Ecology of Dictyosphaerium pulchellum Wood (Chlorophyta, Chlorococcales) in a shallow, acid, forest lake

This study relates to the ecology of Dictyosphaerium pulchellum Wood in Delamere Lake in Cheshire, UK. Dictyosphaerium pulchellum is a cosmopolitan, green colonial phytoplankton species that occasionally forms dense, monospecific populations in lakes. Delamere Lake is a small, shallow, acid lake (mean pH, 4.5) with very high phytoplankton biomass (annual mean chlorophyll a, 290 μg l⁻¹) and devoid of any significant cladoceran population, the efficient grazers of phytoplankton. A predominantly unicellular form of D. pulchellum was the dominant species in Lake Delamere, and it comprised on average ca. 80% (maximum >99%) of the lake phytoplankton biovolume. Laboratory and lake experiments were conducted on this species showed that its pH tolerance varied between 2.4 and 10.7, and its optimum tolerance range between 3.3 and 8.5 depending on other environmental variables. Low pH was not responsible for the unicellular habit of this alga, but a very high nutrient regime could be an important factor. Bioassays revealed that in Delamere Lake this species was limited by nitrogen, but nitrogen did not hamper high growth in the lake. Dictyosphaerium pulchellum can persist at low light levels, tolerate CO₂-deficiency and can grow in polyhumic water with water colour around 300 mg Pt l⁻¹, but probably not in darker waters. The dominance of D. pulchellum in Delamere Lake is apparently due to a combination of several factors: its ability to tolerate both low pH and high turbidity, exploit high nutrient conditions, absence of effective grazing pressure by zooplankton and being a superior competitor.     

Hyalella montezuma,a new species (crustacea: amphipoda) from montezuma well,Arizona

Two distinct species of Hyalella Smith, with no intermediate forms, occur in the Arizona limnocrene known as Montezuma Well. One is referable to the inermis type of H. azteca (Sauss.). The other is a new and presumably endemic species, H. montezuma, characterized by elongate appendages, mucronation of tergites of pereonite 7 and pleonites 1 & 2, and remarkably enlarged mouthparts; the inner plate of maxilla 1 is broad and armed with up to 30 plumose setae, rather than the 2–3 apical setae that typify the genus. H. azteca occurs alone in a small stream emerging from the Well; both species coexist at the weedy margins of the Well; and H. montezuma swims alone in the open water, where it is a member of the plankton. A high level of free CO₂ in the water serves as a barrier to fish, indirectly allowing Hyalella to colonize the limnetic zone where it represents, perhaps, one of four of the World's freshwater planktonic amphipods. Theoretically, the ancestors of H. montezuma arrived at the Well sometime more than 11,000 years BP, when it was a new steep-walled collapse basin lacking fish; H. azteca appeared later after a shallow littoral zone had developed. The two species are reproductively isolated at present.     

Inorganic carbon promotes photosynthesis,growth, and maximum biomass of phytoplankton in eutrophic water bodies

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Seasonal and annual variability in the phytoplankton community of the Raunefjord,west coast of Norway from 2001–2006

Changes in phytoplankton community composition potentially affect the entire marine food web. Because of seasonal cycles and inter-annual variations in species composition, long-term monitoring, covering many sequential years, is required to establish a baseline study and to reveal long-term trends. The current study describes the phytoplankton biomass variations and species composition in relation to hydrographic and meteorological conditions in the Raunefjord, western Norway, over a 6-year period from 2001 to 2006. The extent of inflow or upwelling in the fjord varied from year to year and resulted in pronounced differences in water column stability. The annual phytoplankton community succession showed some repeated seasonal patterns, but also high variability between years. Two to four diatom blooms were observed per year, and the spring blooms occurring before water column stratification in March were dominated by Skeletonema marinoi and Chaetoceros socialis, and other Chaetoceros and Thalassiosira spp. Blooms of the haptophytes Phaeocystis pouchetii and Emiliania huxleyi were irregular and in some years totally absent. Although E. huxleyi was present all year round it appeared in bloom concentrations only in 2003, when the summer was warm and the water column characterized by high surface temperatures and pronounced stratification. The annual average abundance of both diatoms and flagellates increased during the six years. Despite the high variation from year to year, our investigation provides valuable knowledge about annual phytoplankton community patterns in the region, and can be used as a reference to detect possible future changes.     

