Rates Of Gross Photosynthesis Research Articles

Plankton metabolic balance at the margins of very large lakes: temporal variability and evidence for dominance of autochthonous processes

Summary1. Planktonic metabolic balance (PMBm) of the surface mixed layer (SML) was measured as the ratio of areal rates of gross photosynthesis (AGP) to community respiration (AR) to test the idea that previously neglected allochthonous inputs of organic matter may support chronic excess respiration relative to photosynthesis even in very large lakes during the summer (May–October) season. Four Laurentian Great Lakes coastal sites of varying trophic status, physical structure and dissolved organic carbon (DOC) concentration were studied with oxygen light‐and dark bottle and 14C methods, with excess respiration anticipated in the higher DOC sites.2. Planktonic metabolic balance was net autotrophic in 73% of the observations. The calculated mixing depth at which respiration would predominate over photosynthesis was greater than typically observed mixing depths, varying from 11 to 25 m in the more transparent, low DOC (<3 g m−3) sites to 8–15 m in the higher DOC (4–6 g m−3) sites. Biweekly measurements at one higher and one lower DOC site over two successive summer seasons showed that seasonal gross photosynthesis (ΣAGP) exceeded seasonal community respiration (ΣAR). Despite the location of the sites at the periphery of the lakes, where allochthonous influences should be strongest, the measurements indicated prevailing conditions of net autotrophy in the SML.3. Individual measurements of AR from this study and the literature were correlated with AGP but season average values were more tightly correlated, suggesting a tighter coupling of metabolic rates on a larger scale and a looser coupling on a shorter scale. The observed temporal variability was variable in pattern among years, and likely to confound inferences based on limited sampling.4. It is shown that accepted formulations for AGP and AR lead to the conclusion that PMBm should be largely predictable from knowledge of a biological properties ratio (light‐saturated gross photosynthesis to plankton community respiration, Pmax/R) and a physical properties ratio (euphotic to mixing depths, Zeu/Zm) and this prediction was confirmed using data from this study and from the literature. The evident success of this model points to the pre‐eminent importance of plankton biomass and physical conditions in determining metabolic balance. Variation in these fundamental factors appears capable of explaining the diversity of PMBm reported for different Great Lakes.

Effect of salinity changes on the bacterial diversity, photosynthesis and oxygen consumption of cyanobacterial mats from an intertidal flat of the Arabian Gulf

The effects of salinity fluctuation on bacterial diversity, rates of gross photosynthesis (GP) and oxygen consumption in the light (OCL) and in the dark (OCD) were investigated in three submerged cyanobacterial mats from a transect on an intertidal flat. The transect ran 1 km inland from the low water mark along an increasingly extreme habitat with respect to salinity. The response of GP, OCL and OCD in each sample to various salinities (65 per thousand, 100 per thousand, 150 per thousand and 200 per thousand) were compared. The obtained sequences and the number of unique operational taxonomic units showed clear differences in the mats' bacterial composition. While cyanobacteria decreased from the lower to the upper tidal mat, other bacterial groups such as Chloroflexus and Cytophaga/Flavobacteria/Bacteriodetes showed an opposite pattern with the highest dominance in the middle and upper tidal mats respectively. Gross photosynthesis and OCL at the ambient salinities of the mats decreased from the lower to the upper tidal zone. All mats, regardless of their tidal location, exhibited a decrease in areal GP, OCL and OCD rates at salinities > 100 per thousand. The extent of inhibition of these processes at higher salinities suggests an increase in salt adaptation of the mats microorganisms with distance from the low water line. We conclude that the resilience of microbial mats towards different salinity regimes on intertidal flats is accompanied by adjustment of the diversity and function of their microbial communities.

