PHOTOSYNTHESIS,DARK RESPIRATION AND DESICCATION RESISTANCE OF THE INTERTIDAL SEAWEEDS HESPEROPHYCUS HARVEYANUS AND PELVETIA FASTIGIATA F. GRACILIS12

Rates of net photosynthesis and dark respiration were determined under submersed and emerged conditions for Hesperophycus harveyanus S. & G. and Pelvetia fastigiata f. gracilis (Decne.) S. & G. Both species exhibited submersed photosynthesis-light relationships and dark respiration rates similar to those established for other closely related intertidal, fucoids. Maximal net photosynthesis of H. harveyanus (0.21 mmol O₂ g dry wt.^-1· h^-1; 0.18 mmol CO₂ g dry wt.^-1· h^-1) was similar to that of P. fastigiata f. gracilis (0.17 mmol. O₂ g dry wt.^-1· h^-1; 0.14 mmol CO₂ g dry wt. ^-1· h^-1). Light saturation occurred between 150 and 250 μE · m^-2· s^-1 for H. harveyanus and between 75 and 150 μE · m^-2· s^-1 for P. fastigiata f. gracilis; photon flux densities required for compensation were 6.4 and 9.2 μE · m^-2· s^-1, respectively. Photoinhibition was not observed for either species. The light-saturated, submersed net photosynthetic performances of both species varied significantly with temperature. Greatest photosynthetic rates were obtained at 23° C for H. harveyanus and at 18° C for P. fastigiata f. gracilis. Under emersed conditions, the maximal net photosynthetic rate and the photon flux densities required for saturation were greater for H. harveyanus (0.08 mmol CO₂ g dry wt.^-1· h^-1; 260 to 700 μE · m^-2· s^-1) than for P. fastigiata f. gracilis (0.02 mmol CO₂g dry wt.^-1· h^-1; 72 to 125 μE · m^-2· s^-1). However, for both species, emersed photosynthetic rates were much lower (14–44%) than those obtained under submersed conditions. Desiccation negatively influenced emersed photosynthesis, of both species, but H. harveyanus thalli contained more water when fully hydrated and lost water more slowly during dehydration, thus suggesting greater photosynthetic potential during field conditions of emersion.     

COMPONENTS OF PHOTOSYNTHESIS IN THE INTERTIDAL SACCATE ALGA HALOSACCION AMERICANUM (RHODOPHYTA,PALMARIALES)1

Rates of photosynthesis for the intertidal saccate alga Halosaccion americanum Lee were determined under submersed and emersed conditions. By fitting the data to a hyperbolic tangent function, P _max was 4.08 mmol CO₂. m^?2. h^?1 and I_k was 116.4 μE. m^?2. s^?1. under submersed conditions. Under emersed conditions, P _max was 1.89 mmol CO₂. m^?2. h^?1 and I_k was 22.9 μE. m^?2. s^?1. Dark fixation represented 3.7% of P_max in submersed thalli, whereas it equalled 33.3% of P_max in emersed thalli. Photosynthetic uptake from the thallus cavity represented a significant source of carbon, achieving 68.8% of that from the atmosphere and 29.4% of that from seawater. Retained seawater also greatly reduced drying under emersed conditions. Experimental thalli lost 70.4% of their water after 120 min under desiccating conditions, whereas control thalli lost only 6.3%. Emersed photosynthetic rates were enhanced by desiccation, At times, rates for desiccated thalli were two times those of fully-hydrated ones. Only after water loss exceeded 47% did photosynthetic rates fall below fully-hydrated rates. Utilizing data from this study a model was constructed to determine total photosynthetic production of H. americanum over a single daylight period. These caluclations demonstrate that photosynthetic contributions from emersed photosynthesis and retained seawater are significant. Because production from all sources is almost equal, total photosynthesis over a single day does not change greatly regardless of the time spent in air or in water.     