Differentiation among populations of Sedum wrightii (Crassulaceae) in response to limited water availability: water relations,CO2 assimilation,growth and survivorship

Summary Sedum wrightii is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO₂ assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO₂ exchange patterns, in biomass ¹³C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO₂ uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited.     

High CO2 concentration and iron availability determine the metabolic inventory in an Emiliania huxleyi-dominated phytoplankton community

Ocean acidification (OA), a consequence of anthropogenic carbon dioxide (CO₂) emissions, strongly impacts marine ecosystems. OA also influences iron (Fe) solubility, affecting biogeochemical and ecological processes. We investigated the interactive effects of CO₂ and Fe availability on the metabolome response of a natural phytoplankton community. Using mesocosms we exposed phytoplankton to ambient (390 μatm) or future CO₂ levels predicted for the year 2100 (900 μatm), combined with ambient (4.5 nM) or high (12 nM) dissolved iron (dFe). By integrating over the whole phytoplankton community, we assigned functional changes based on altered metabolite concentrations. Our study revealed the complexity of phytoplankton metabolism. Metabolic profiles showed three stages in response to treatments and phytoplankton dynamics. Metabolome changes were related to the plankton group contributing respective metabolites, explaining bloom decline and community succession. CO₂ and Fe affected metabolic profiles. Most saccharides, fatty acids, amino acids and many sterols significantly correlated with the high dFe treatment at ambient pCO₂. High CO₂ lowered the abundance of many metabolites irrespective of Fe. However, sugar alcohols accumulated, indicating potential stress. We demonstrate that not only altered species composition but also changes in the metabolic landscape affecting the plankton community may change as a consequence of future high-CO₂ oceans.     

The endemic spring snail Pyrgulopsis montezumensis in a high CO2 environment: Importance of extreme chemical habitats as refugia

1. We examined the distribution of the endemic spring snail, Pyrgulopsis montezumensis, along a gradient of dissolved free CO₂ and on selected substrata in the collapsed travertine springmound of Montezuma Well, Arizona, U.S.A. 2. Dissolved CO₂ concentration ranged from 480 (SE ± 56) mg L^??1 near the bottom spring vents in the limnetic zone (12 m deep) of the well to < 20 mg L^??1 in an irrigation canal about 1400 m downstream from the well outlet. Spring snails occurred in habitats with 110–315 mg L^??1 dissolved CO₂. 3. Laboratory experiments showed that P. montezumensis tolerated a dissolved CO₂ concentration up to about 400 mg L^??1 for 11 days, but displayed 40% (SE ± 4) mortality after 3 days at 728 mg L^??1 and 100% mortality after 24 h at 1200 mg L^??1. We also measured 100% mortality after 10 days at ambient concentrations (< 10 mg L^??1) of dissolved CO₂. 4. Although the concentration of dissolved CO₂ was within the tolerance range for spring snails in the irrigation canal, inadequate substrata and increased predation probably restricted their distribution in the lower reaches of the canal. Both field measurements and laboratory selection experiments showed a significant (P < 0.001) preference for limestone substrata over submerged vegetation and fine sediments. 5. We propose that springs with high dissolved CO₂ may provide refugia from invertebrate and/or vertebrate predators not physiologically adjusted to the extreme chemical environment. These extreme chemical habitats probably reduce competition for resources and predation and allow resistant populations like P. montezumensis to attain high density.     