Partitioning of river metabolism identifies phytoplankton as a major contributor in the regulated Murray River (Australia)

Summary1. River metabolism was measured over an annual cycle at three sites distributed along a 1000 km length of the lowland Murray River, Australia.2. Whole system metabolism was measured using water column changes in dissolved oxygen concentrations while planktonic and benthic metabolism were partitioned using light‐dark bottles and benthic chambers.3. Annual gross primary production (GPP) ranged from 775 to 1126 g O2 m−2 year−1 which in comparison with rivers of similar physical characteristics is moderately productive.4. Community respiration (CR) ranged from 872 to 1284 g O2 m−2 year−1 so that annual net ecosystem production (NEP) was near zero, suggesting photosynthesis and respiration were balanced and that allochthonous organic carbon played a minor role in fuelling metabolism.5. Planktonic rates of gross photosynthesis and respiration were similar to those of the total channel, indicating that plankton were responsible for much of the observed metabolism.6. Respiration rates correlated with phytoplankton standing crop (estimated as the sum of GPP plus the chlorophyll concentration in carbon units), yielding a specific respiration rate of −1.1 g O2 g C−1 day−1. The respiration rate was equivalent to 19% of the maximum rate of phytoplankton photosynthesis, which is typical of diatoms.7. The daily GPP per unit phytoplankton biomass correlated with the mean irradiance of the water column giving a constant carbon specific photon fixation rate of 0.35 gO2 g Chl a−1 day−1 per μmole photons m−2 s−1 (ca. 0.08 per mole photons m−2 on a carbon basis) indicating that light availability determined daily primary production.8. Annual phytoplankton net production (NP) estimates at two sites indicated 25 and 36 g C m−2 year−1 were available to support riverine food webs, equivalent to 6% and 11% of annual GPP.9. Metabolised organic carbon was predominantly derived from phytoplankton and was fully utilised, suggesting that food‐web production was restricted by the energy supply.

Intra-colonial variability in light acclimation of zooxanthellae in coral tissues of Pocillopora damicornis

We investigated heterogeneity of light acclimation of photosynthesis in sun- and shade-adapted coenosarc and polyp tissues of Pocillopora damicornis. The zooxanthellar community within P. damicornis colonies at Heron Island is genetically uniform, yet they showed a large degree of plasticity in their photo-physiological acclimation linked to light microclimates characterised by fibre-optic microprobes. Microscale scalar irradiance measurements showed higher absorption in polyp than coenosarc tissues and higher absorption in the more densely pigmented shade-adapted polyps than in sun-adapted polyps. The combination of an O2 microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabled parallel measurements of O2 concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface under steady-state conditions as a function of increasing irradiances. Lower O2 levels at the tissue surface and higher compensation irradiance indicated a higher respiration activity in sun-adapted polyp tissue as compared to shade-adapted polyps. Shade-adapted coenosarc and polyp tissues exhibited lower maxima of relative electron transport rates (rETRmax) (84±15 and 41±10, respectively) than sun-adapted coenosarc and polyp tissues (136±14 and 77±13, respectively). Shade-adapted tissues showed stronger decrease of rETR at high scalar irradiances as compared to sun-adapted tissues. The relationship between the relative PSII electron transport and the rate of gross photosynthesis, as well as O2 concentration, was non-linear in sun-adapted tissues over the entire irradiance range, whereas for shade-adapted tissues the relationship became non-linear at medium to high scalar irradiances >200 μmol photons m−2 s−1. This suggests that rETR measurements should be used with caution in corals as a proxy for photosynthesis rates. The apparently high rates of photosynthesis (oxygen evolution rates) suggest that there must be a considerable electron transport rate through the photosystems that is not observed by the rETR measurements. This may be accounted for by vertical heterogeneity of zooxanthellae in the tissue and the operation of an alternative electron pathway such as cyclic electron flow around PSII.

Isidia ontogeny and its effect on the CO2 gas exchanges of the epiphytic lichen Pseudevernia furfuracea (L.) Zopf

The development of isidia in thalli of Pseudevernia furfuracea from the Carnic Alps (North-eastern Italy), and the effects of these structures on CO2 gas exchanges were investigated. The ontogenetic events were studied by comparison of sections stained with different histochemical tests and SEM observations. A high cell turnover rate in both symbiotic partners is the first sign of isidium development, followed by an increased aplanosporogenesis of algae and growth of neighbouring medullary hyphae which become oriented upwards. Large nuclei and an intense cytoplasm activity characterize the mycobiont cells. The surface of very young isidia shows an irregular structure of spherical to ovoid protruding tips of perpendicular cortical hyphae, that are later organised in a pseudomeristematic area similar to that observed in the apex of growing lobes. CO2 gas exchange measurements carried out in the laboratory confirmed the high metabolic activity of isidia. At optimal water content and favourable light conditions, isolated isidia had rates of gross photosynthesis and dark respiration that were twice those of non-isidiate lobes. Isolated isidia also had a very low CO2 saturation point, probably because of their favourable surface/volume ratio, and a high light saturation, probably linked to their high content of photosynthetic pigments. The different roles played by isidia in the biology of Pseudevernia furfuracea, and particularly their rejuvenating effect on aged lobes, are discussed, and the presence of thalloconidia is briefly mentioned.

