Photosynthetic Production Rate Research Articles

An HD-Zip transcription factor, VvHDZ4, in grapes (Vitis vinifera L.) confers enhanced drought tolerance in transgenic tomato

BACKGROUND: HD-Zip genes encode several large and highly conserved protein families of transcription factors that play important roles in plant development and responses to environmental stress. To date, information about the involvement of HD-Zip in grape to drought response is limited. OBJECTIVE: The production of grapes is limited by a range of biotic and abiotic stresses, which cause significant losses in yield every year as well as a reduction in fruit quality. Identification and analysis of stress related genes in grapes are very important for cultivating more robust varieties with environmental stress resistance METHODS: We isolated a homeodomain-leucine zipper gene (HD-Zip), VvHDZ4, from grape, Vitis vinifera L. “Yatomi Rose”, and characterized VvHDZ4 based on analyses of its expression patterns, subcellular localization, transcription activity, and overexpression. RESULTS: We found that VvHDZ4 was highly expressed in roots and leaves, localized to nucleus, and activated the expression of reporter genes in yeast. The overexpression (OE) of VvHDZ4 led to enhanced drought stress tolerance in Solanum lycopersicum L. “Ailsa Craig” (tomato). The drought-tolerant phenotypes of the OE lines exhibited decreases in electrolyte leakage and rate of water loss, higher photosynthetic production and net photosynthesis rate, and showed up-regulation of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX), all of which are key antioxidant enzymes known to be activated during drought stress. Moreover, we observed the up-regulation of four additional drought stress indicator genes, SIDREB2A, SIAREB, SIRD29 and SIERD10, in the OE lines. VvHDZ4 improved drought tolerance in tomato, and the modes of action possibly consist of reducing membrane damage, increasing photosynthetic productivity, and modulating the expression of stress defense genes. CONCLUSIONS: This work increases our understanding of the important roles HD-Zip transcription factors in the responses of plants to the environment, especially abiotic stress.

Estimation of linear and cyclic electron flows in photosynthesis based on 13C-metabolic flux analysis

Photosynthetic organisms produce ATP and NADPH using light as an energy source and further utilize these cofactors during metabolism. Photosynthesis involves linear and cyclic electron flows; as the cyclic electron flow produces ATP more effectively than the linear electron flow without NADPH, the cell efficiently adjusts ATP and NADPH production using the two different pathways. Nevertheless, direct measurement of ATP and NADPH production during photosynthesis has been difficult. In the present study, the photosynthetic ATP and NADPH production rates of Synechocystis sp. PCC 6803 under three different single peak wavelength lights (blue: 470nm, R630: 630nm, and R680: 680nm) were evaluated based on 13C-metabolic flux analysis (13C-MFA) by considering the mass balance of ATP and NADPH between photosynthesis and metabolism. The ratios of ATP/NADPH production via photosynthesis were estimated as 3.13, 1.70, and 2.10 under blue, R630, and R680 light conditions, respectively. Moreover, the linear and cyclic electron flow ratios were estimated to be 1.1-2.2, 0.2-0.5, and 0.5-1.0 under blue, R630, and R680 light conditions, respectively. The predicted linear and cyclic electron flow ratios were consistent with the excitation ratio between photosystems I and II, as observed in the steady-state fluorescence spectra.

Modeling the oxygen inhibition in microalgae: An experimental approach based on photorespirometry

Microalgae cultivation has been the object of relevant interest for many industrial applications. Where high purity of the biomass/product is required, closed photobioreactors (PBRs) appear to be the best technological solution. However, as well as cost, the major drawback of closed systems is oxygen accumulation, which is well known to be responsible for growth inhibition. Only a few quantitative approaches have attempted to describe and model oxygen inhibition, which is the result of different biological mechanisms. Here, we have applied a photorespirometric protocol to assess and quantify the effect of high oxygen concentration on photosynthetic production rate. In particular, the effects of light intensity and biomass concentration were assessed, resulting in different maximum inhibitory oxygen concentrations. Literature models available were found not to fully represent experimental data as a function of concentration and light. Accordingly, a new formulation was proposed and validated to describe the photosynthetic rate as a function of external oxygen concentration.