The effects of nutrient addition and pH manipulation in bag experiments on the phytoplankton of a small acidic lake in West Virginia,USA

In situ bag experiments were performed during summer and autumn in a small acidic lake, Tibbs Run Lake, West Virginia, USA. The objective was to evaluate phytoplankton responses to pH manipulation and nutrient addition. Increasing the pH from below 4.5 to over 6.3 resulted in great declines in phytoplankton biovolume. There was also a succession from dinoflagellates (Peridinium inconspicuum to small chlorophytes. The trend was more rapid where phosphorus (P) additions were made along with pH enhancement. During summer, P limitation was indicated, while nitrogen (N) appeared to limit production in autumn. In both seasons, nutrient additions greatly altered the phytoplankton composition in high pH treatments, but had no discernable effects at (the natural) low pH. A low pH, P addition treatment in autumn was the single exception. When N was subsequently added, phytoplankton composition changed dramatically, probably because the proceeding P additions caused severe secondary N-limitation. In general, however, the results supported the view that phytoplankton compositional responses to nutrient additions are suppressed in low pH, relative to high pH lake water.     

Diel vertical migration of a pelagic amphipod in the absence of fish predation

The pelagic amphipod, Hyalella montezuma, migrates vertically into the surface waters at twilight in Montezuma Well, Arizona, USA despite the absence of fish predators or thermal stratification. We suggest that a persistent, dense, neustonic algal assemblage may provide a food resource incentive for the twilight ascent.     

Diel, episodic and seasonal changes in pH and concentrations of inorganic carbon in a productive lake

• 1 Two pH electrodes and a thermistor were used to record conditions in the surface of Esthwaite Water every 15 min over a 12-month period. Combined with approximately weekly measurements of alkalinity they allowed inorganic carbon speciation to be calculated.
• 2 Large changes in pH from 7.1 to nearly 10.3, and hence in concentrations of inorganic carbon species, were measured over a year. Carbon speciation and pH varied on a diel, episodic and seasonal basis. Diel variation of up to pH 1.8 was recorded, although typical daily variation was between 0.03 and 1.06 (5 and 95 percentiles). Daily change in concentration of inorganic carbon varied between 4 and 63 mmol m^-3 (5 and 95 percentiles).
• 3 During lake stratification, episodes of high pH, typically of 1–2 weeks' duration were interspersed with episodes of lower pH. These changes appeared to relate to the weather: e.g. low wind velocity, high pressure, low rainfall and high sunshine hours correlated with periods of high pH.
• 4 Seasonal progression of carbon depletion generally followed stratification and the development of high phytoplankton biomass. When the lake was isothermal, the phytoplankton biomass caused relatively small amounts of carbon depletion.
• 5 During autumn, winter and spring, the lake had concentrations of CO₂* (free CO₂) up to 0.12 mol m^-3 which is nearly seven times the calculated atmospheric equilibrium concentration so the lake will accordingly be losing carbon to the atmosphere. In contrast, during periods of elevated pH the concentration of CO₂* was reduced close to zero and the lake will take up atmospheric CO₂. The rates of transfer between water and the atmosphere were estimated using a chemical equilibrium model with three boundary layer thicknesses. The calculations show that over a year the lake loses CO₂ to the atmosphere with the current mean atmospheric level of 360 μmol mol^-1, at between 0.28 and 2.80 mol m^-2 yr^-1. During elevated pH, rates of CO₂-influx increased up to nearly tenfold as a result of chemical-enhancement by parallel flux of HCO^-₃. Input of CO₂* to the lake from the catchment is suggested to be the main source of the carbon lost to the atmosphere.
• 6 The turnover time for CO₂ between the air and water was calculated to be 1 year for the gross influx and 3.3 years for the net flux. These values are less than the average water residence time of 0.25 years, which indicates that over a year inflow from streams is a more important source of inorganic carbon than the atmosphere.
• 7 Influx of CO₂ from the atmosphere was calculated to be roughly equivalent to between 1 and 4% of the rates of production in the water during mid-summer indicating that this source of inorganic carbon is not a major one in this lake.
• 8 Influx of CO₂ from the hypolimnion was estimated on one occasion to be 6.9 10^-9 mol m^-2 s^-1 using transfer values based on mass eddy-diffusion. These rates are equivalent to 23% of the rate of influx of CO₂ from the atmosphere on this occasion which suggests that the hypolimnion is probably a small source of inorganic carbon to the epilimnion. The exception appears to be during windy episodes when pH is depressed. Calculations based on depth-profiles of CO₂* and HCO^-₃ suggest that the measured changes in pH can be accounted for by entrainment of hypolimnetic water into the epilimnion.
• 9 The solubility product for calcite was exceeded by up to about sixfold which, although insufficient to allow homogeneous precipitation, may have allowed heterogeneous precipitation around algal particles.