Plant Species Numbers Predicted by a Topography-based Groundwater Flow Index

We measured net ecosystem exchange of carbon dioxide (CO2) (NEE) during wet and dry summers (2000 and 2001) across a range of plant communities at Mer Bleue, a large peatland near Ottawa, southern Ontario, Canada. Wetland types included ombrotrophic bog hummocks and hollows, mineral-poor fen, and beaver pond margins. NEE was significantly different among the sites in both years, but rates of gross photosynthesis did not vary spatially even though species composition at the sites was variable. Soil respiration rates were very different across sites and dominated interannual variability in summer NEE within sites. During the dry summer of 2001, net CO2 uptake was significantly smaller, and most locations switched from a net sink to a source of CO2 under a range of levels of photosynthetically active radiation (PAR). The wetter areas—poor fen and beaver pond margin— had the largest rates of CO2 uptake and smallest rates of respiratory loss during the dry summer. Communities dominated by ericaceous shrubs (bog sites) maintained similar rates of gross photosynthesis between years; by contrast, the sedge-dominated areas (fen sites) showed signs of early senescence under drought conditions. Water table position was the strongest control on respiration in the drier summer, whereas surface peat temperature explained most of the variability in the wetter summer. Q 10 temperature-respiration quotients averaged 1.6 to 2.2. The ratio between maximum photosynthesis and respiration ranged from 3.7:1 in the poor fen to 1.2:1 at some bog sites; it declined at all sites in the drier summer owing to greater respiration rates relative to photosynthesis in evergreen shrub sites and a change in both processes in sedge sites. Our ability to predict ecosystem responses to changing climate depends on a more complete understanding of the factors that control NEE across a range of peatland plant communities.

What is NPP? Inconsistent accounting of respiratory fluxes in the definition of net primary production

Primary production represents a globally important flux of carbon between the atmosphere and the biosphere. From an ecological perspective, it measures the rate at which solar energy is stored by plants as organic matter, and is therefore a measure of the rate at which solar energy is captured and made available to the rest of the food chain (Odum 1971). From a biogeochemical perspective, primary production provides links between the biosphere and the climate system through the global cycling of C, water and nutrients (Roy et al . 2001). Gross primary production (GPP) is the total amount of C assimilated by plants within a given area over a specified timeframe. (Here GPP refers to the total C fixed by photosynthesis, minus the C lost by photorespiration. We have adopted the convention of specifying productivity in units of C mass per unit ground area per unit time, although we could have alternatively used units of total biomass, or energy: Odum 1971.) Net primary production (NPP) is GPP less the flux of autotrophic respiration of assimilate used for the plant’s own metabolism ( R ), therefore:

Initial Measurements of Benthic Photosynthesis and Respiration in Lake Erie

The first reported in situ measurements of benthic primary production in Lake Erie were made on rocky substrata using oxygen and dissolved inorganic carbon change in 1997 and 1998. The productivity quotient for net benthic algal growth was determined to be 1.1 based on contemporaneous measurement of oxygen production and carbon uptake. Highest rates of gross photosynthesis were > 300 mg O 2/m 2/h on rocky substrata with mixed stands of Cladophora and dreissenid mussels. These stands also exhibited the greatest rates of oxygen consumption in the dark. Bare rock substrata had lower rates of gross photosynthesis than rock occupied by mussels or Cladophora, but even these habitats had rates that would place them among the highest rates measured in fresh water. Areal benthic photosynthetic rates would greatly exceed areal pelagic photosynthesis in depths shallower than 5 m depth in the eastern basin; and, at greater depths, the importance of declining benthic areal photosynthesis relative to generally increasing integral pelagic production with depth would depend on substratum type and water clarity. Dreissenid mussels had a positive effect on benthic photosynthesis that was especially evident in stimulating Cladophora growth over the course of the 1998 season.

