Improvement of photosynthesis in dense microalgal suspension by reduction of light harvesting pigments

The effects of light-harvesting pigments (LHP) inmicroalgal cells on photosynthetic activity in adense cell suspension were examined. The results suggest that a lower LHP content should result in higher photosynthetic productivity under high light intensity. The idea was first proposed by Lien and San Pietro in 1975 that photosynthesis could be improved by reducing the LHP content in microalgal cells, but this has not been demonstrated in detail. Experiments to evaluate the idea were conducted with Synechocystis PCC6714 and Chlorellapyrenoidosa. In the experiments with PCC 6714, photosynthesis of a phycocyanin-deficient mutant was compared with that of the wild type. In the experiments with C. pyrenoidosa, the LHP content was controlled by the light intensity in the algalculture. The maximum photosynthetic activity was 20–30% higher in the dense suspension of cells having a lower LHP content with both organisms. These results indicate that the idea of reducing the LHP contentcould be applicable to a wide variety of photosynthetic organisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal acclimation and photoacclimation of photosynthesis in the brown alga Laminaria saccharina

Thermal acclimation and photoacclimation of photosynthesis were compared in Laminaria saccharina sporophytes grown at temperatures of 5 and 17 °C and irradiances of 15 and 150μmol photons m^?2 s^?1. When measured at a standard temperature (17°C), rates of light-saturated photosynthesis (P_max) were higher in 5 °C-grown algae (c. 3.0 μmol O₂ m^?2 s^?1) than in 17 °C-grown algae (c. 0.9 μmol O₂ m_-2 s_-1). Concentrations of Rubisco were also 3-fold higher (per unit protein) in 5 °C-grown algae than in algae grown at 17 °C. Light-limited photosynthesis responded similarly to high temperature and low light Photon yields (α) were higher in algae grown at high temperature (regardless of light), and at 5 °C in low light, than in algae grown at 5 °C in high light Differences in a were correlated with light absorption; both groups of 17 °C algae and 5 °C low-light algae absorbed c. 75% of incident light, whereas 5 °C high-light algae absorbed c. 55%. Increased absorption was correlated with increases in pigment content PSII reaction centre densities and the fucoxanthin-Chl ale protein complex (FCP). Changes in a were also attributed, in part, to changes in the maximum photon yield of photosynthesis (0_max). PSI reaction centre densities were unaffected by growth temperature, but the areal concentration of PSI in low-light-grown algae was twice that of high-light-grown algae (c. 160.0 versus 80.0 nmol m^?2). We suggest that complex metabolic regulation allows L, saccharina to optimize photosynthesis over the wide range of temperatures and light levels encountered in nature.     

The role of phytoplankton photosynthesis in global biogeochemical cycles

Phytoplankton biomass in the world's oceans amounts to only 1–2% of the total global plant carbon, yet these organisms fix between 30 and 50 billion metric tons of carbon annually, which is about 40% of the total. On geological time scales there is profound evidence of the importance of phytoplankton photosynthesis in biogeochemical cycles. It is generally assumed that present phytoplankton productivity is in a quasi steady-state (on the time scale of decades). However, in a global context, the stability of oceanic photosynthetic processes is dependent on the physical circulation of the upper ocean and is therefore strongly influenced by the atmosphere. The net flux of atmospheric radiation is critical to determining the depth of the upper mixed layer and the vertical fluxes of nutrients. These latter two parameters are keys to determining the intensity, and spatial and temporal distributions of phytoplankton blooms. Atmospheric radiation budgets are not in steady-state. Driven largely by anthropogenic activities in the 20th century, increased levels of IR- absorbing gases such as CO₂, CH₄ and CFC's and NO_x will potentially increase atmospheric temperatures on a global scale. The atmospheric radiation budget can affect phytoplankton photosynthesis directly and indirectly. Increased temperature differences between the continents and oceans have been implicated in higher wind stresses at the ocean margins. Increased wind speeds can lead to higher nutrient fluxes. Throughout most of the central oceans, nitrate concentrations are sub-micromolar and there is strong evidence that the quantum efficiency of Photosystem II is impaired by nutrient stress. Higher nutrient fluxes would lead to both an increase in phytoplankton biomass and higher biomass-specific rates of carbon fixation. However, in the center of the ocean gyres, increased radiative heating could reduce the vertical flux of nutrients to the euphotic zone, and hence lead to a reduction in phytoplankton carbon fixation. Increased desertification in terrestrial ecosystems can lead to increased aeolean loadings of essential micronutrients, such as iron. An increased flux of aeolean micronutrients could fertilize nutrient-replete areas of the open ocean with limiting trace elements, thereby stimulating photosynthetic rates. The factors which limit phytoplankton biomass and photosynthesis are discussed and examined with regard to potential changes in the Earth climate system which can lead the oceans away from steady-state. While it is difficult to confidently deduce changes in either phytoplankton biomass or photosynthetic rates on decadal time scales, time-series analysis of ocean transparency data suggest long-term trends have occurred in the North Pacific Ocean in the 20th century. However, calculations of net carbon uptake by the oceans resulting from phytoplankton photosynthesis suggest that without a supply of nutrients external to the ocean, carbon fixation in the open ocean is not presently a significant sink for excess atmospheric CO₂.The submitted paper has been authored under Contract No. DE-AC02-76H00016 with the US Department of Energy. Accordingly, the US Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes.     