     

Structural and metabolic properties of green tissue cultures from a CAM plant, Kalanchoë blossfeldiana hybr. Montezuma

Abstract Crassulacean acid metabolism (CAM) was studied in mixotrophic callus tissue cultures of Kalanchoë blossfeldiana hybr. Montezuma and compared with plants propagated from the calli. The ultrastructural properties of the green callus cells are similar to mesophyll cells of CAM plants except that occasionally abnormal mitochondria were observed. There was permanent net CO₂ output by the calli in light and darkness, which was lower in darkness than in light. The calli exhibited a diurnal rhythm of malic acid, with accumulation during the night and depletion during the day. ¹⁴C previously incorporated by dark CO₂ fixation into malate was transferred upon subsequent illumination into end products of photosynthesis. All these data indicate that CAM operates in the calli tissue. The results revealed that the capacity for CAM is obviously lower in the calli compared with plantlets developing from the calli, or with ‘adult’ plants. The data suggest also that CAM in the calli was not limited by the activities of CAM enzymes.     

Short-term interactive effects of ultraviolet radiation,carbon dioxide and nutrient enrichment on phytoplankton in a shallow coastal lagoon

The main goal of this study was to evaluate short-term interactions between increased CO₂, UVR and inorganic macronutrients (N, P and Si) on summer phytoplankton assemblages in the Ria Formosa coastal lagoon (SW Iberia), subjected to intense anthropogenic pressures and highly vulnerable to climate change. A multifactorial experiment using 20 different nutrient-enriched microcosms exposed to different spectral and CO₂ conditions was designed. Before and after a 24-h in situ incubation, phytoplankton abundance and composition were analysed. Impacts and interactive effects of high CO₂, UVR and nutrients varied among different functional groups. Increased UVR had negative effects on diatoms and cyanobacteria and positive effects on cryptophytes, whereas increased CO₂ inhibited cyanobacteria but increased cryptophyte growth. A positive synergistic interaction between CO₂ and UVR was observed for diatoms; high CO₂ counteracted the negative effects of UVR under ambient nutrient concentrations. Nutrient enrichments suppressed the negative effects of high CO₂ and UVR on cyanobacteria and diatoms, respectively. Beneficial effects of CO₂ were observed for diatoms and cryptophytes under combined additions of nitrate and ammonium, suggesting that growth may be limited by DIC availability when the primary limitation by nitrogen is alleviated. Beneficial effects of high CO₂ and UVR in diatoms were also induced or intensified by ammonium additions.     

FUNCTIONAL GENETIC DIVERGENCE IN HIGH CO2 ADAPTED EMILIANIA HUXLEYI POPULATIONS

Predicting the impacts of environmental change on marine organisms, food webs, and biogeochemical cycles presently relies almost exclusively on short‐term physiological studies, while the possibility of adaptive evolution is often ignored. Here, we assess adaptive evolution in the coccolithophore Emiliania huxleyi, a well‐established model species in biological oceanography, in response to ocean acidification. We previously demonstrated that this globally important marine phytoplankton species adapts within 500 generations to elevated CO₂. After 750 and 1000 generations, no further fitness increase occurred, and we observed phenotypic convergence between replicate populations. We then exposed adapted populations to two novel environments to investigate whether or not the underlying basis for high CO₂‐adaptation involves functional genetic divergence, assuming that different novel mutations become apparent via divergent pleiotropic effects. The novel environment “high light” did not reveal such genetic divergence whereas growth in a low‐salinity environment revealed strong pleiotropic effects in high CO₂ adapted populations, indicating divergent genetic bases for adaptation to high CO₂. This suggests that pleiotropy plays an important role in adaptation of natural E. huxleyi populations to ocean acidification. Our study highlights the potential mutual benefits for oceanography and evolutionary biology of using ecologically important marine phytoplankton for microbial evolution experiments.     

