Improving photosynthesis for algal biofuels: toward a green revolution

Biofuels derived from marine algae are a potential source of sustainable energy that can contribute to future global demands. The realisation of this potential will require manipulation of the fundamental biology of algal physiology to increase the efficiency with which solar energy is ultimately converted into usable biomass. This 'photosynthetic solar energy conversion efficiency' sets an upper limit on the potential of algal-derived biofuels. In this review, we outline photosynthetic molecular targets that could be manipulated to increase the efficiency and yield of algal biofuel production. We also highlight modern 'omic' and high-throughput technologies that might enable identification, selection and improvement of algal cell lines on timescales relevant for achieving significant contributions to future energy solutions.     

Photoadaptation in marine phytoplankton : changes in spectral absorption and excitation of chlorophyll a fluorescence

The optical properties of marine phytoplankton were examined by measuring the absorption spectra and fluorescence excitation spectra of chlorophyll a for natural marine particles collected on glass fiber filters. Samples were collected at different depths from stations in temperate waters of the Southern California Bight and in polar waters of the Scotia and Ross Seas. At all stations, phytoplankton fluorescence excitation and absorption spectra changed systematically with depth and vertical stability of the water columns. In samples from deeper waters, both absorption and chlorophyll a fluorescence excitation spectra showed enhancement in the blue-to-green portion of the spectrum (470-560 nm) relative to that at 440 nm. Since similar changes in absorption and excitation were induced by incubating sea water samples at different light intensities, the changes in optical properties can be attributed to photoadaptation of the phytoplankton. The data indicate that in the natural populations studied, shade adaptation caused increases in the concentration of photosynthetic accessory pigments relative to chlorophyll a. These changes in cellular pigment composition were detectable within less than 1 day. Comparisons of absorption spectra with fluorescence excitation spectra indicate an apparent increase in the efficiency of sensitization of chlorophyll a fluorescence in the blue and green spectral regions for low light populations.     

Assessment of a microscopic photobleaching technique for measuring the spectral absorption efficiency of individual phytoplankton cells

Measurements of the absorptance of photosynthetic pigments inindividual phytoplankton cells were made using an epifluorescencemicroscope equipped with a spectrograph and CCD array detector.Correction for light loss due to scattering was achieved bybleaching the cells with intense light from a mercury arc lamp,and using the bleached cells as a spectrophotometric blank.Absorption efficiency factors were calculated from knowledgeof the geometrical cross-section of the cells obtained fromcalibratedvideo images acquired at the time of measurement. The single-cellefficiency factors were consistent with the average absorptionefficiencies of cell suspensions measured using a spectrophotometerover most of the visible spectrum, but they were significantlylower below 420 nm. Cells of the diatom Cyclotella crypticaand the chlorophyte Chlorella salina showed clear spectral differencesin spectral shape that could be related to taxonomic differencesin pigment content, but absorption efficiency factors of approximately0.4 at 675 nm were found for both species.     

Light absorption by phytoplankton and the filter amplification correction: cell size and species effects

Filter amplification corrections are presented for eight marine centric diatoms. These functions are required to correct the amplified optical path-length associated with the glass-fiber filter used in the measurement of phytoplankton absorption. Correction factors constructed from phytoplankton cultures in the laboratory are often applied to phytoplankton assemblages in the field. This study demonstrates significant differences in the filter amplification correction correlated to species and cell volume. This variation in the filter amplification correction can result in significant error in estimated absorption coefficients, compromising subsequent estimates of quantum yield and primary production.     

Effect of photoinhibition on algal photosynthesis: a dynamic model

Recent evidence from algal physiology and molecular biologyconfirms that photoinhibition is directly related to D1 proteindamage and recovery, and D1 protein damage leads to a decreasein electron transfer or an increase in turnover time of theelectron transfer chain. In this study, the turnover time ofthe electron transfer chain is defined as a function of therelative concentration of D1 protein in reaction centre II andthe photoinhibition processes due to D1 protein degradationare incorporated into a model of photosynthesis, initiated byDubinsky et al. (Plant Cell Physiol., 27, 1335–1349, 1986)and developed by Sakshaug et al. (Limnol. Oceanogr., 34, 198–205,1989). D1 protein damage is assumed to be both light and D1protein concentration dependent, and to be proportional to thecross-section of PSII (     