Influence of tidal location on morphology, photosynthesis and pigments of the agarophyte, Gelidiella acerosa, from Northern Philippines

The red seaweed agarophyte, Gelidiella acerosa (Forssk?l) (Feldmann& Hamel) was collected from tidepools, high intertidal rocks. and shallow subtidal are as along a reef flat in Ilocos Norte, northern Philippines. The three populations were compared during the summer (dry) and rainy (wet) seasons to determine changes in morphology and photoacclimation capacity as possible use in mariculture. During summer months (February toApril) after exposure to environmental extremes (i.e. the highest percent of minus tides during daylight, high light regimes, desiccation, and solar bleaching), the populations differed in their morphologies and responses to increasing irradiance levels (P–I curve). Tidepool plants were the tallest, bushiest, and with increased diameter of cortical cells; while,high intertidal plants were the shortest, with sparse branching pattern and decreased diameter of cortical cells. Although their saturation irradiances indicated shade tolerance (Ik = 52 − 112 μmol photon m ^-2 s^-1). their differential light saturation curves (P-I curves) suggested a capacity to acclimate to ambient light regimes. For example, plants from the high intertidal zone showed higher photosynthetic rates and saturation irradiances, slightly lower initial slopes of the P-I curves and levels of light harvesting accessory pigments, rphycoerhythrin (R-PE) and rphycocyanin (R-PC), after being exposed to higher light regimes. In contrast, plants from tidepools and shallow subtidal areas had lower photosynthetic rates and saturation irradiances, slightly steeper initial slopes of the P-I curves and levels of R-PE and R-PC, having been exposed to lower light regimes. During the rainy months (June to November) no significant responses in these parameters were recorded. Comparison of the P-I responses of vegetative and tetrasporic plants showed these to vary with season. The data suggest that when plants became reproductive their physiological fitness either was unchanged or slightly enhanced. These results indicate that all three populations of G. acerosa could be used as seed stock for mariculture. This revised version was published online in August 2006 with corrections to the Cover Date.     

Can photosynthesis provide a `biological blueprint'' for the design of novel solar cells?

The primary photochemical reactions in purple-bacterial photosynthesis take place in discrete, membrane-bound pigment–protein complexes called reaction centres and light-harvesting complexes. The detailed information on their structure and function now available is being used to aid the design and construction of novel solar-energy converters.     

The golden age of biochemical research in photosynthesis

The perspectives and enthusiasms recorded in this review describe the events I witnessed and, in small ways, contributed to. Two great rewards emerged from my experiences – the pleasure of doing experiments and the great wealth of friendships with students and colleagues. As a graduate student, phenomena appeared at the bench before me which clarified the coupling of electron transport to ATP synthesis. My first PhD graduate student measured concentrations of pyridine nucleotides in chloroplasts and his results have been often confirmed and well used. All of the many graduate students who followed contributed to our understanding of photosynthesis. I have taken much pleasure from documenting the details of photosynthetic phosphorylation and electron transport in cyanobacteria. Studies of the `c' type cytochromes in these organisms continue to fascinate me. My experiences in government in its efforts to promote research are unusual, perhaps unique. A rare event outside the laboratory – a natural bloom of cyanobacteria – stimulated new thoughts and special opportunities for laboratory science. Photosynthesis seems magisterial in its shaping of our planet and its biology and in the details of its cleverness that were revealed in the time of my witness. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of salt stress on photosynthesis and chlorophyll fluorescence in Medicago truncatula