Spatial distribution of calcification and photosynthesis in the scleractinian coral Galaxea fascicularis

The spatial heterogeneity of photosynthesis and calcification of single polyps of the coral Galaxea fascicularis was investigated. Photosynthesis was investigated with oxygen microsensors. The highest rates of gross photosynthesis (Pg) were found on the tissue covering the septa, the tentacles, and the tissues surrounding the mouth opening of the polyp. Lower rates were found on the tissues of the wall and the coenosarc. Calcification was investigated by radioactive tracers. The incorporation pattern of 45Ca and 14C in the corallites was imaged with use of a Micro-Imager. The β-images obtained showed that the incorporation of the radioactive tracers coincided with the Pg distribution pattern with the highest incorporation rates found in the corallite septa. Thus, the high growth rate of the septa is supported by the high rates of Pg by the symbiont in the adjacent tissues. The total incorporation rates were higher in light than in dark, however, the distribution pattern of the radioisotope incorporation was not affected by illumination. This further emphasizes the close relation between calcification and photosynthesis.

Diffusive boundary layers and photosynthesis of the epilithic algal community of coral reefs

The effects of mass transfer resistance due to the presence of a diffusive boundary layer on the photosynthesis of the epilithic algal community (EAC) of a coral reef were studied. Photosynthesis and respiration of the EAC of dead coral surfaces were investigated for samples from two locations: the Gulf of Aqaba, Eilat (Israel), and One Tree Reef on the Great Barrier Reef (Australia). Microsensors were used to measure O2 and pH at the EAC surface and above. Oxygen profiles in the light and dark indicated a diffusive boundary layer (DBL) thickness of 180–590 μm under moderate flow (~0.08 m s−1) and >2,000 μm under quasi-stagnant conditions. Under light saturation the oxygen concentration at the EAC surface rose within a few minutes to 200–550% air saturation levels under moderate flow and to 600–700% under quasi-stagnant conditions. High maximal rates of net photosynthesis of 8–25 mmol O2 m−2 h−1 were calculated from measured O2 concentration gradients, and dark respiration was 1.3–3.3 mmol O2 m−2 h−1. From light–dark shifts, the maximal rates of gross photosynthesis at the EAC surface were calculated to be 16.5 nmol O2 cm−3 s−1. Irradiance at the onset of saturation of photosynthesis, E k, was <100 µmol photons m−2 s−1, indicating that the EAC is a shade-adapted community. The pH increased from 8.2 in the bulk seawater to 8.9 at the EAC surface, suggesting that very little carbon in the form of CO2 occurs at the EAC surface. Thus the major source of dissolved inorganic carbon (DIC) must be in the form of HCO3 −. Estimates of DIC fluxes across the DBL indicate that, throughout most of the daytime under in situ conditions, DIC is likely to be a major limiting factor for photosynthesis and therefore also for primary production and growth of the EAC.

Leaf factors affecting the relationship between chlorophyll fluorescence and the rate of photosynthetic electron transport as determined from CO 2 uptake

CO2 uptake and chlorophyll fluorescence were measured under non-photorespiratory conditions in leaves from 14 plant species. The rate of CO2-dependent electron transport (JCO2) was calculated as four times rate of gross photosynthesis. The quantum yield of electron transport in photosystem II was estimated from the ratio delta F/Fm', where delta F is the difference between steady-state and maximal fluorescence in the light. As photon flux density (PFD) increased, JCO2 increased linearly first, and then reached saturation. The product (delta F/Fm')PFD, which is a function of electron transport rate, showed a similar response. Therefore, the relationship between (delta F/Fm') PFD versus JCO2 was proportional. However, under high light, a linear correlation was not always maintained. Factors affecting the linear correlation were analyzed by measuring CO2 uptake and chlorophyll fluorescence under illumination from either the upper (adaxial) or lower (abaxial) leaf surface, and by using plants with anatomically symmetric leaves having palisade tissues on both sides. Consequently, it was shown that the parameter delta F/Fm' is based on chlorophyll fluorescence emitted from chloroplasts present near the illuminated surface. Further, it was suggested that this restriction of the origin of fluorescence actually measured is significant in a leaf with high chlorophyll content, resulting in the deviation from linearity in the relationship between JCO2 and (delta F/Fm')PFD.