Predictable ecological response to rising CO2 of a community of marine phytoplankton

Rising atmospheric CO₂ and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO₂ related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO₂. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO₂. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 μatm) and future (1,000 μatm) CO₂ levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high‐nitrogen batch cultures and lower‐nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO₂ independent of the culture regime. We also find that, despite species‐specific variation in growth response to high CO₂, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO₂. The CO₂‐induced growth response is a good predictor of CO₂‐induced changes in competition (R² > .93) and community composition (R² > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO₂ levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction.     

Phytoplancton du lac Ichkeul (Tunisie): Composition,structure et activité photosynthétique Phytoplankton of Lake Ichkeul (Tunesia): Composition,Structure and Photosynthetic Activity

Phytoplankton evolution was established during a study carried out on lake Ichkeul (Tunisia) from January 1985 to June 1988. During this period, the flora, composed of 51 species is distributed principally between Diatomophyceae and Chlorophyceae. Succesional changes of phytoplankton were often unpredictable and characterized by populations with monospecific trend. Photosynthetic activity and the parameters A_opt and I_K were studied regularly along the year 1985 by oxygen method (GOLTERMANN , 1969). The relations between phytoplankton composition and photosynthetic activity were discussed.     

Composition and Productivity of the Benthic Macroinvertebrate Community of a Subtropical Reservoir

Physical-chemical conditions, phytoplankton productivity, community structure and productivity of the macroinvertebrate benthic community were determined during 1976–77 in a subtropical reservoir. Physical-chemical results revealed high nitrate and phosphate concentrations with highest values in the riverine segment. Large phytoplankton populations were present during most of the year. Phytoplankton productivity was high, producing an annual mean of 87 mg C · m⁻³ · h⁻¹ (12 hours light day). High turbidity in the riverine segment limited phytoplankton productivity during winter and spring. Macrobenthos was dominated by chironomids (Chironomus, Procladius, Coelotanypus and Tanypus) and oligochaetes (Limnodrilus). The annual mean benthic population was estimated at 1,626 · m⁻² with a mean dry weight of 0.66 g · m⁻². Mean benthic species diversity was 1.80. A lacustrine-riverine community gradient was revealed. Benthic productivity was 6.8 g · m⁻² · yr⁻¹ (dry weight) with a P: B ratio of 10. A low correlation was observed between benthic and phytoplankton productivity, and between phytoplankton standing crop and benthic macroinvertebrate numbers throughout the reservoir. Algal food supplies had little impact on the benthic community which was composed predominately of species which fed mostly on organic detritus. Stressful conditions caused by low dissolved oxygen concentrations probably inhibited development of the benthic community throughout the reservoir during summer months, while high sedimentation rates limited development in the head waters.     

Patterns of dark14CO2 incorporation by natural marine phytoplankton communities

The rates of dark¹⁴CO₂ fixation by natural phytoplankton communities growing in eutrophic and oligotrophic waters were studied with short-term in situ experiments. Three aspects were investigated: (1) the time course incorporation of¹⁴CO₂ in darkness, (2) the depth variability in dark¹⁴CO₂ fixation, and (3) the variability in¹⁴CO₂ fixation within a year. The highest dark¹⁴CO₂ incorporation rates were observed during the first interval of incubation (20 min) after which they approached a constant rate with time. The observed differences in dark¹⁴CO₂ fixation rates between populations from different depths were associated with differences in species composition as well as with physiological differences caused by exposure to different illumination conditions prior to their exposure to darkness. Autocorrelation coefficients were computed for the analysis of variability of dark¹⁴CO₂ fixation rates within a year. It was suggested that dark¹⁴CO₂ incorporation might be a periodic phenomenon depending mainly on the productive capacity of the phytoplankton community.     