Marine system dynamical response to a changing climate in frame of power law, exponential decay, and Mittag‐Leffler kernel

The increase of sea surface temperature in ocean changes the photosynthetic production rate of phytoplankton. Therefore, it is crucial to understand the relation between temperature and phytoplanktons photosynthesis to deal the extinction caused by excessive increase in temperature. It is worth observing that temperature is one of the most principal limiting factors for phytoplanktons production due to photosynthetic enzymes work at their optimum temperature levels. In this study, the fractional oxygen‐phytoplankton‐zooplankton model is considered by singular and nonsingular fractional operators within Caputo, Caputo‐Fabrizio, and Atangana‐Baleanu in Caputo sense. The rate of oxygen production is considered by a function of temperature account for the sea surface warming. At first, the temperature function is constant and then it starts to increase, after a certain time of increase, before the oxygen depletion begins, the temperature is set to a higher secure value. With this temperature function choice, detailed numerical simulations are carried out to provide details of the internal structure of the system. We observe that the species are more sustainable in Caputo model than its corresponding integer‐order model.

Selection of photosynthesis and respiration models to assess the effect of environmental conditions on mixed microalgae consortia grown on wastewater

This study aimed at evaluating the effects of different environmental conditions (irradiance, temperature, pH and dissolved oxygen) on a microalgae-bacteria consortium cultivated in a pilot-scale open pond and fed on the liquid fraction of anaerobic digestate. A standardized photo-respirometry protocol was followed to evaluate the activity of microalgae under different conditions. Two datasets (specific photosynthetic oxygen production rates and respiratory oxygen consumption rates) were obtained for each environmental parameter, throughout the entire range of conditions found in the outdoor cultivation system. Different kinetic models available in literature were fitted to experimental data and the resulting outputs were compared through model selection estimators, in order to select the most appropriate equations. The proposed set of equations constitute a modelling tool for the prediction of algal growth rates in algae-bacteria systems, as a function of environmental conditions.

Analysis of chlorophyll and photosynthesis of a tomato chlorophyll-deficient mutant induced by EMS.

We analyzed the growth, leaf chlorophyll content, and photosynthetic parameters of a tomato leaf yellowing mutant (Y55) induced by ethyl methane sulfonate (EMS) from the cultivar "Heinz 1706" (WT). Results showed that the plant height, stem diameter, and fresh mass of Y55 significantly . The contents of chlorophyll a, chlorophyll b, carotenoid, total chlorophyll and the chlorophyll a/b ratio of the mutant were significantly lower than those of WT. The contents of all precursor materials of chlorophyll synthesis, especially porphyrinogen III and those involved in the chlorophyll biosynthesis pathway, were significantly lower in Y55 than those in WT. Moreover, the net photosynthesis (Pn), transpiration rate (Tr), intercellular CO2 concentration (Ci), and conductance to H2O (gs) significantly in Y55. The maximum photosynthetic rate, CO2 saturation and compensation point, and light saturation and compensation point. The Fv/Fm significantly, whereas the Fo significantly in Y55. The photosynthetic electron production and electron transport rates of PSII and PSI also significantly decreased. The total photosynthetic pigment molecules (No) and the minimum average lifetime of photosynthetic pigment molecules in the excited state (τmin) significantly in Y55. All these results suggest that blocking the synthesis of porphyrinogen III ould decrease the chlorophyll content in the mutant Y55. Furthermore, the reduced amount of leaf pigment could affect photosynthesis in leaves and slow down the growth of mutant plants.