Photosynthesis of Desiccating Leaves of Poplar

The relation between photosynthesis and water content was investigated using detached leaves of Populus euramericana (Dode) Guinier cv. Robusta. The time course of photosynthesis was measured at different light intensities, at different CO₂ contents of the air and at constant temperature during the desiccation of the leaves. The time course of decreasing water content was obtained from continuous measurement of water transpired from the leaves. A large reduction of light saturated (400 W × m⁻²) photosynthetic rates was observed with decreasing water contents between 78 and 64% (water potential between −14 and −24 atm (bar)). This reduction was much greater in air with 0.3 % CO₂ than in air with 5 % CO₂, indicating a significant influence of CO₂ diffusion resistance on rate of photosynthesis. The reduction of the rate of light and CO₂ saturated photosynthesis (at 400 W × m^–2 and 5% CO₂ in the air) is a measure of the inactivation of the photosynthetic enzyme system by desiccation. A proportional reduction of the light saturated and light limited rate of photosynthesis (for different H₂O contents) was found, when measured in air containing a saturating amount of CO₂ (5 %). The reduction of the light limited rate of photosynthesis (at 20 W × m⁻²) was the same at both CO₂ levels.     

Photosynthetic use of light and inorganic carbon in air and water by the intertidal alga Petalonia fascia (Scytosiphonales, Phaeophyta)

The photosynthetic performance of the intertidal alga Petalonia fascia (0. F. Muller) Kuntze (Scytosiphona-ceae, Phaeophyta) has been investigated, both in air and water, by analyzing the relationship between apparent photosynthesis rate and photon irradiance and inorganic carbon. In relation to the use of photon irradiance, it was found that the net photosynthetic capacity in water was 5.7 times that in air (fully hydrated thallus). The light compensation point was achieved at 5.9 and 3.0 μmol photons m^?2 s^?1 in air and water, respectively. The light onset-saturation parameter and the photosynthetic efficiency were 77% and 25% greater in water than in air, respectively. The dark respiration rate was one-third greater when emersed in comparison to submersion conditions. These data suggest that the photosynthetic response to irradiance in P. fascia is similar to that in infralittoral species rather than the intertidal species. This assessment can be explained by the winter seasonality of the bladed stage of growth, when storms and waves permit a permanent hydrated status of P. fascia that in the intertidal zone. Moreover, the minimum tissue water content that permitted active photosynthesis in the alga was around 20%. The net photosynthetic capacity as a function of inorganic carbon (C) concentration in water was 1.5 times that in air. Photosynthesis was saturated in both media with respect to the availability of inorganic C in natural conditions. The affinity to inorganic C, and the carbon conductance, were two orders of magnitude higher in air than in water. However, the higher photosynthetic capacity when submerged in comparison to emersion conditions suggests that P. fascia can assimilate the external HCO₃,– or the occurrence of a CO₂ concentrating mechanism in this species.     

PHOTOSYNTHESIS AND CALCIFICATION BY EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) AS A FUNCTION OF INORGANIC CARBON SPECIES

To test the possibility of inorganic carbon limitation of the marine unicellular alga Emiliania huxleyi (Lohmann) Hay and Mohler, its carbon acquisition was measured as a function of the different chemical species of inorganic carbon present in the medium. Because these different species are interdependent and covary in any experiment in which the speciation is changed, a set of experiments was performed to produce a multidimensional carbon uptake scheme for photosynthesis and calcification. This scheme shows that CO₂ that is used for photosynthesis comes from two sources. The CO₂ in seawater supports a modest rate of photosynthesis. The HCO is the major substrate for photosynthesis by intracellular production of CO₂ (HCO+ H⁺→ CO₂+ H₂O → CH₂O + O₂). This use of HCO is possible because of the simultaneous calcification using a second HCO, which provides the required proton (HCO+ Ca²⁺→ CaCO₃+ H⁺). The HCO is the only substrate for calcification. By distinguishing the two sources of CO₂ used in photosynthesis, it was shown that E. huxleyi has a K_½ for external CO₂ of “only” 1.9 ± 0.5 μM (and a V_max of 2.4 ± 0.1 pmol·cell⁻¹·d⁻¹). Thus, in seawater that is in equilibrium with the atmosphere ([CO₂]= 14 μM, [HCO]= 1920 μM, at fCO₂= 360 μatm, pH = 8, T = 15° C), photosynthesis is 90% saturated with external CO₂. Under the same conditions, the rate of photosynthesis is doubled by the calcification route of CO₂ supply (from 2.1 to 4.5 pmol·cell⁻¹·d⁻¹). However, photosynthesis is not fully saturated, as calcification has a K_½ for HCO of 3256 ± 1402 μM and a V_max of 6.4 ± 1.8 pmol·cell⁻¹·d⁻¹. The H⁺ that is produced during calcification is used with an efficiency of 0.97 ± 0.08, leading to the conclusion that it is used intracellularly. A maximum efficiency of 0.88 can be expected, as NO uptake generates a H⁺ sink (OH⁻ source) for the cell. The success of E. huxleyi as a coccolithophorid may be related to the efficient coupling between H⁺ generation in calcification and CO₂ fixation in photosynthesis.     