Kinetics of photoacclimation in corals

Traditional models describing the relationship between photosynthesis (P) and irradiance (I) do not account for photoacclimation to short-term variation in irradiance. Here we develop and test a model that predicts the rate of photosynthesis under fluctuating irradiances at the scale of days to weeks. Using oxygen respirometry, we measured the rates of change in the P-I model parameters P(max) (maximum rate of gross photosynthesis) and I(k) (sub-saturation irradiance) of the photo-symbiotic coral Turbinaria mesenterina (Lamarck) following large and small increases and decreases in growth irradiance. We analyse the behaviour of the dynamic P-I model in turbid-water conditions using a dataset of 3-month continuous irradiance as the input variable. In response to upward or downward changes in experimental growth irradiance, I(k) values decreased or increased exponentially, reaching new and stable levels within 5-10 days. I(k) responded 4 times stronger than P(max) to changes in growth irradiance. The kinetics of I(k) did not show hysteresis, and changed in similar ways when irradiance was increased or decreased in small or large amounts. This suggests that mechanisms associated with photo-protection during increases in irradiance, and the maximisation of photosynthetic efficiency during decreases in irradiance, are equally potent. On the scale of months, the dynamic P-I model did not predict higher rates of photosynthesis than the static P-I model, but buffered the variation in photosynthesis during periods of reduced irradiance. Fourier analysis indicated that the kinetics of I(k) closely matches the main periodicities in daily irradiance (1-2 weeks). The recorded kinetics of photoacclimation in the Turbinaria-zooxanthella symbiosis is comparable to that of free-living phytoplankton and faster than that of higher plants.

Environmental regulation of carbon dioxide exchange at the forest floor in a boreal black spruce ecosystem

Measurements of CO 2 exchange at the forest floor were made in shady dry areas dominated by feather mosses and open wet areas dominated by Sphagnum mosses within a 120-year-old black spruce forest in central Saskatchewan during 1996. Our objective was to study the environmental regulation of moss photosynthesis and forest floor (moss and soil) respiration, in order to calculate the contribution of moss photosynthesis to the carbon balance of the forest ecosystem. We used chamber gas exchange techniques to measure net carbon exchange at the forest floor during three periods that spanned the growing season. Measurements were also made with the clear chamber darkened, to obtain total respiration rates. Maximum measured rates of forest floor CO 2 efflux were approximately 7 μmol m −2 s −1 in both feather moss and Sphagnum areas. We developed and applied a two-term (shallow component and deep component) exponential model to describe the temperature dependence of forest floor respiration. We interpret the shallow stratum component as live moss respiration, and our field estimates of moss respiration were consistent with laboratory studies on isolated moss samples. The average (±S.D.) proportion of total respiration originating from the shallow stratum (at a uniform soil temperature of 10°C) was 7.0±15% in the feather moss community and 21±20% in the Sphagnum community. Gross photosynthesis was calculated as the difference between chamber measurements made in the light and dark. A rectangular hyperbole was fitted to the light response curves for moss gross photosynthesis. Sphagnum had higher maximum rates of gross photosynthesis than feather moss, and Sphagnum showed a pronounced seasonal change in photosynthetic capacity. The photosynthesis and respiration models were used, along with environmental data, to calculate the carbon balance for the forest floor. Our data indicated that the forest floor lost 255.4 g C m −2 during May–October. This net loss of carbon was the difference between moss gross photosynthesis of −140.7 g C m −2 and total forest floor respiration of 396.1 g C m −2 during May–October. Moss photosynthesis contributed approximately 13% of the total ecosystem gross production. We calculated moss net primary productivity to be 228 and 80 g C m −2 in Sphagnum and feather moss communities, respectively.

The effect of high levels of visible and ultra-violet radiation on the photosynthesis of phytoplankton from a freshwater lake