Trace metal limitations (Co, Zn) increase PIC/POC ratio in coccolithophore Emiliania huxleyi

Abstract The effect of cobalt (Co), zinc (Zn) or cadmium (Cd) availability on the growth and particulate inorganic (PIC) to organic (POC) carbon ratio of the cosmopolitan coccolithophore Emiliania huxleyi was examined using batch cultures. Growth co-limitation by Co and Zn occurred at low free metal ion concentrations below 10–12 pM. The two metals could replace one another in biochemical functions: full replacement of Zn by Co was possible, but the 20% higher growth rate observed with high Co levels in the absence of Zn than observed in the presence of high Zn but no added Co suggested that there was an absolute growth requirement of Co that could only partly be met by Zn. In contrast, Cd had no effect on growth in the absence of Co and Zn, indicating that this strain cannot use Cd under these conditions. The co-limitation by inorganic Co and Zn led to highly calcified cells containing a large quota of nitrogen, but diminished organic carbon content, likely due to a lower photosynthetic production rate. Deficiency of Zn and Co cofactors could have dramatically decreased the activity of carbonic anhydrase, lowering CO2 availability for photosynthesis. In contrast to the large decreases in carbon fixation rates at low Zn and Co ion concentrations, calcification rates were unaffected at low Zn levels and were only minimally decreased by low Co. As a result the PIC:POC ratios in E. huxleyi increased by up to 8-fold under Zn limitation and 3–5-fold under Co limitation to values where coccolithophores act as a source of atmospheric CO2. As a result, low Co and Zn availability in seawater could affect the efficiency of the biological carbon pump, and atmospheric CO2 levels. Thus, the impact of Co and Zn limitation may need to be considered to improve assessments of the impact of E. huxleyi and other biogenic calcifiers on oceanic sequestration of CO2.

A steady-state stomatal model of balanced leaf gas exchange, hydraulics and maximal source-sink flux.

Trees must simultaneously balance their CO2 uptake rate via stomata, photosynthesis, the transport rate of sugars and rate of sugar utilization in sinks while maintaining a favourable water and carbon balance. We demonstrate using a numerical model that it is possible to understand stomatal functioning from the viewpoint of maximizing the simultaneous photosynthetic production, phloem transport and sink sugar utilization rate under the limitation that the transpiration-driven hydrostatic pressure gradient sets for those processes. A key feature in our model is that non-stomatal limitations to photosynthesis increase with decreasing leaf water potential and/or increasing leaf sugar concentration and are thus coupled to stomatal conductance. Maximizing the photosynthetic production rate using a numerical steady-state model leads to stomatal behaviour that is able to reproduce the well-known trends of stomatal behaviour in response to, e.g., light, vapour concentration difference, ambient CO2 concentration, soil water status, sink strength and xylem and phloem hydraulic conductance. We show that our results for stomatal behaviour are very similar to the solutions given by the earlier models of stomatal conductance derived solely from gas exchange considerations. Our modelling results also demonstrate how the 'marginal cost of water' in the unified stomatal conductance model and the optimal stomatal model could be related to plant structural and physiological traits, most importantly, the soil-to-leaf hydraulic conductance and soil moisture.

The dual effect of a ferredoxin-hydrogenase fusion protein in vivo: successful divergence of the photosynthetic electron flux towards hydrogen production and elevated oxygen tolerance.

BackgroundHydrogen photo-production in green algae, catalyzed by the enzyme [FeFe]-hydrogenase (HydA), is considered a promising source of renewable clean energy. Yet, a significant increase in hydrogen production efficiency is necessary for industrial scale-up. We have previously shown that a major challenge to be resolved is the inferior competitiveness of HydA with NADPH production, catalyzed by ferredoxin-NADP+-reductase (FNR). In this work, we explored the in vivo hydrogen production efficiency of Fd-HydA, where the electron donor ferredoxin (Fd) is fused to HydA and expressed in the model organism Chlamydomonas reinhardtii.ResultsWe show that once the Fd-HydA fusion gene is expressed in micro-algal cells of C. reinhardtii, the fusion enzyme is able to intercept photosynthetic electrons and use them for efficient hydrogen production, thus supporting the previous observations made in vitro. We found that Fd-HydA has a ~4.5-fold greater photosynthetic hydrogen production rate standardized for hydrogenase amount (PHPRH) than that of the native HydA in vivo. Furthermore, we provide evidence suggesting that the fusion protein is more resistant to oxygen than the native HydA.ConclusionsThe in vivo photosynthetic activity of the Fd-HydA enzyme surpasses that of the native HydA and shows higher oxygen tolerance. Therefore, our results provide a solid platform for further engineering efforts towards efficient hydrogen production in microalgae through the expression of synthetic enzymes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0601-3) contains supplementary material, which is available to authorized users.