EFFECTS OF CO2 ENRICHMENT ON THE BLOOM‐FORMING CYANOBACTERIUM MICROCYSTIS AERUGINOSA (CYANOPHYCEAE): PHYSIOLOGICAL RESPONSES AND RELATIONSHIPS WITH THE AVAILABILITY OF DISSOLVED INORGANIC CARBON1

Microcystis aeruginosa Kütz. 7820 was cultured at 350 and 700 μL·L ^{? 1} CO₂ to assess the impacts of doubled atmospheric CO₂ concentration on this bloom‐forming cyanobacterium. Doubling of CO₂ concentration in the airflow enhanced its growth by 52%–77%, with pH values decreased and dissolved inorganic carbon (DIC) increased in the medium. Photosynthetic efficiencies and dark respiratory rates expressed per unit chl a tended to increase with the doubling of CO₂. However, saturating irradiances for photosynthesis and light‐saturated photosynthetic rates normalized to cell number tended to decrease with the increase of DIC in the medium. Doubling of CO₂ concentration in the airflow had less effect on DIC‐saturated photosynthetic rates and apparent photosynthetic affinities for DIC. In the exponential phase, CO₂ and HCO₃ ^? levels in the medium were higher than those required to saturate photosynthesis. Cultures with surface aeration were DIC limited in the stationary phase. The rate of CO₂ dissolution into the liquid increased proportionally when CO₂ in air was raised from 350 to 700 μL·L ^{? 1}, thus increasing the availability of DIC in the medium and enhancing the rate of photosynthesis. Doubled CO₂ could enhance CO₂ dissolution, lower pH values, and influence the ionization fractions of various DIC species even when the photosynthesis was not DIC limited. Consequently, HCO₃ ^? concentrations in cultures were significantly higher than in controls, and the photosynthetic energy cost for the operation of CO₂ concentrating mechanism might decrease.     

The role of leaf and canopy-level gas exchange in the replacement of Quercus virginiana (Fagaceae) by Juniperus ashei (Cupressaceae) in semiarid savannas

Photosynthesis, transpiration, and leaf area distribution were sampled in mature Quercus virginiana and Juniperus ashei trees to determine the impact of leaf position on canopy-level gas exchange, and how gas exchange patterns may affect the successful invasion of Quercus communities by J. ashei. Sampling was conducted monthly over a 2-yr period in 12 canopy locations (three canopy layers and four cardinal directions). Photosynthetic and transpiration rates of both species were greatest in the upper canopy and decreased with canopy depth. Leaf photosynthetic and transpiration rates were significantly higher for Q. virginiana (4.1–6.7 μmol CO₂·m⁻²·s⁻¹ and 1.1–2.1 mmol H₂O·m⁻²·s⁻¹) than for J. ashei (2.1–2.8 μmol CO₂·m⁻²·s⁻¹ and 0.7–1.0 mmol H₂O·m⁻²·s⁻¹) in every canopy level and direction. Leaves on the south and east sides of both species had higher gas exchange rates than leaves on the north and west sides. Although Quercus had a greater mean canopy diameter than Juniperus (31.3 vs. 27.7 m²), J. ashei had significantly greater leaf area (142 vs. 58 m²/tree). A simple model combining leaf area and gas exchange rates for different leaf positions demonstrated a significantly greater total canopy carbon dioxide uptake for J. ashei compared to Q. virginiana (831 vs. 612 g CO₂·tree⁻¹·d⁻¹, respectively). Total daily water loss was also greater for Juniperus (125 vs. 73 Ltree⁻¹·d⁻¹). Differences in leaf gas exchange rates were poor predictors of the relationship between the invasive J. ashei and the codominant Q. virginiana. Leaf area and leaf area distribution coupled with leaf gas exchange rates were necessary to demonstrate the higher overall competitive potential of J. ashei.     