Light absorption and photosynthesis of a benthic moss community: importance of spectral quality of light and implications of changing light attenuation in the water column

1. The underwater light climate and benthic moss communities of Grane Langsø were investigated in May 1997 to determine the potential effects on benthic production of changing water column attenuation and spectral quality of light.
2. A reduction in water clarity in the lake since the 1960s was manifested as a marked increase in the attenuation of blue light, relative to red light, which can be attributed to increased dissolved organic carbon.
3. The biomass of the benthic moss community ranged from a maximum of 195 gDW m⁻² at a depth of 4 m to 39 g DW m⁻² at a depth of 10 m and Drepanocladus exannulatus contributed 70% of the biomass at all depths.
4. Absorption of PAR by D. exannulatus was maximal in the highly pigmented youngest parts of the plant and these correspondingly showed the highest rates of net photosynthesis. The absolute amount of chlorophyll- a per g dry weight was greater at 10 m than 2 m, but the ratio of accessory pigments to chlorophyll- a did not change. Deep growing plants did not show adaptation to changed light quality.
5. Increased attenuation of blue light and the resultant overall decrease in water clarity is likely to impact negatively on net annual production of benthic macrophytes of Grane Langsø. Any further increase in dissolved organic carbon concentration has the potential to markedly decrease the depth to which mosses grow by reducing the length of time in a year during which net photosynthesis occurs.     

Photophysiology in two major southern ocean phytoplankton taxa: photosynthesis and growth of Phaeocystis antarctica and Fragilariopsis cylindrus under different irradiance levels

The Ross Sea, Antarctica, supports two distinct populations of phytoplankton, one that grows well in sea ice and blooms in the shallow mixed layers of the Western marginal ice zone and the other that can be found in sea ice but thrives in the deeply mixed layers of the Ross Sea. Dominated by diatoms (e.g. Fragilariopsis cylindrus) and the prymnesiophyte Phaeocystis antarctica, respectively, the processes leading to the development of these different phytoplankton assemblages are not well known. The goal of this article was to gain a better understanding of the photophysiological characteristics that allow each taxon to dominate its specific habitat. Cultures of F. cylindrus and P. antarctica were each grown semi-continuously at four different constant irradiances (5, 25, 65, and 125 μmol quanta/m2/s). Fragilariopsis cylindrus produced far less photosynthetic pigment per cell than did P. antarctica but much more photoprotective pigment. Fragilariopsis cylindrus also exhibited substantially lower rates of photosynthesis and growth but also was far less susceptible to photoinhibition of cell growth. Excess photosynthetic capacity, a measure of the ability of phytoplankton to exploit variable light environments, was significantly higher in both strains of P. antarctica than in F. cylindrus. The combination of these characteristics suggests that F. cylindrus has a competitive advantage under conditions where mixed layers are shallow and light levels are relatively constant and high. In contrast, P. antarctica should dominate waters where mixed layers are deep and light levels are variable. These results are consistent with distributions of phytoplankton in the Ross Sea and suggest that light is the primary factor determining composition of phytoplankton communities.     

An analytical flat-plate photobioreactor with a spectrally attenuated light source for the incubation of phytoplankton under dynamic light regimes

An inexpensive and simple, analytical microalgal photobioreactor with a highly controllable, dynamic, spectrally attenuated light source is described. Spectral attenuation is achieved through the introduction of a variable thickness of CuSO₄ solution between the photobioreactor and a light source. The level of liquid is precisely determined via a computer-controlled peristaltic pump, which can be programmed to pump at a variety of rates. The resultant light fields consist of a wide range of irradiance intensities with concomitant spectral narrowing, which closely mimics modeled clear water attenuation patterns. Irradiance dynamics associated with virtually any mixing regime can be achieved. The culturing apparatus of the analytical photobioreactor is based on traditional flat-plate, photobioreactor design, but with several modifications: (1) The light path has been reduced to 1 cm to assure a uniform light field is experienced by all phytoplankton at relatively low cell densities; (2) carbon dioxide concentrations are kept constant through a negative feedback mechanism that pulses CO₂ into a constant air stream when culture media pH rises above a set point; (3) temperature is controlled in a similar manner, through the addition of cooling water to a water jacket in response to an increase in culture media temperature. This design is intended for use in photophysiological and bio- physical studies of microalgae under highly controlled culture conditions. It should prove easily adaptable to any number of more complex configurations.     

