Photoinhibition in Chlamydomonas reinhardtii: Effect on state transition, intersystem energy distribution and Photosystem I cyclic electron flow

Photosynthetic adaptation strategy of Ulva prolifera floating on the sea surface to environmental changes

For 8 consecutive years, a green tide has originated in the southern Yellow Sea and spread to the Qingdao offshore area. The causative species, Ulva prolifera, always forms a very thick thallus mat that is capable of drifting long distances over long periods. During this process, although the thalli face disturbance by complex environmental factors, they maintain high biomass and proliferation. We hypothesized that some form of photosynthetic adaptation strategy must exist to protect the thalli. Therefore, we studied the different photosynthetic response characteristics of the surface and lower layers of the floating thallus mats, and investigated the physiological and molecular-level adaptation mechanisms. The results showed that: (1) U. prolifera has strong photosynthetic capability that ensures it can gain sufficient energy to increase its biomass and adapt to long-distance migration. (2) Surface layer thalli adapt to the complex environment by dissipating excess energy via photosynthetic quantum control (energy quenching and energy redistribution between PSII/PSI) to avoid irreversible damage to the photosynthetic system. (3) Lower layer thalli increase their contents of Chlorophyll a (Chl a) and Chlorophyll b (Chl b) and decrease their Chl a/Chl b ratio to improve their ability to use light energy. (4) U. prolifera has strong photosynthetic plasticity and can adapt to frequent exchange between the surface and lower layer environments because of wave disturbance. Pigment component changes, energy quenching, and energy redistribution between PSII/PSI contribute to this photosynthetic plasticity.

Purification and photobiochemical profile of photosystem 1 from a high-salt tolerant, oleaginous Chlorella (Trebouxiophycaea, Chlorophyta)

The eukaryotic green alga Chlamydomonas reinhardtii has been studied extensively within the biofuel industry as a model organism, as researchers look towards algae to provide chemical feedstocks (i.e., lipids) for the production of liquid transportation fuels. C. reinhardtii, however, is unsuitable for high-level production of such precursors due to its relatively poor lipid accumulation and fresh-water demand. In this study we offer insight into the primary light harvesting and electron transfer reactions that occur during phototropic growth in a high-salt tolerant strain of Chlorella (a novel strain introduced here as NE1401), a single-celled eukaryotic algae also in the phylum Chlorophyta. Under nutrient starvation many eukaryotic algae increase dramatically the amount of lipids stored in lipid bodies within their cell interiors. Microscopy and lipid analyses indicate that Chlorella sp. NE1401 may become a superior candidate for algal biofuels production. We have purified highly active Photosystem 1 (PS1) complexes to study in vitro, so that we may understand further the photobiochemisty of this promising biofuel producer and how its characteristics compare and contrast with that of the better understood C. reinhardtii. Our findings suggest that the PS1 complex from Chlorella sp. NE1401 demonstrates similar characteristics to that of C. reinhardtii with respect to light-harvesting and electron transfer reactions. We also illustrate that the relative extent of the light state transition performed by Chlorella sp. NE1401 is smaller compared to C. reinhardtii, although they are triggered by the same dynamic light stresses.

Populations of photoinactivated photosystem II reaction centers characterized by chlorophyll a fluorescence lifetime in vivo

Photosystem (PS) II centers, which split water into oxygen, protons, and electrons during photosynthesis, require light but are paradoxically inactivated by it. Prolonged light exposure concomitantly decreased both the functional fraction of PSII reaction centers and the integral PSII chlorophyll (Chl) a fluorescence lifetime in leaf segments of Capsicum annuum L. Acceleration of photoinactivation of PSII by a pretreatment with the inhibitors/uncoupler lincomycin, DTT, or nigericin further reduced PSII Chl a fluorescence lifetimes. A global analysis of fluorescence lifetime distributions revealed the presence of at least two distinct populations of photoinactivated PSII centers, one at 1.25 ns, and the other at 0.58 ns. Light treatment first increased the 1.25-ns component, a weak quencher, at the expense of a component at 2.22 ns corresponding to functional PSII centers. The 0.58-ns component, a strong quencher, emerged later than the 1.25-ns component. The strongly quenching PSII reaction centers could serve to avoid further damage to themselves and protect their functional neighbors by acting as strong energy sinks.

Short-term responses of Photosystem I to heat stress : Induction of a PS II-independent electron transport through PS I fed by stromal components

When 23°C-grown potato leaves (Solanum tuberosum L.) were exposed for 15 min to elevated temperatures in weak light, a dramatic and preferential inactivation of Photosystem (PS) II was observed at temperatures higher than about 38°C. In vivo photoacoustic measurements indicated that, concomitantly with the loss of PS II activity, heat stress induced a marked gas-uptake activity both in far-red light (>715 nm) exciting only PS I and in broadband light (350-600 nm) exciting PS I and PS II. In view of its suppression by nitrogen gas and oxygen and its stimulation by high carbon-dioxide concentrations, the bulk of the photoacoustically measured gas uptake by heat-stressed leaves was ascribed to rapid carbon-dioxide solubilization in response to light-modulated stroma alkalization coupled to PS I-driven electron transport. Heat-induced gas uptake was observed to be insensitive to the PS II inhibitor diuron, sensitive to the plastocyanin inhibitor HgCl2 and saturated at a rather high photon flux density of around 1200 μE m(-2) s(-1). Upon transition from far-red light to darkness, the oxidized reaction center P700(+) of PS I was re-reduced very slowly in control leaves (with a half time t1/2 higher than 500 ms), as measured by leaf absorbance changes at around 820 nm. Heat stress caused a spectacular acceleration of the postillumination P700(+) reduction, with t1/2 falling to a value lower than 50 ms (after leaf exposure to 48°C). The decreased t1/2 was sensitive to HgCl2 and insensitive to diuron, methyl viologen (an electron acceptor of PS I competing with the endogenous acceptor ferredoxin) and anaerobiosis. This acceleration of the P700(+) reduction was very rapidly induced by heat treatment (within less than 5 min) and persisted even after prolonged irradiation of the leaves with far-red light. After heat stress, the plastoquinone pool exhibited reduction in darkness as indicated by the increase in the apparent Fo level of chlorophyll fluorescence which could be quenched by far-red light. Application (for 1 min) of far-red light to heat-pretreated leaves also induced a reversible quenching of the maximal fluorescence level Fm, suggesting formation of a pH gradient in far-red light. Taken together, the presented data indicate that PS I responded to the heat-induced loss of PS II photochemical activity by catalyzing an electron flow from stromal reductants. Heat-stress-induced PS I electron transport independent of PS II seems to constitute a protective mechanism since block of this electron pathway in anaerobiosis was observed to result in a dramatic photoinactivation of PS I.