In the present study, photosynthetic parameters including gas exchanges, pigment contents, and chlorophyll fluorescence, were compared in two contrasting local Medicago truncatula lines TN6.18 and TN8.20, in response to salt added to the nutrient solution. Plants were cultivated under symbiotic nitrogen fixation (SNF) after inoculation with a reference strain Sinorhizobium meliloti 2011, a very tolerant strain to salinity (700 mM NaCl), and grown in a controlled glasshouse. On one month old plants (with active SNF), salt treatment (75 mM NaCl) was gradually applied. Photosynthesis, assimilating pigments and chlorophyll fluorescence were monitored throughout the experiment during both short and long terms, compared to control (non-saline) conditions. A genotypic variation in salt tolerance was found; TN6.18 was the more sensitive to salinity. The relative tolerance of TN8.20 was concomitant with the highest photochemical quenching coefficient (q_P) affecting the maximum quantum yield of PSII (Y); the real quantum yield (?_exc) was the most affected in the sensitive line. Moreover, stomatal and PSII reaction centers activities differed clearly between the studied lines. We found that the effect of salinity on photosynthesis of M. truncatula was related to PSII activity reduction rather than to stomatal conductance limitation. Photosynthesis was reduced by the inhibition of CO₂ assimilation caused by PSII damage. This was clearly estimated by the Y, ?_exc and especially by the quantum yield of electron transport of PSII (Φ_PSII). Thus, on the basis of our results on the two local M. truncatula lines, we recommend the use of chlorophyll fluorescence as non-destructive screening method to discriminate susceptible and resistant legumes to salt stress.     

Host preference and specialization in Gnathia sp., a common parasitic isopod of coral reef fishes

A gnathiid species (Crustacea: Isopoda; one of the most common ectoparasites of coral reef fishes) from the Great Barrier Reef, Australia, was allowed to choose among fishes from three different families to feed on (using two species of fishes per family). Gnathiids showed a strong preference for labrids, rarely feeding on pomacentrids or apogonids. In a separate experiment, gnathiid host preference did not vary among three labrid fish species. Gnathiids that fed on labrids had higher survival than those that fed on apogonids. Male gnathiids that fed on labrids also moulted to the adult stage more quickly. This suggests that host specialization and local adaptation might be occurring between these ectoparasites and their host fishes at the host fish family level.     

Photoinhibition of photosynthesis in vivo: The role of protein turnover in photosystem II

Divergent theories on the mechanism behind, and the nature of, photoinhibition are discussed, especially in relation to observations made in higher plant leaves. Comparisons are made with 'lower' plant groups and results of in vivo and in vitro experiments are considered. Irradiance-induced mechanisms involved in the regulation of PSII function and structure are discussed in connection with turnover of the DI protein. A model is presented in which a structural change in DI protein facilitates the formation of a population of dissipative PSII centres that do not participate in linear electron transport to PSI. We suggest a sophisticated regulatory mechanism whereby this variable PSII function is controlled with respect to both incident light and biochemical demand, a control which relies on feedback from both light and dark reactions.     

Is photoinhibition of zooxanthellae photosynthesis the primary cause of thermal bleaching in corals?

The bleaching of corals in response to increases in temperature has resulted in significant coral reef degradation in many tropical marine ecosystems. This bleaching has frequently been attributed to photoinhibition of photosynthetic electron transport and the consequent photodamage to photosystem II (PSII) and the production of damaging reactive oxygen species (ROS) in the zooxanthellae (Symbiodinium spp.). However, these events may be because of perturbations of other processes occurring within the zooxanthellae or the host cells, and consequently constitute only secondary responses to temperature increase. The processes involved with the onset of photoinhibition of electron transport, photodamage to PSII and pigment bleaching in coral zooxanthellae are reviewed. Consideration is given to how increases in temperature might lead to perturbations of metabolic processes in the zooxanthellae and/or their host cells, which could trigger events leading to bleaching. It is concluded that production of ROS by the thylakoid photosynthetic apparatus in the zooxanthellae plays a major role in the onset of bleaching resulting from photoinhibition of photosynthesis, although it is not clear which particular ROS are involved. It is suggested that hydrogen peroxide generated in the zooxanthellae may have a signalling role in triggering the mechanisms that result in expulsion of zooxanthellae from corals.     

Physiological nitrogen efficiency in rice: Nitrogen utilization,photosynthesis, and yield potential

Characteristics of rice (Oryza sativa) as a crop plant are briefly introduced, and the relationship between formation of yield potential and nitrogen (N) nutrition is described on the basis of studies using 15N as a tracer. In addition, the relationship between the leaf photosynthetic capacity and leaf N, and the factors limiting leaf photosynthesis under different growth conditions are reviewed. Finally, targets for improving rice yield potential are discussed with a focus on the role of increased photosynthesis efficiency in relation to leaf N status and the photosynthetic components in the leaves.     