Quantification of algal biofilms colonising building materials: chlorophyll a measured by PAM-fluorometry as a biomass parameter

The aim of this study was to quantify algal colonisation on anthropogenic surfaces (viz. building facades and roof tiles) using chlorophyll a (chl a) as a specific biomarker. Chl a was estimated as the initial fluorescence F0 of 'dark adapted' algae using a pulse-modulated fluorometer (PAM-2000). Four isolates of aeroterrestrial green algae and one aquatic isolate were included in this study. The chl a concentration and F0 showed an exponential relationship in the tested range between 0 and 400 mg chl a m(-2). The relationship was linear at chl a concentrations <20 mg m(-2). Exponential and linear models are presented for the single isolates with large coefficients of determination (exponential: r2 > 0.94, linear: r2 > 0.92). The specific power of this fluorometric method is the detection of initial algal colonisation on surfaces in thin or young biofilms down to 3.5 mg chl a m(-2), which corresponds to an abundances of the investigated isolates between 0.2 and 1.5 million cells cm(-2).     

Leaf development and phenology of Triticum aestivum and T. durum under different moisture regimes

Wheat grain yield production in the rain-fed areas is limited by water deficits during crop growth. A greenhouse experiment was conducted during spring 1992 at ICARDA, Tel Hadya, Syria, with eight genotypes representing two Triticum species (Triticum turgidum var. durum and Triticum aestivum L.) under four soil-moisture regimes (95%, 75%, 55%, and 35% field capacity) to study the effect of water deficit on leaf development. The phyllochron was similar in the two species across the watering regimes. The range in variation in phyllochron among the genotypes was similar in the two species. Phyllochron response to water stress among genotypes was distinct in the driest regime in both species. Cham 6 (T. aestivum) and Gallareta (T. turgidum var. durum) had similar phyllochron across all moisture regimes whereas in other genotypes phyllochron was higher in the dries regime. Leaf area decreased with increasing moisture stress. Triticum turgidum var. durum genotypes were later in flowering as they had, on average, one leaf more than Triticum aestivum genotypes with similar leaf appearance rates.     

Viability and sporulating capability of Coelomycetes preserved under a range of different storage regimes

The viability and sporulating capability of 45 Coelomycetes strains were evaluated. Strain subcultures were maintained under mineral oil, in soil and on agar slant for different periods of time lasting as long as 50 years, 39 years and 2 years, respectively. Of the 34 strains preserved under mineral oil, 20 maintained their viability but lost the sporulating capability with exception of one strain of Pestalotiopsis guepinii. Of the 16 strains also preserved in soil only one was viable and it was not able to sporulate. All 12 endophytic strains, 11 preserved on agar slant and one under mineral oil remained viable; however, the strain preserved under mineral oil lost its sporulating capability, while the strains on agar slant were only able to sporulate after culturing on sterilized alfalfa twigs. The results demonstrate that routine monitoring, and the use of different preservation methods, specially with the addition of sterilized plant tissue on the culture media for promoting conidiomata formation, is necessary for the success of the Coelomycetes long-term preservation.     

Water uptake efficiency and above- and belowground biomass development of sweet sorghum and maize under different water regimes

Background and aims

Lately sweet sorghum (S) has attracted great interest as an alternative feedstock for biofuel production due to its high yielding potential and better adaptation to drought than maize (M). However, little is known about the response of newly developed sweet sorghum genotypes to water deficits, especially at the root level and its water uptake patterns. The objective of this study was to compare the water uptake capacity, growth and developmental characteristics at the root and canopy levels of a sweet sorghum hybrid (Sorghum bicolor cv. Sucro 506) with those of maize (Zea mays cv. PR32F73) at two water regimes.

Methods

The trial was setup in a total of 20 rhizotrons (1?m³), where calibrated soil moisture probes were installed for monitoring and adjusting the soil moisture content to 25% (well-watered, W) and 12% (drought stress, D).

Results

DS was able to sustain its physiological activity close to that of WS plants, while maize was not. The biomass production potential of DS was reduced about 38%, while in maize the reduction was 47%. The water use efficiency (WUE), however, was increased by 20% in sweet sorghum and reduced in 5% in maize. Moreover, in contrast to maize the root length density and water uptake capacity of DS was enhanced. Root water uptake efficiency in DM was sustained close to its potential, but not in sweet sorghum.