Light adaptation of cyclic electron transport through Photosystem I in the cyanobacterium Synechococcus sp. PCC 7942

Photosystem I-driven cyclic electron transport was measured in intact cells of Synechococcus sp PCC 7942 grown under different light intensities using photoacoustic and spectroscopic methods. The light-saturated capacity for PS I cyclic electron transport increased relative to chlorophyll concentration, PS I concentration, and linear electron transport capacity as growth light intensity was raised. In cells grown under moderate to high light intensity, PS I cyclic electron transport was nearly insensitive to methyl viologen, indicating that the cyclic electron supply to PS I derived almost exclusively from a thylakoid dehydrogenase. In cells grown under low light intensity, PS I cyclic electron transport was partially inhibited by methyl viologen, indicating that part of the cyclic electron supply to PS I derived directly from ferredoxin. It is proposed that the increased PSI cyclic electron transport observed in cells grown under high light intensity is a response to chronic photoinhibition.

Variable thermal dissipation in a Photosystem I submembrane fraction

Photoacoustic spectroscopy was used to study the thermal deactivation processes in a Photosystem I submembrane fraction isolated from spinach. A large part of the thermal dissipation was variable. The yield of this variable thermal emission depended on the redox state of the Photosystem. It increased with the measuring modulated light intensity coinciding with the gradual closure of the reaction centers. Thermal deactivation was maximal when the reaction centers were closed by a saturating illumination. Extrapolation of the data at zero light intensity indicated that the yield of non-variable thermal emission represented about 37% of the maximal emission. The presence of methylviologen as artificial electron acceptor decreased the yield of variable thermal emission whereas inhibition following heat stress treatments increased it. The significance of the variable and non-variable components of thermal dissipation is discussed and the measured energy storage is suggested to originate from the reduction of the plastoquinone pool during cyclic electron transport around Photosystem I.

Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O2 in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b 6-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b 6-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and 'poises' the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO2 can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic photophosphorylation will depend on experiments made on living cells and measurements of cyclic photophosphorylation in vivo.

Thylakoid membrane energization and swelling in photoinhibited Chlamydomonas cells is prevented in mutants unable to perform cyclic electron flow

Photoinhibition of Photosystem II in unicellular algae in vivo is accompanied by thylakoid membrane energization and generation of a relatively high ΔpH as demonstrated by (14)C-methylamine uptake in intact cells. Presence of ammonium ions in the medium causes extensive swelling of the thylakoid membranes in photoinhibited Chlamydomonas reinhardtii but not in Scenedesmus obliquus wild type and LF-1 mutant cells. The rise in ΔpH and the related thylakoid swelling do not occur at light intensities which do not induce photoinhibition. The rise in ΔpH and membrane energization are not induced by photoinhibitory light in C. reinhardtii mutant cells possessing an active Photosystem II but lacking cytochrome b6/f, plastocyanin or Photosystem I activity and thus being unable to perform cyclic electron flow around Photosystem I. In these mutants the light-induced turnover of the D1 protein of Reaction Center II is considerably reduced. The high light-dependent rise in ΔpH is induced in the LF-1 mutant of Scenedesmus which can not oxidize water but otherwise possesses an active Reaction Center II indicating that PS II-linear electron flow activity and reduction of plastoquinone are not required for this process. Based on these results we conclude that photoinhibition of Photosystem II activates cyclic electron flow around Photosystem I which is responsible for the high membrane energization and ΔpH rise in cells exposed to excessive light intensities.

Correlation between chlorophyll fluorescence and photoacoustic signal transients in spinach leaves

Chlorophyll fluorescence and photoacoustic transients from dark adapted spinach leaves were measured and analyzed using the saturating pulse technique. Except for the first 30 s of photosynthetic induction, a good correlation was found between photoacoustically detected oxygen evolution at 35 Hz modulation frequency and electron flow calculated from the fluorescence quenching coefficients qP and qN. The induction kinetics of the photothermal signal, i.e., the photoacoustic signal at 370 Hz, reveal a fast (t r <10 ms) and a slow (t r ≈1 s) rise component. The fast component is suggested to be composed of the minimal thermal losses in photosynthesis and thermal losses from non-photosynthetic processes. The slow phase is attributed to variable thermal losses in photosynthesis. The variable thermal losses were normalized by measuring the minimal photothermal signal (H0) in the dark-adapted state and the maximal photothermal signal (Hm) during a saturating light pulse. The kinetics of the normalized photochemical loss (H-H0)/(Hm-H0) obtained from high-frequency PA measurements were found to correlate with the kinetics of oxygen evolution measured at low frequency.