Surface water CO2 concentration influences phytoplankton production but not community composition across boreal lakes

Recent experimental evidence suggests that changes in the partial pressure of CO₂ (pCO₂), in concert with nutrient fertilisation, may result in increased primary production and shifted phytoplankton community composition that favours species lacking adaptations to low CO₂ environments. It is not clear whether these results apply in ambient freshwaters, which are already often supersaturated in CO₂, and where phytoplankton structure and activity are under complex control of diverse local and regional factors. Here, we use a large‐scale comparative study of 69 boreal lakes to explore the influence of existing CO₂ gradients (c. 50–2300 μatm) on phytoplankton community composition and biomass production. While community composition did not respond to pCO₂ gradients, gross primary production was enhanced, but only in lakes already supersaturated in CO₂, demonstrating that environmental context is key in determining pCO₂–phytoplankton interactions. We further argue that increased atmospheric CO₂ is unlikely to influence phytoplanktonic composition and production in northern lakes.     

Nitrogen and Carbon Concentrations, and Stable Isotope Ratios in Mediterranean Shrubs Growing in the Proximity of a CO2 spring

Seasonal changes in foliage nitrogen (N) and carbon (C) concentrations and δ¹⁵N and δ¹³C ratios were monitored during a year in Erica arborea, Myrtus communis and Juniperus communis co-occurring at a natural CO₂ spring (elevated [CO₂], about 700 μmol mol⁻¹) and at a nearby control site (ambient [CO₂], 360 μmol mol⁻¹) in a Mediterranean environment. Leaf N concentration was lower in elevated [CO₂] than in ambient [CO₂] for M. communis, higher for J. communis, and dependent on the season for E. arborea. Leaf C concentration was negatively affected by atmospheric CO₂ enrichment, regardless of the species. C/N ratio varied concomitantly to N. Leaves in elevated [CO₂] showed lower δ¹³C, and therefore likely lower water use efficiencies than leaves at the control site, regardless of the species, suggesting substantial photosynthetic acclimation under long-term CO₂-enriched atmosphere. Leaves of E. arborea showed lower values of δ¹⁵N under elevated [CO₂], but this was not the case of M. communis and J. communis foliage. The use of the resources and leaf chemical composition are affected by elevated [CO₂], but such an effect varies during the year, and is species-dependent. The seasonal dependency and species specificity suggest that plants are able to exploit different available water and N resources within Mediterranean sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interactions of glycolate-, HCO 3 - -, Cl--, and H+-balance of Scenedesmus obliquus

Excretion and absorption of glycolate by young cells of Scenedesmus obliquus (Turp.) Krüger strain D₃ grown synchronously with 2% CO₂ was compared after no pretreatment with air (CO₂-adapted) or after a 2 h adaptation to normal air (0.03% CO₂) (air-adapted). At 21% O₂, excretion occurred only from CO₂-adapted cells at high pH (pH 8.0). Under conditions where no excretion occurred, external glycolate (0.2 mM) was taken up by both air-and CO₂-adapted cells at a much faster rate at pH 5 than at pH 8. The uptake was accompanied by an apparent stoichiometric uptake of H⁺. CO₂-adapted algae exhibited high uptake rates that were even higher in the dark than in the light. Air-adapted algae showed high uptake rates in the light but only minimal uptake in the dark. The uptake rate was decreased to about 1/3 with 5% CO₂, except with CO₂-adapted cells in the light, in which a slight stimulation occurred. Cl^- ions inhibited glycolate uptake by air-adapted cells in the light; conversely, light-stimulated Cl^- uptake of these cells was inhibited by glycolate. A hypothesis is discussed according to which the internal pH regulates the uptake and release of Cl^-, HCO ₃ ^- , and glycolate.Abbreviations DCMU 3-(3,4 dichlorophenyl)-1, 1-dimethyl urea - FCCP carbonyl cyanide p-trifluoro-methoxyphenylhydrazone - HEPES 2-(4-(2-hydroxyethyl)-piperazinyl) ethanesulfonic acid - HPMS -hydroxypyridinemethanesulfonate - MES 2-morpholinoethanesulfonic acid - PCV packed cell volume