Variability in photosynthetic production of dissolved and particulate organic carbon in the North Pacific Subtropical Gyre

The partitioning of photosynthetically-derived organic carbon between particulate and dissolved phases has important implications for marine carbon cycling. In this study we utilized 14C-bicarbonate assimilation to quantify rates of photosynthetic production of both particulate and dissolved organic carbon (DOC) at Station ALOHA (22˚45’N, 158˚W) in the North Pacific Subtropical Gyre (NPSG). At near-monthly time scales over ~5 years, we examined retention of 14C-labeled organic matter by both glass fiber filters and 0.2 µm pore size polycarbonate membrane filters that are commonly used for measurements of 14C-based plankton productivity. Use of polycarbonate filters resulted in significantly lower (averaging 60%) estimates of 14C-production compared to glass fiber filters. Coincident measurements of chlorophyll a concentrations from both 0.2 µm polycarbonate and glass fiber filters were not significantly different, suggesting the differences in 14C-productivity between these filter-types did not derive from differences in retention of photosynthetic biomass by these filters. Moreover, consistent with previous studies, results from experiments aimed at quantifying retention of organic matter by these filters suggested the difference between these two types of filters resulted from retention of DOC by glass fiber filters. We also quantified rates of 14C-DOC production to evaluate the partitioning of photosynthetic production between dissolved and particulate phases over daily to monthly time scales in this ecosystem. Unlike the strong depth-dependence observed in measurements of particulate organic carbon production, measured rates of 14C-DOC demonstrated no clear depth-dependence. On average, depth-integrated (0-75 m) rates of 14C-DOC production rates were equivalent to 18 ± 10% of the total (particulate and dissolved) productivity. Our findings indicate that in this oligotrophic ecosystem, rates of dissolved and particulate production can be temporally decoupled over daily to monthly time scales.

Physiological growth and gas exchange response of saffron (Crocus sativus L.) to irrigation water salinity, manure application and planting method

The objective of the present study is to investigate the effects of irrigation water salinity, cow manure levels and different planting methods on saffron growth rate and gas exchange. A split-split plot arrangement was conducted in randomized complete block design with irrigation water salinity levels (0.45 (fresh water, S1), 1.0 (S2), 2.0 (S3), and 3.0 (S4) dSm−1) as the main plot, cow manure levels (30 (F1) and 60 (F2) Mgha−1) as the subplot and planting method (basin (P1) and in-furrow (P2)) as the sub-subplot in three replications. Results showed that initial amount of planted corms were degraded by using saline water (EC higher than 2.0dSm−1) in the basin planting method and under F1 and F2 cow manure levels in consecutive growing seasons; however, we observed an acceptable increase in corm growth in high salinity levels (S3 and S4) under the in-furrow planting method. Assessment of the ratio of corm DW to the leaf DM showed that corms propagation were higher in the first year; however, in the second year higher weight proportions of dry matter were allocated to the leaves. Maximum saffron LAI and crop growth rate (CGR) decreased significantly as about 26% by using the highest water salinity level and these parameters increased as 37 and 61%, respectively by using the in-furrow planting method. Higher cow manure (F2) also increased LAI and CGR by 16 and 13%, respectively. Results of gas exchange parameters showed that photosynthesis rate (An) decreased as about 32% in the highest water salinity level. Higher manure application increased the An as about 18%. Also, An increased significantly by 29% under the in-furrow planting method compared with that in the basin. An was 17% higher in the second year than that in the first year and the conversion rate of photosynthetic production to shoot dry weight for developed saffron increased by about 47% due to leaf area index increase. High salt sensitivity of saffron could be remediated by using the in-furrow planting method and higher cow manure application rate.

