR above the compensation point during 5%, 6% and 0% of the time in July, August and September, respectively. Optimal hydration of tissues was infrequent. Hence PAR, temperature and water limit CO₂ uptake for most of the growing season. Our data suggest that the higher than normal ambient CO₂ concentration in the immediate environment of the plant counteracts some of the limitations in PAR supply that it experiences in its habitat. This species already experiences concentrations of atmospheric CO₂ predicted to occur over the next 50 years.