Rates of oxygen production of a natural population of phytoplankton from a freshwater lake were measured in situ at mid-summer under near-maximal irradiances for PAR (400-700 nm), UV-B (280-315 nm) and UV-A (320-400 nm), The radiation climate was altered by incubation at different depths, and hence different PAR, and by using flasks which cut-off different amounts of UV-radiation: quartz flasks transmitted all wavelengths >280 nm, glass flasks had a 50 % transmission at 309 nm and glass flasks with a plastic shield had 50 % transmission at 354 nm. At 0.01 and 0.11 m, rates of oxygen consumption in the light were greater than dark respiration in all flasks, but were particularly high in the full UV treatment, possibly as a result of photochemical degradation of dissolved organic carbon. Gross oxygen production was achieved in all flask types at depths of 0.41, 0.53 and 1.1 m but rates were lowest under the full UV-spectrum. An ANOVA showed a significant variation in rate of gross photosynthesis with depth and type of flask. A multiple-regression against received flux of PAR, UV.9 and UV-B explained 81 % of the variation in gross photosynthesis but co-variation of irradiance with depth prevented the effect of the different wavebands to be separated statistically. The biological weighting function of CULLEN et al (1992) for the inhibition of photosynthesis by ultraviolet radiation for a marine phytoplankter, explained 80% of the variation in our data but we could not distinguish between two scenarios with different sensitivity to UV radiation. Extrapolation to in-situ conditions suggested that UV-A radiation was more inhibitory than UV-B radiation.

Irradiance and temperature regulation of oxygenic photosynthesis and O 2 consumption in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

Short-term effects of temperature and irradiance on oxygenic photosynthesis and O2 consumption in a hypersaline cyanobacterial mat were investigated with O2 microsensors in a laboratory. The effect of temperature on O2 fluxes across the mat–water interface was studied in the dark and at a saturating high surface irradiance (2162 μmol photons m−2 s−1) in the temperature range from 15 to 45 °C. Areal rates of dark O2 consumption increased almost linearly with temperature. The apparent activation energy of 18 kJ mol−1 and the corresponding Q 10 value (25 to 35 °C) of 1.3 indicated a relative low temperature dependence of dark O2 consumption due to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperature up to 40 °C and exhibited a Q 10 value (20 to 30 °C) of 2.8. Both O2 dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 °C, with the most pronounced increase of gross photosynthesis at the mat surface between 25 and 35 °C (Q 10 of 3.1). In another mat sample, measurements at increasing surface irradiances (0 to 2319 μmol photons m−2 s−1) were performed at 25, 33 (the in situ temperature) and 40 °C. At all temperatures, areal rates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of saturation (E k = 148 to 185 μmol photons m−2 s−1) estimated from P versus E d curves showed no clear trend with temperature, while maximal photosynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 μmol photons m−2 s−1, where oxygenic gross photosynthesis and also the thickness of the photic zone was significantly reduced at 40 °C. The compensation irradiance increased with temperature, from 32 μmol photons m−2 s−1 at 25 °C to 77 μmol photons m−2 s−1 at 40 °C, due to increased rates of O2 consumption relative to gross photosynthesis. Areal rates of O2 consumption in the illuminated mat were higher than dark O2 consumption at corresponding temperatures, due to an increasing O2 consumption in the photic zone with increasing irradiance. Both light and temperature enhanced the internal O2 cycling within hypersaline cyanobacterial mats.

The responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain).

We have evaluated the effects of short-term changes in incident irradiance and temperature on oxygenic photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat from the Ebro Delta, Spain, in which Microcoleus chthonoplastes was the dominant phototrophic organism. The mat was incubated in the laboratory at 15, 20, 25 and 30 degrees C at incident irradiances ranging from 0 to 1,000 micromol photons m(-2) s(-1). Oxygen microsensors were used to measure steady-state oxygen profiles and the rates of gross photosynthesis, which allowed the calculation of areal gross photosynthesis, areal net oxygen production, and oxygen consumption in the aphotic layer of the mat. The lowest surface irradiance that resulted in detectable rates of gross photosynthesis increased with increasing temperature from 50 micromol photons m(-2) s(-1) at 15 degrees C to 500 micromol photons m(-2) s(-1) at 30 degrees C. These threshold irradiances were also apparent from the areal rates of net oxygen production and point to the shift of M. chthonoplastes from anoxygenic to oxygenic photosynthesis and stimulation of sulphide production and oxidation rates at elevated temperatures. The rate of net oxygen production per unit area of mat at maximum irradiance, J0, did not change with temperature, whereas, JZphot, the flux of oxygen across the lower boundary of the euphotic zone increased linearly with temperature. The rate of oxygen consumption per volume of aphotic mat increased with temperature. This increase occurred in darkness, but was strongly enhanced at high irradiances, probably as a consequence of increased rates of photosynthate exudation, stimulating respiratory processes in the mat. The compensation irradiance (Ec) marking the change of the mat from a heterotrophic to an autotrophic community, increased exponentially in this range of temperatures.