Mannitol metabolism of the epiphytic mangrove red alga Caloglossa leprieurii (Ceramiales): A long-term field study

The concentrations of mannitol and floridean starch were determined in a 1–year field study of the epiphytic red alga Caloglossa leprieurii (Montagne) J. Agardh from warm temperate waters of eastern Australia. Seasonal environmental data for air and water temperature, day length and rainfall were recorded. The mannitol content and the floridean starch content varied significantly between collections but no seasonal responses were observed, nor were the contents correlated with any of the abiotic factors. A possible function of the starch pool as a supply for respiratory substrates under emergent conditions is discussed. All data indicate that productivity and biomass of C. leprieurii are affected by short-term abiotic and/or biotic conditions rather than controlled directly by seasonally fluctuating environmental factors. In addition, the activity of three highly specific enzymes (mannitol-1–phosphate dehydroge-nase, mannitol-1–phosphatase, mannitol-dehydroge-nase) and one non-specific enzyme (hexokinase), all of which are involved in the mannitol cycle, were measured in cell-free extracts every 2 weeks. All enzymes showed marked changes in activity over the year, but again no clear patterns emerged, neither with season nor in relationship to one another. On the basis of the results here, C. leprieurii is considered to be a 'season responded rather than a ‘season anticipator’.     

Primary production of phytoplankton in Lake Valencia (Venezuela)

Lake Valencia is heavily polluted by waste water of domestic, agricultural and industrial origin. The high organic load may have produced important changes in the limnological properties. Cyanobacteria dominated in numbers and biomass (over 90% throughout the year). Chlorophyll-a content averaged 37.7 + 15 μg · 1⁻¹. Maximum concentrations of 50–80 μg · 1⁻¹ were found near the inflows affected by organically polluted affluents. There has been a 50% reduction in the euphotic zone in only 13 years. The maximum rate of gross photosynthesis per hour at light saturation was determined within the uppermost 1-meter layer. The highest value was 16,290 mg O₂ · m⁻³ · h⁻¹. Lake Valencia is among the most productive lakes in the world, with areal net photosynthesis averaging 7.5 g C · m⁻² · d⁻¹.     

Differential inhibition of photosynthesis during pre-flowering drought stress in Sorghum bicolor genotypes with different senescence traits

Young (16-day-old) Sorghum bicolor plants of a late- and slow-senescing Texas A&M line (B 35) and of an early- and fast-senescing descendant of an Ethiopian landrace (E 36-1) were subjected to drought stress by decreasing the soil water content to 30% field capacity over 6 days. Plant water potentials decreased from − 2 bar (controls) to − 10 to − 18 bar, and this drought stress resulted in: (1) differential phenotypic reactions and (2) differential decreases in photosynthesis rates in the two cultivars. While E 36-1 tended to lose viable leaf area from the leaf tips downwards, B 35 showed a gradual overall drying of the leaf. At the same time, photosynthesis rates decreased from 31.5 ± 1.6 to 12.3 ± 5.0 µmol CO₂ m⁻² s⁻¹ (E 36-1) and from 30.5 ± 1.6 to 3.3 ± 2.6 µmol CO₂ m⁻² s⁻¹ (B 35), respectively. In vitro enzyme activities of phosphoenolpyruvate carboxylase (PEPCase), malate dehydrogenase (MDH) and malic enzyme (ME) on a leaf area basis exceeded the photosynthesis rates. Pyruvate phosphate dikinase (PPDK) activity was close to the photosynthesis rates in control plants and higher than the photosynthesis rates in drought-stressed plants. Thus, none of the enzymes appeared to limit photosynthesis under drought stress, and likely bottleneck enzyme activities of the C3 pathway in the bundle-sheath cells, i.e. ribulose-1,5-bisphosphate carboxylase (RubisCO) and stromal fructose-1,5-bisphosphatase (sFBPase), also showed sufficient activities to sustain higher photosynthesis rates than those observed in the stressed plants. However, under drought stress, total leaf malate concentrations were higher in B 35 (up to 33.1 µmol g⁻¹ FW) than in E 36-1 (up to 22.4 µmol g⁻¹ FW). In particular, at the presumed cytosolic pH of 7.0–7.3, S. bicolor PEPCase was strongly inhibited by malate. In contrast with the in vitro PEPCase enzyme activities, the A/C_i curves suggested a stronger decrease in the in vivo activity of the enzyme in B 35 under drought stress than in E 36-1. It is therefore suggested that photosynthesis under drought stress may be inhibited differentially through feedback malate inhibition of PEPCase in S. bicolor.     