English

With the limited fossil fuel resources and aggravating energy crisis, coupled with the concern about the climate change caused by greenhouse gases, many people hope that renewable fuels will be developed as an alternative to fossil fuels, with special attention being paid to bioethanol.1,2 Compared to fossil fuels, biofuels emit less ozone, benzene, carbon dioxide and other harmful pollutants. For a long time, bioethanol has been raising world-wide attention and many researchers are searching for alternative biomass sources for the production of bioethanol, such as corn,4 wood,5 sugarcane6 switch grass,7 rice straw,8 corn straw9 and wheat straw.10 Today, about 30 % of the corn currently grown is used for ethanol production, and more corn is needed to meet the increasing demand for bioethanol.11 The higher amounts of corn turned to biofuel production could have devastating effects on food supply around the world and cause conflicts in the food vs. fuel dilemma. Ethanol production from lignocellulose is a promising alternative but the current technologies for lignocellulose fermentation have to overcome the cost of the complex processes needed to release simple sugars from recalcitrant polysaccharides.12 With limited land area, pretreatment technical difficulties and low conversion rate, much more needs to be done in bioethanol production from lignocellulose. And the increasing need for energy consumption is expected to continue as the world’s population is expected to increase. In order to meet the expected increasing demand for bioethanol, there is a need to find alternative biomass sources, particularly those that do not rely on using large amounts of agricultural land. Marine algae are attractive renewable energy resources due to their abundance, high photosynthetic efficiency and production rate. Algae contain a low concentration of lignin and sugars can be easily released by simple operations such as milling or crushing, so seaweeds are proposed as one of the most promising biomass materials for ethanol production.13 Marine algae are classified into three groups by their colors: green, brown, and red. Brown algae, as the second most abundant marine biomass, have several key features of an ideal feedstock for biofuel production. They do not require arable land, fertilizer, or fresh water, they are of The Isolation and Performance Studies of an Alginate Degrading and Ethanol Producing Strain

Enhanced Raman multigas sensing – a novel tool for control and analysis of13CO2labeling experiments in environmental research

Cavity-enhanced Raman multigas spectrometry is introduced as a versatile technique for monitoring of (13)CO2 isotope labeling experiments, while simultaneously quantifying the fluxes of O2 and other relevant gases across a wide range of concentrations. The multigas analysis was performed in a closed cycle; no gas was consumed, and the gas composition was not altered by the measurement. Isotope labeling of plant metabolites via photosynthetic uptake of (13)CO2 enables the investigation of resource flows in plants and is now an important tool in ecophysiological studies. In this experiment the (13)C labeling of monoclonal cuttings of Populus trichocarpa was undertaken. The high time resolution of the online multigas analysis allowed precise control of the pulse labeling and was exploited to calculate the kinetics of photosynthetic (13)CO2 uptake and to extrapolate the exact value of the (13)CO2 peak concentration in the labeling chamber. Further, the leaf dark respiration of immature and mature leaves was analyzed. The quantification of the photosynthetic O2 production rate as a byproduct of the (13)CO2 uptake correlated with the amount of available light and the leaf area of the plants in the labeling chamber. The ability to acquire CO2 and O2 respiration rates simultaneously also simplifies the determination of respiratory quotients (rate of O2 uptake compared to CO2 release) and thus indicates the type of combusted substrate. By combining quantification of respiration quotients with the tracing of (13)C in plants, cavity enhanced Raman spectroscopy adds a valuable new tool for studies of metabolism at the organismal to ecosystem scale.