Seasonal courses of CO(2) exchange and carbon balance in fruits of Cinnamomum camphora.

Carbon dioxide exchange in fruits of Cinnamomum camphora Sieb. was followed over a growing season from July to December 1992. Dark respiration was exponentially related to temperature, with a Q(10) value near 2. Light dependence of photosynthetic CO(2) refixation, i.e., the ratio of gross photosynthesis to dark respiration, was approximated by a hyperbolic function. Seasonal variation in maximum CO(2) refixation capacity ranged between 52 and 174%, reaching a maximum in early August. Daily photosynthetic CO(2) refixation ranged between 17 and 51% over the growth period. We evaluated seasonal variation in translocation rate to the fruit on the basis of the seasonal rates of gross photosynthesis, dark respiration and increase in fruit dry weight, and used the results to develop a simple carbon flow model of fruit development. Seasonal changes in translocation rate paralleled those in fruit growth rate, with two peaks during the periods before and after September. Seed formation took place in the period between the two peaks. The relationship between fruit growth rate and translocation rate was approximated by a linear function. The carbon flow model estimated that, over the reproductive period, the amount of assimilate translocated to each fruit was 377.2 mg dry weight, of which 58.5% was accounted for by weight growth and 41.5% was consumed by net respiration. Carbon dioxide refixation accounted for 22.9% of the carbon balance of the fruit.

Effect of mineral nutrition content on oxygen exchange and chlorophyll a fluorescence in needles of Norway spruce.

Photosynthetic O(2) evolution at high irradiances (approximately 600-1000 micro mol m(-2) s(-1)) and O(2) uptake in darkness were measured in needles of control, irrigated and irrigated-fertilized trees of Norway spruce (Picea abies (L.) Karst.). Measurements were made at 20 degrees C and at high CO(2) concentrations. The results suggest that, at given times of the year, a major part of the variation in gross photosynthesis of current-year and one-year-old needles across treatments is associated with differences in needle N content. Furthermore, the rate of O(2) uptake measured after 5 or 10 min in darkness was positively correlated with both the preceding rate of gross O(2) evolution and the N content in fully expanded current-year needles. Measurements of chlorophyll a fluorescence, taken simultaneously with measurements of O(2) evolution in current-year sun needles, showed that Stern-Volmer quenching of minimum fluorescence and the ratio of variable to maximum fluorescence in the dark- and light-adapted state were strongly correlated with the gross rate of O(2) evolution. This suggests that the increased rate of gross photosynthesis in needles of irrigated-fertilized trees was associated with adjustments in the thermal energy dissipation within photosystem II.

Primary production of phytoplankton in Lake Valencia (Venezuela)

AbstractLake 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.

MICROENVIRONMENTS AND MICROSCALE PRODUCTIVITY OF CYANOBACTERIAL DESERT CRUSTS1

ABSTRACTWe used microsensors to characterize physicochemical microenvironments and photosynthesis occurring immediately after water saturation in two desert soil crusts from southeastern Utah, which were formed by the cyanobacteria Microcoleus vaginatus Gomont, Nostoc spp., and Scytonema sp. The light fields within the crusts presented steep vertical gradients in magnitude and spectral composition. Near‐surface light‐trapping zones were formed due to the scattering nature of the sand particles, but strong light attenuation resulted in euphotic zones only ca. 1 mm deep, which were progressively enriched in longer wavelengths with depth. Rates of gross photosynthesis (3.4–9.4 mmol O2·m−2·h−1) and dark respiration (0.81–3.1 mmol O−2·m−2·h−1) occurring within 1 to several mm from the surface were high enough to drive the formation of marked oxygen microenvironments that ranged from oxygen supersaturation to anoxia. The photosynthetic activity also resulted in localized pH values in excess of 10, 2–3 units above the soil pH. Differences in metabolic parameters and community structure between two types of crusts were consistent with a successional pattern, which could be partially explained on the basis of the microenvironments. We discuss the significance of high metabolic rates and the formation of microenvironments for the ecology of desert crusts, as well as the advantages and limitations of microsensor‐based methods for crust investigation.