PHOTOPHYSIOLOGICAL EVIDENCE OF NUTRIENT LIMITATION OF PLATELET ICE ALGAE IN MCMURDO SOUND, ANTARCTICA

Seasonally changing photophysiological and biochemical characteristics of sea ice microalgae are interpreted with respect to light availability and measurements of nutrient concentration made at high vertical resolution (12.5 cm) during a dense bloom in the platelet ice layer of McMurdo Sound during a 6-week study in austral spring of 1989. Platelet ice algae remained highly shade adapted throughout the spring as shown by their low photoadaptive index (E_k, 3.7–8.4 μmol photons·m⁻²·s⁻¹), low mean specific absorption coefficient (<0.009 m² mg⁻¹ Chl a), high optical cross-sectional area of photosystem II (σ_PSII, 3.0–8.2), and high molar ratio of fucoxanthin:chlorophyll a (mean = 1.62 ± 0.15 SD). Between 24 October and 8 November, the algae exhibited a photoacclimative response that was marked by a 30% decrease in photosynthetic efficiency (α^B), a 75% decrease in maximum photosynthetic rate (P^B/_m), and a 60% increase in σ_PSII. The photochemical conversion efficiency at photosystem II (F_v/F_m= ca. 0.5) and the quantum yield of photosynthesis (Ø_C= 0.062– 0.078 mol C mol⁻¹ photons) were ca. 80% of their maximal values. After 8 November, changes in algal photophysiology and biochemistry, which were inconsistent with a photoacclimation response, suggest that the platelet ice algae near the platelet/congelation ice interface became increasingly nutrient limited. The number of pennate diatoms increased threefold to 150 × 10⁹ cells m⁻³ between 8 and 14 November, then remained unchanged throughout the remainder of the field season. Following the increase in cell number, F_v/F_m, Ø_C, and C:Chla decreased by >40%, σ_PSII increased by 70%; and the biochemical ratios C:N and C:Si increased 25%–30%. Nutrient depletion was apparent from the high-resolution vertical profiles, but nutrient concentrations limiting algal growth were not observed. However, nutrient concentrations at the likely site of nutrient limitation near the platelet/congelation ice interface were not measured, indicating that higher resolution sampling is necessary to fully characterize this highly variable habitat.     

Interactions of blue light and inorganic carbon supply in the control of light-saturated photosynthesis in brown algae

Photosynthetic capacities of five species of brown algae in red light were found to be strongly limited by the inorganic carbon supply of natural sea water. Under these conditions, pH 8·2 and dissolved inorganic carbon concentration (DIG) of 2·1 mol m^?3, a short pulse of blue light was found to increase the subsequent rate of photosynthesis in saturating red light. The degree of blue light stimulation varied between species, ranging from an increase of over 200% of the original rate in Colpomenia peregrins to only 10% in Dictyota dichotoma. Increasing the DIG concentration of sea water by bicarbonate addition resulted in carbon saturation of photosynthesis in all five species. Blue light stimulation was greatly reduced at these higher DIG concentrations. The response in Laminaria digitata was examined in more detail by manipulation of pH and DIG to produce solutions with different concentrations of dissolved CO₂. At a CO₂ concentration typical of normal sea water (12·4 mmol m^?3), blue light treatment increased photosynthetic rate by approximately 50%. Blue light stimulation was increased to over 150% at CO₂ concentrations below that of sea water, whereas at concentrations above that of sea water, the effect was diminished. Therefore, the effect of blue light on photosynthetic capacity appears to involve an increase in the rate of supply of carbon dioxide to the plant.     

RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND CELL SIZE OF MARINE DIATOMS12

Three photosynthetic parameters of 7 species of marine diatoms were studied using Na₂¹⁴CO₃ at 5–8 C using log phase axenic cultures. The cell volumes of the different species varied from 70 μm³ to 40 × 10⁵μm³. The present experiment is consistent with the interpretation that the initial slope α (mg C · [mg chl a]^?1· h^?1· w^?1· m²) of photosynthesis vs. light curves is controlled by self-shading of chlorophyll a in the cell. Pm, the rate of photosynthesis at light saturation (mg C · [mg cell, C]^?1· h^?1) and R, the intercept at zero light intensity (mg C · [mg cell C]^?1· H^?1) are both dependent on the ratio of surface area to volume of cell.     

Strong seasonal patterns of DOC release by a temperate seaweed community: Implications for the coastal ocean carbon cycle

Release of dissolved organic carbon (DOC) by seaweed underpins the microbial food web and is crucial for the coastal ocean carbon cycle. However, we know relatively little of seasonal DOC release patterns in temperate regions of the southern hemisphere. Strong seasonal changes in inorganic nitrogen availability, irradiance, and temperature regulate the growth of seaweeds on temperate reefs and influence DOC release. We seasonally surveyed and sampled seaweed at Coal Point, Tasmania, over 1 year. Dominant species with or without carbon dioxide (CO₂) concentrating mechanisms (CCMs) were collected for laboratory experiments to determine seasonal rates of DOC release. During spring and summer, substantial DOC release (10.06–33.54 μmol C · g DW⁻¹ · h⁻¹) was observed for all species, between 3 and 27 times greater than during autumn and winter. Our results suggest that inorganic carbon (C_i) uptake strategy does not regulate DOC release. Seasonal patterns of DOC release were likely a result of photosynthetic overflow during periods of high gross photosynthesis indicated by variations in tissue C:N ratios. For each season, we calculated a reef-scale net DOC release for seaweed at Coal Point of 7.84–12.9 g C · m⁻² · d⁻¹ in spring and summer, which was ~16 times greater than in autumn and winter (0.2–1.0 g C · m⁻² · d⁻¹). Phyllospora comosa, which dominated the biomass, contributed the most DOC to the coastal ocean, up to ~14 times more than Ecklonia radiata and the understory assemblage combined. Reef-scale DOC release was driven by seasonal changes in seaweed physiology rather than seaweed biomass.     

PRODUCTIVITY OF THE FILAMENTOUS ALGA PITHOPHORA OEDOGONIA (CHLOROPHYTA) IN SURREY LAKE,INDIANA1

Biomass, akinete numbers, net photosynthesis, and respiration of Pithophora oedogonia were monitored over two growing seasons in shallow Surrey Lake, Indiana. Low rates of photosynthesis occurred from late fall to early spring and increased to maximum levels in late spring to summer (29–39 mgO₂·g^?1 dry wt·h^?1). Areal biomass increased following the rise in photosynthesis and peaked in autumn (163–206g dry wt·m^?2). Photosynthetic rates were directly correlated with temperature, nitrogen, and phosphorus over the entire annual cycle and during the growing season. Differences in photosynthetic activity and biomass between the two growing seasons (1980 and 1981) were apparently related to higher, early spring temperatures and higher levels of NO₃-N and PO₄-P in 1981. Laboratory investigations of temperature and light effects on Pithophora photosynthesis and respiration indicated that these processes were severely inhibited below 15°C. The highest P_max value occurred at 35°C (0.602 μmol O₂·mg^?1 chl a·min^?1). Rates of dark respiration did not increase above 25°C thus contributing to a favorable balance of photosynthetic production to respiratory utilization at high temperatures. Light was most efficiently utilized at 15°C as indicated by minimum values of I_k(47 μE·m^?2·s^?1) and I_c (6 μE·m^?2·s^?1). Comparison of P. oedogonia and Cladophora glomerata indicated that the former was more tolerant of temperatures above 30°C. Pithophora's tolerance of high temperature and efficient use of low light intensity appear to be adaptive to conditions found within the dense, floating algal mats and the shallow littoral areas inhabited by this filamentous alga.     