An all-atom structure-based potential for proteins: bridging minimal models with all-atom empirical forcefields

Protein dynamics take place on many time and length scales. Coarse-grained structure-based (Go) models utilize the funneled energy landscape theory of protein folding to provide an understanding of both long time and long length scale dynamics. All-atom empirical forcefields with explicit solvent can elucidate our understanding of short time dynamics with high energetic and structural resolution. Thus, structure-based models with atomic details included can be used to bridge our understanding between these two approaches. We report on the robustness of folding mechanisms in one such all-atom model. Results for the B domain of Protein A, the SH3 domain of C-Src Kinase, and Chymotrypsin Inhibitor 2 are reported. The interplay between side chain packing and backbone folding is explored. We also compare this model to a C(alpha) structure-based model and an all-atom empirical forcefield. Key findings include: (1) backbone collapse is accompanied by partial side chain packing in a cooperative transition and residual side chain packing occurs gradually with decreasing temperature, (2) folding mechanisms are robust to variations of the energetic parameters, (3) protein folding free-energy barriers can be manipulated through parametric modifications, (4) the global folding mechanisms in a C(alpha) model and the all-atom model agree, although differences can be attributed to energetic heterogeneity in the all-atom model, and (5) proline residues have significant effects on folding mechanisms, independent of isomerization effects. Because this structure-based model has atomic resolution, this work lays the foundation for future studies to probe the contributions of specific energetic factors on protein folding and function.     

Refugia under threat: Mass bleaching of coral assemblages in high‐latitude eastern Australia

Environmental anomalies that trigger adverse physiological responses and mortality are occurring with increasing frequency due to climate change. At species' range peripheries, environmental anomalies are particularly concerning because species often exist at their environmental tolerance limits and may not be able to migrate to escape unfavourable conditions. Here, we investigated the bleaching response and mortality of 14 coral genera across high‐latitude eastern Australia during a global heat stress event in 2016. We evaluated whether the severity of assemblage‐scale and genus‐level bleaching responses was associated with cumulative heat stress and/or local environmental history, including long‐term mean temperatures during the hottest month of each year (SST_LTMAX), and annual fluctuations in water temperature (SST_VAR) and solar irradiance (PARZ_VAR). The most severely‐bleached genera included species that were either endemic to the region (Pocillopora aliciae) or rare in the tropics (e.g. Porites heronensis). Pocillopora spp., in particular, showed high rates of immediate mortality. Bleaching severity of Pocillopora was high where SST_LTMAX was low or PARZ_VAR was high, whereas bleaching severity of Porites was directly associated with cumulative heat stress. While many tropical Acropora species are extremely vulnerable to bleaching, the Acropora species common at high latitudes, such as A. glauca and A. solitaryensis, showed little incidence of bleaching and immediate mortality. Two other regionally‐abundant genera, Goniastrea and Turbinaria, were also largely unaffected by the thermal anomaly. The severity of assemblage‐scale bleaching responses was poorly explained by the environmental parameters we examined. Instead, the severity of assemblage‐scale bleaching was associated with local differences in species abundance and taxon‐specific bleaching responses. The marked taxonomic disparity in bleaching severity, coupled with high mortality of high‐latitude endemics, point to climate‐driven simplification of assemblage structures and progressive homogenisation of reef functions at these high‐latitude locations.     

Interacting effects of CO2 partial pressure and temperature on photosynthesis and calcification in a scleractinian coral

We show here that CO₂ partial pressure (pCO₂) and temperature significantly interact on coral physiology. The effects of increased pCO₂ and temperature on photosynthesis, respiration and calcification rates were investigated in the scleractinian coral Stylophora pistillata. Cuttings were exposed to temperatures of 25°C or 28°C and to pCO₂ values of ca. 460 or 760 μatm for 5 weeks. The contents of chlorophyll c₂ and protein remained constant throughout the experiment, while the chlorophyll a content was significantly affected by temperature, and was higher under the ‘high‐temperature–high‐pCO₂’ condition. The cell‐specific density was higher at ‘high pCO₂’ than at ‘normal pCO₂’ (1.7 vs. 1.4). The net photosynthesis normalized per unit protein was affected by both temperature and pCO₂, whereas respiration was not affected by the treatments. Calcification decreased by 50% when temperature and pCO₂ were both elevated. Calcification under normal temperature did not change in response to an increased pCO₂. This is not in agreement with numerous published papers that describe a negative relationship between marine calcification and CO₂. The confounding effect of temperature has the potential to explain a large portion of the variability of the relationship between calcification and pCO₂ reported in the literature, and warrants a re‐evaluation of the projected decrease of marine calcification by the year 2100.