Conclusions

In summary, the better adaptation to drought of sweet sorghum is explained by increased WUE, sustained physiological activity and enlarged root system. It is also associated with a reduced water uptake efficiency compared to its control but maintained compared to maize.     

Production of biomass and nutraceutical compounds by Spirulina platensis under different temperature and nitrogen regimes

The cyanobacterium Spirulina platensis has been used by humans because of its nutritional and possibly medicinal effects. Our study evaluated the influence of temperature and nitrogen concentration in the medium on the production of biomass by this cyanobacterium and the biomass composition in protein, lipid and phenolic compounds. We found that at 35 degrees C there was a negative effect on biomass production but a positive effect on the production of protein, lipids and phenolics, the highest levels of these compounds being obtained in Zarrouk's medium containing 1.875 or 2.500 g l(-1) sodium nitrate. Higher biomass densities and productivity were obtained at 30 degrees C than at 35 degrees C, but nitrogen concentration appeared to have no effect on the amount of protein, lipid or phenolics, indicating that at 30 degrees C the concentration of sodium nitrate in Zarrouk's medium (2.50 g l(-1)) can be reduced without loss of productivity, an important cost-saving factor in large-scale cultivation.     

Polymixis and algal production in a tropical lake: latitudinal effects on the seasonality of photosynthesis

SUMMARY. 1. Phytoplankton production was measured during 1981–82 in Bahia de Puno. a large polymictic system that is part of LakeTiticaca in the tropical Andes (Peru-Bolivia), Photosynthesis followed aweak seasonal pattern through each 12-month cycle but the largestvariations occurrud between years. 2. The large temporal variations in productivity appeared to becontrolled by biological events rather than by the direct effeets ofseasonal or aseasonal physical processes. Major shifts in production overthe 2-year period were attributable to the proportional abundance andactivity of N₂-fixing blue-green algae. Production was uncorrelated withthe seasonal radiation cycle. 3. In vivo fluorescence and photosynthetic assays revealed largechanges in phytoplankton community structure over time and depth.The shifts in vertical structure were associated with changes in transparency and water column stabilily.Physiological as well as floristiceffects dictated the variations in fluorescence per unit chlorophyll a. 4. An analysis of twenty-seven primary productivity data sets fromdifferent latitudes shtiwed that Bahia dc Puno resembled other tropicallakes in its relatively weak coupling between algal photosynthesis andseasonal variations in light. Productivity in the temperate zone correlated with the annual cycle of incoming radiation while the tropicalproduction data did not. For many of the tropical lakes, but fewtemperate lakes, the between-month variance in primary production wassignificantly higher than the between-month variance for incomingradiation. There was a significant positive relationship between maximum production variance explained hy light (maximum r² for each dataset) and latitude. 5. As in many tropical lakes. Bahia de Puno experiences moderatelylarge variations in algal photosynthesis.The muted seasonal cycle of light explains only a small portion of the total variation In productivity, and other factors such as weuther-related events are disproportionately important.     

Limitation of coffee leaf photosynthesis by stomatal conductance and light availability under different shade levels

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (A _n) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee A _n “spot” measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to A _n by low light availability (photon flux density, PFD) and stomatal conductance (g _s). These gas exchange measurements were carried out during two consecutive coffee growing seasons in a commercial plantation in the Orosi valley of Costa Rica. Levels of A _n were related to PFD and g _s in order to calculate envelope functions which were used to establish PFD or g _s limitations to A _n. Under the growing conditions of the present trial, mean leaf A _n remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by ~20% at 19% GI. Limitation to A _n due to g _s was strong in full sun and decreasing with increasing shade levels. On the other hand, limitation due to PFD remained at a similar level for all shade treatments. These different evolutions of limitations of A _n by PFD and g _s in response to shade explain the absence of a decrease in coffee leaf A _n with a shade level up to 55%. Consequently, these results confirm that Arabica coffee is a shade-adapted plant with leaves that can maintain a high photosynthetic performance under low light availability.     