MODELLING THE EFFECT OF XYLEM AND PHLOEM TRANSPORT ON LEAF GAS EXCHANGE

Whole tree water and carbon relations are mediated between the leaves and carbon sinks in the form of water potential and osmotic concentration through long distance xylem and phloem transport within the tree. Here we present a whole tree level model which couples these source, sink and transport processes. We demonstrate how xylem and phloem transport capacity, together with soil water and sink sugar status, set limits to the maximum leaf gas exchange that can be sustained by a plant in steady state. We further show that when down-regulation of photosynthetic capacity depending on leaf sugar concentration is added to the model formulation, the typically observed stomatal behaviour in relation to environmental drivers and sink status can be qualitatively understood by maximization of the simultaneous photosynthesis production. In our steady state formulation, the rate of photosynthetic production equals the phloem transport rate, and the rate of sugar utilization in the sinks.

Reply to comment by J. Marra on “Measuring primary production rates in the ocean: Enigmatic results between incubation and non‐incubation methods at Station ALOHA”

Reply to comment by J. Marra on “Measuring primary production rates in the ocean: Enigmatic results between incubation and non‐incubation methods at Station ALOHA”

Biological production in the NE Pacific and its influence on air‐sea CO2 flux: Evidence from dissolved oxygen isotopes and O2/Ar

We determine rates of gross photosynthetic O2 production (GOP) and net community O2 production (NCP) using the triple oxygen isotope and O2/Ar approach on two spring and two late summer meridional transects of the NE Pacific. Observed GOP and NCP in the subtropical (89 ± 9 and 8.3 ± 1.3 mmol O2 m−2 d−1, respectively) and subarctic (193 ± 16 and 16.3 ± 3.8 mmol O2 m−2 d−1) were in agreement with rates previously determined at time series stations in each region, validating the regional representativeness of these sites. At the transition zone chlorophyll front (TZCF), which migrates seasonally from 32°N in spring to 40°N in summer, GOP and NCPwere elevated by 2–4× compared to adjacent areas. Coincident with the TZCF, increases in surface nitrate concentration and extensive changes in phytoplankton community composition were observed. HPLC pigment data indicated substantial increases in a prymnesiophyte (e.g., coccolithophore) biomarker at the TZCF on a spring and summer cruise, and a diatom biomarker on the spring cruise. Increases in remotely sensed surface particulate inorganic carbon concentration were also observed at the TZCF on all four cruises, indicating that coccolithophore production may contribute to increased productivity at the TZCF. Meridional trends in observed air‐sea CO2 flux on each cruise resembled those of the biologically induced CO2 flux (NCP), but with an overprinting of the response of air‐sea CO2 exchange to summer warming. A simple carbon budget based on regional CO2 flux climatology demonstrates the importance of NCPfor net annual air‐sea CO2uptake, although slow air‐sea equilibration and seasonal solubility effects obscure this term.

Primary production and carbon export rates across the subpolar N. Atlantic Ocean basin based on triple oxygen isotope and dissolved O2 and Ar gas measurements

Gross photosynthetic O2 production (GOP) rates in the subpolar North Atlantic Ocean were estimated using the measured isotopic composition of dissolved oxygen in the surface layer on samples collected on nine transits of a container ship between Great Britain and Canada during March 2007 to June 2008. The mean basin‐wide GOP rate of 226 ± 48 mmol O2 m−2 d−1 during summer was double the winter rate of 107 ± 41 mmol O2 m−2 d−1. Converting these GOP rates to equivalent 14C‐based PP (14C‐PPeqv) yielded rates of 1005 ± 216 and 476 ± 183 mg C m−2 d−1 in summer and winter, respectively, that generally agreed well with previous 14C‐based PP estimates in the region. The 14C‐PPeqv estimates were 1–1.6× concurrent satellite‐based PP estimates along the cruise track. A net community production rate (NCP) of 87 ± 12 mmol O2 m−2 d−1 (62 ± 9 mmol C m−2 d−1) and NCP/GOP of 0.35 ± 0.06 in the mixed layer was estimated from O2/Ar and 17Δ measurements (61°N 26°W) during spring bloom conditions in May 2008. Contrastingly, a much lower long‐term annual mean NCP or organic carbon export rate of 2.8 ± 2.7 mol C m−2 yr−1 (8 ± 7 mmol C m−2 d−1) and NCP/GOP of 0.07 ± 0.06 at the winter mixed layer depth was estimated from 15 years of surface O2 data in the subpolar N. Atlantic collected during the CARINA program.