THE EFFECTS OF TEMPERATURE ON THE PHOTOSYNTHETIC PARAMETERS AND RECOVERY OF TWO TEMPERATE BENTHIC MICROALGAE,AMPHORA CF. COFFEAEFORMIS AND COCCONEIS CF. SUBLITTORALIS (BACILLARIOPHYCEAE)1

Temperature and irradiance are the most important factors affecting marine benthic microalgal photosynthetic rates in temperate intertidal areas. Two temperate benthic diatoms species, Amphora cf. coffeaeformis (C. Agardh) Kütz. and Cocconeis cf. sublittoralis Hendey, were investigated to determine how their photosynthesis responded to temperatures ranging from 5°C to 50°C after short‐term exposure (1 h) to a range of irradiance levels (0, 500, and 1,100 μmol photons · m^?2 · s^?1). Significant differences were observed between the temperature responses of maximum relative electron transport rate (rETRmax), photoacclimation index (E_k), photosynthetic efficiency (α), and effective quantum yield (ΔF/F_m’) in both species. A. coffeaeformis had a greater tolerance to higher temperatures than C. sublittoralis, with nonphotochemical quenching (NPQ) activated at temperatures of 45°C and 50°C. C. sublittoralis, however, demonstrated a more rapid rate of recovery at ambient temperatures. Temperatures between 10°C and 20°C were determined to be optimal for photosynthesis for both species. High temperatures and irradiances caused a greater decrease in ΔF/F_m’ values. These results suggest that the effects of temperature are species specific and that short‐term exposure to adverse temperature slows the recovery process, which subsequently leads to photoinhibition.     

Two Steps of Gas Exchange in Leaf Photosynthesis

Photosynthesis and transpiration of excised leaves of Taraxacum officinale L. and a few other species of plants were measured, using an open gas analysis system. The rates of CO₂ uptake and transpiration increased in two steps upon illumination of stomata-bearing epidermis of these leaves at a light intensity of 50 mW × cm⁻². Abscisic acid inhibited only the second step of gas exchange. Illumination of the astomatous epidermis of hypostomatous leaves caused only the first step of gas exchange. These data indicate that the first and second steps arise from cuticular and stomatal gas exchange, respectively. The rate of the cuticular photosynthesis in a Taraxacum leaf reached saturation at a light intensity of 5 mW × cm⁻², and the rates of the stomatal photosynthesis and transpiration reached saturation at a higher intensity of 35 mW × cm⁻². The cuticular photosynthesis of a Taraxacum leaf was 18% of the stomatal photosynthesis at 50 mW × cm⁻² and 270% at 5 mW × cm⁻². The other species of leaves showed the same trend. The importance of cuticular CO₂ uptake in leaf photosynthesis, especially under low light intensity was stressed from these data.     

PHOTOSYNTHESIS IN PROTOPLASTS OF MACROCYSTIS PYRIFERA (PHAEOPHYTA)1

Photosynthetic rates measured in protoplasts isolated from the broivn alga Macrocystis pyrifera (L.) Ag. were compared to those for intact tissue. Both ¹⁴C incorporation and O₂ evolution gave similar rates of light-saturated protoplast photosynthesis (approximately 0.4 mmol-g chl a^?1· min^?1). Light saturated photosynthetic rates (P_max) and light harvesting efficiencies (α) of protoplasts were approximately 40% those of intact tissue. In contrast, protoplasts had a greater substrate affinity for photosynthetic HCO₃ uptake (lower K_0.5) than intact tissue (0.87 and 4.1 mMolar, respectively), presumably because of a reduction in the thickness of the unstirred boundary layer in the absence of the cell wall. Overall, the data suggest that protoplasts isolated from Macrocystis pyrifera are of valur in the study of photosynthesis. However, experiments with intact tissue are necessary as controls to aid interpretation of protoplast data.     

Can light-saturated photosynthesis in lowland tropical forests be estimated by one light level?