Light sensitivity of na, rb, and k absorption by different tissues of bean leaves

Autoradiographs of ²²Na-loaded bean (Phaseolus vulgaris L. cv. `Brittle Wax') leaf slices showed that most of the tracer was concentrated in vascular tissue. Rubidium-86 was uniformly distributed in slices that had been incubated in darkness; after incubation in the light many small dark spots appeared on the autoradiographs, apparently corresponding with the stomata. Autoradiographs of ⁴²K-loaded slices showed a rather uniform distribution of the tracer, whether the slices had been incubated in light or in darkness.     

Performance of a pyrethroid-resistant strain of the predator mite Typhlodromus pyri (Acari: Phytoseiidae) under different insecticide regimes

An organophosphate pyrethroid-resistant strain of Typhlodromus pyri Scheuten imported from New Zealand was reared on potted apple trees in an outdoor insectary. From 1988 to 1995, the population was selected one to three times per year with a dilute solution (1.7 ppm) of the pyrethroid cypermethrin. Petri dish bioassays with cypermethrin in 1995 indicated that the insectary-reared T. pyri had an LC50 of 81 ppm versus 0.006 ppm for native T. pyri taken from a research orchard. The bioassays suggested that recommended orchard rates of cypermethrin would cause heavy mortality in native populations of T. pyri but only moderate losses in the imported New Zealand strain. Bioassays in 1996 with the organophosphate insecticide dimethoate indicated both New Zealand and native T. pyri were susceptible and that recommended orchard rates of dimethoate likely would cause high mortality of T. pyri in apple orchards. These findings from bioassays were supported by data from orchard trials. In June and July 1993, insectary-reared New Zealand T. pyri were placed on five apple trees in each of eight 38-tree plots in the research orchard. In late August 1994, New Zealand T. pyri from orchard trees that had been sprayed twice by airblast sprayer with the full recommended rate of 50 g (AI)/ha (83 ppm) cypermethrin were placed on the other 33 trees in each of six plots. In the summers of 1994-1996, plots were treated with one of the following insecticide regimes: (1) conventional integrated pest management (IPM) (registered neurotoxic insecticides considered harmless or slightly toxic to T. pyri); (2) advanced IPM (use of newer, more selective insecticides); (3) pyrethroid (at least one full-rate application of cypermethrin); (4) dimethoate; and (5) dimethoate plus pyrethroid. Densities of European red mite, Panonychus ulmi (Koch), were highest in all plots treated with dimethoate and in pyrethroid plots not yet inoculated with New Zealand T. pyri. Densities of apple rust mite, Aculus schlechtendali (Nalepa), and of the stigmaeid predator Zetzellia mali (Ewing) were highest in plots treated with dimethoate and were nearly absent in the IPM plots. Densities of T. pyri were high enough for effective biocontrol in the IPM plots and in the pyrethroid plots 1-2 yr after release of the New Zealand strain, provided pyrethroid was applied just before the resistant strain was released in the orchard. A recurring theme of this study was the generally negative association between densities of phytophagous mites and those of T. pyri, suggesting the ability of this predator to suppress their prey. In contrast, the positive association between phytophagous mites and Z. mali suggests the inability of this predator to regulate their prey at least under the conditions of this study.     

Comparison of radioactive and stable isotope tracer techniques for measuring photosynthesis: 13C and 14C uptake by marine phytoplankton

The stable carbon isotope ¹³C has been used in the open oceanto estimate the inorganic carbon uptake by phytoplankton andthis technique has been compared with the ¹³C tracer method.An overall correlation coefficient of 0.806 and a regressionslope of 1.29 were calculated from 50 sample pairs gatheredduring three cruises in widely different oceanic areas rangingin production rates from 0.01 to 6 mgC m^–3 h^–1.However, significant differences between the two methods wereapparent for cruises located in nutrient-depleted areas. Possibleexplanations lie either in a volume effect, the high silicatecontent of the ¹⁴C solution which could stimulate the ¹⁴C uptakeor in errors associated with the particulate carbon measurementswhich are necessary to convert specific uptake rates to absoluteuptake rates and to yield compatible units for the comparison,in laboratory cultures the ¹⁴C technique overestimated the netparticulate carbon increase by — 16%. ⁺Present address: Laboratoire marin CNRS-IFREMER, L'Houmeau,17137 Nieul-sur-Mer, France.^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.