Field measurements of key parameters associated with nocturnal OBT formation in vegetables grown under Canadian conditions

The objective of this study was to provide the parameter values required to model OBT formation in the edible parts of plants following a hypothetical accidental tritium release to the atmosphere at night. The parameters considered were leaf area index, stomatal resistance, photosynthesis rate, the photosynthetic production rate of starch, the nocturnal hydrolysis rate of starch, the fraction of starch produced daily by photosynthesis that appears in the fruits, and the mass of the fruit. Values of these parameters were obtained in the summer of 2002 for lettuce, radishes and tomatoes grown under typical Canadian environmental conditions. Based on the maximum observed photosynthetic rate and growth rate, the fraction of starch translocated to the fruit was calculated to be 17% for tomato fruit and 14% for radish root.

Oligotrophication trend of Lake Ziway, Ethiopia

Some ecological changes have been noted in Lake Ziway since the 1980s, such as lowering of the lake level, introduction and dominance of the catfish Clarias gariepinus in the fishery (53%) and establishment of cladocerans such as Daphnia barbata and Ceriodaphnia cornuta. This prompted us to study the phytoplankton biomass to see whether these changes were cascaded through the trophic food chain of Lake Ziway. The dynamics of some chemical and biological parameters were studied in two littoral and one offshore sites in the lake from November 2003 to August 2004. Nutrients showed temporal as well as spatial variations. Nitrate increased in the offshore whereas soluble reactive phosphate (SRP) was high in littoral sites (20–380 µg/L), indicating anthropogenic impacts. Mean phytoplankton biomass was high at the offshore (43.85 mg Chl a m-3) and almost similar at the littoral sites (Mean values 33.73 and 33.68 mg Chl a m-3), but much lower than values reported earlier. Hourly rate of integral photosynthesis (∑a) ranged from 57.4–726 mg O2 m-2 h-1 at the offshore and 95–300 mg O2 m-2 h-1 at littoral sites, respectively. Biomass-specific rate of photosynthetic production at light saturation, (photosynthetic capacity, Φmax) ranged from 5.06–28.8 mg O2 (mg Chl a)-1 h-1, slightly higher than values reported in the 1980s (9.6–22.5), due to depressed algal biomass. Although nutrients have increased, phytoplankton biomass (as Chl a) has decreased over the last two decades, possibly due to heavy grazing by zooplankton and introduced fish. If this continues for some time, Lake Ziway will head towards oligotrophication, instead of eutrophication, as speculated by previous workers.

Thermodynamics of within-water processes in freshwater ecosystems under anthropogenic impact

A series of field experiments were carried out in natural model ecosystems (mesocosms) with the aim to study the effect of pollutants on processes proceeding within water body. The generalized thermodynamic state parameters used in the study were the rate of photosynthetic production and destruction of organic matter. The ratio of destruction to primary production and the rate of changes in this ratio were used to construct phase portraits of ecosystems. Analysis showed that the dynamics of ecosystem states described by those characteristics corresponds to the general regularities in the evolution of the state of dissipative structures, hence the rate of changes in the destruction-to-production ratio can be taken as a thermodynamic criterion of evolution of aquatic ecosystems. The proposed approach allows this criterion to be used to develop methods for standardization of ecological loads.