Leaf-level net photosynthesis (A_n) estimates and associated photosynthetic parameters are crucial for accurately parameterizing photosynthesis models. For tropical forests, such data are poorly available and collected at variable light conditions. To avoid over- or underestimation of modeled photosynthesis, it is critical to know at which photosynthetic photon flux density (PPFD) photosynthesis becomes light-saturated. We studied the dependence of A_n on PPFD in two tropical forests in French Guiana. We estimated the light saturation range, including the lowest PPFD level at which A_sat (A_n at light saturation) is reached, as well as the PPFD range at which A_sat remained unaltered. The light saturation range was derived from photosynthetic light-response curves, and within-canopy and interspecific differences were studied. We observed wide light saturation ranges of A_n. Light saturation ranges differed among canopy heights, but a PPFD level of 1,000 µmol m⁻² s⁻¹ was common across all heights, except for pioneer trees species that did not reach light saturation below 2,000 µmol m⁻² s⁻¹. A light intensity of 1,000 µmol m⁻² s⁻¹ sufficed for measuring A_sat of climax species at our study sites, independent of the species or the canopy height. Because of the wide light saturation ranges, results from studies measuring A_sat at higher PPFD levels (for upper canopy leaves up to 1,600 µmol m⁻² s⁻¹) are comparable with studies measuring at 1,000 µmol m⁻² s⁻¹.     

Photosynthetic gas exchange under emersed conditions in eulittoral and normally submersed members of the Fucales and the Laminariales: interpretation in relation to C isotope ratio and N and water use efficiency

Summary Ten species of brown macroalgae (five eulittoral and one submersed species of the Fucales; four submersed species of the Laminariales) from a rocky shore at Arbroath, Scotland, were examined for characteristics of emersed photosynthesis in relation to the partial pressure of CO₂ and O₂. The five eulittoral species of the Fucaceae were approaching CO₂ saturation for light-saturated photosynthesis at normal air levels of CO₂ (35 Pa) in 21 kPa O₂. The normally submersed algae are further from CO₂ saturation under these conditions, especially in the case of the four members of the Laminariales. The rate of net photosynthesis in the Fucaceae is O₂-independent in the range 2–21 kPa O₂ over the entire range of CO₂ partial pressure tested (compensation up to 95 Pa). For the other five algae tested, net photosynthesis is slightly inhibited by O₂ at 21 kPa relative to 2 kPa over the entire range of CO₂ partial pressures tested (compensation up to 95 Pa). CO₂ compensation partial pressures are low (<0.5 Pa) for the Fucaceae and independent of O₂ in the range 2–42 kPa. For the other five algae, the CO₂ compensation partial pressure are higher, and increased with O₂ partial pressure in the range 2–42 kPa. These gas exchange data show that the Fucaceae exhibit more C₄-like characteristics of their photosynthetic physiology than do the other five species tested, although even the Laminariales and Halidrys siliquosa are not classic C₃ plants in their photosynthetic physiology. These data suggest that, in emersed conditions as well as in the previously reported work on submersed photosynthesis, a CO₂ concentrating mechanism is operating which, by energized transmembrane transport of inorganic C, accumulates CO₂ at the site of RUBISCO and, at least in part, suppresses the oxygenase activity. Work with added extracellular carbonic anhydrase (CA), and with a relatively membrane-impermeant inhibitor of the native extracellular CA activity (acetazolamide), suggests that, in emersed conditions as well as in the previously reported work on algae submersed in seawater at pH 8, HCO _inf3 ^sup– is the major inorganic C species entering the cell. At optimal hydration, the rate of emersed photosynthesis in air is not less than the rate of photosynthesis when submersed in seawater, at least for the Fucaceae. ¹³C ratios of organic C for the Fucaceae are slightly more negative than is the case for the other five algae; these data are consitent with substantial (half or more of the entering inorganic C) leakage of CO₂ from the accumulated pool, and with some contribution of atmospheric CO₂ to the organic C gain by the eulittoral algae. The predicted increase in N use efficiency of photosynthesis in the Fucaceae, with their more strongly developed CO₂ concentrating mechanism, is consistent with data on emersed, but not submersed, photosynthesis for the algae collected from the wild and thus at a poorly defined N status. The more C₄-like gas exchange charateristics of photosynthesis in the eulittoral Fucaceae may be important in increasing the water use efficiency of emersed photosynthesis from the limited capital of water available for transpiration by a haptophyte.