TY - JOUR A1 - Clegg, Mark R. A1 - Gaedke, Ursula A1 - Böhrer, Bertram A1 - Spijkerman, Elly T1 - Complementary ecophysiological strategies combine to facilitate survival in the hostile conditions of a deep chlorophyll maximum JF - Oecologia N2 - In the deep, cooler layers of clear, nutrient-poor, stratified water bodies, phytoplankton often accumulate to form a thin band or "deep chlorophyll maximum" (DCM) of ecological importance. Under such conditions, these photosynthetic microorganisms may be close to their physiological compensation points and to the boundaries of their ecological tolerance. To grow and survive any resulting energy limitation, DCM species are thought to exhibit highly specialised or flexible acclimation strategies. In this study, we investigated several of the adaptable ecophysiological strategies potentially employed by one such species, Chlamydomonas acidophila: a motile, unicellular, phytoplanktonic flagellate that often dominates the DCM in stratified, acidic lakes. Physiological and behavioural responses were measured in laboratory experiments and were subsequently related to field observations. Results showed moderate light compensation points for photosynthesis and growth at 22A degrees C, relatively low maintenance costs, a behavioural preference for low to moderate light, and a decreased compensation point for photosynthesis at 8A degrees C. Even though this flagellated alga exhibited a physiologically mediated diel vertical migration in the field, migrating upwards slightly during the day, the ambient light reaching the DCM was below compensation points, and so calculations of daily net photosynthetic gain showed that survival by purely autotrophic means was not possible. Results suggested that strategies such as low-light acclimation, small-scale directed movements towards light, a capacity for mixotrophic growth, acclimation to low temperature, in situ exposure to low O-2, high CO2 and high P concentrations, and an avoidance of predation, could combine to help overcome this energetic dilemma and explain the occurrence of the DCM. Therefore, corroborating the deceptive ecophysiological complexity of this and similar organisms, only a suite of complementary strategies can facilitate the survival of C. acidophila in this DCM. KW - DCM KW - Photosynthesis KW - Growth KW - Behaviour KW - Phytoplankton Y1 - 2012 U6 - https://doi.org/10.1007/s00442-011-2225-4 SN - 0029-8549 VL - 169 IS - 3 SP - 609 EP - 622 PB - Springer CY - New York ER - TY - JOUR A1 - Götze, Jan Philipp A1 - Saalfrank, Peter T1 - Quantum chemical modeling of the kinetic isotope effect of the carboxylation step in RuBisCO JF - Journal of molecular modeling N2 - Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the most important enzyme for the assimilation of carbon into biomass, features a well-known isotope effect with regards to the CO2 carbon atom. This kinetic isotope effect alpha = k (12)/k (13) for the carboxylation step of the RuBisCO reaction sequence, and its microscopic origin, was investigated with the help of cluster models and quantum chemical methods [B3LYP/6-31G(d,p)]. We use a recently proposed model for the RuBisCO active site, in which a water molecule remains close to the reaction center during carboxylation of ribulose-1,5-bisphosphate [B. Kannappan, J.E. Gready, J. Am. Chem. Soc. 130 (2008), 15063]. Alternative active-site models and/or computational approaches were also tested. An isotope effect alpha for carboxylation is found, which is reasonably close to the one measured for the overall reaction, and which originates from a simple frequency shift of the bending vibration of (CO2)-C-12 compared to (CO2)-C-13. The latter is the dominant mode for the product formation at the transition state. KW - Cluster model KW - Dark reactions KW - Densityfunctional theory KW - Isotope effect KW - Photosynthesis KW - Quantum chemistry KW - RuBisCO Y1 - 2012 U6 - https://doi.org/10.1007/s00894-011-1207-0 SN - 1610-2940 VL - 18 IS - 5 SP - 1877 EP - 1883 PB - Springer CY - New York ER - TY - JOUR A1 - Kröner, Dominik A1 - Götze, Jan Philipp T1 - Modeling of a violaxanthin-chlorophyll b chromophore pair in its LHCII environment using CAM-B3LYP JF - Journal of photochemistry and photobiology : B, Biology N2 - Collecting energy for photosystem II is facilitated by several pigments, xanthophylls and chlorophylls, embedded in the light harvesting complex II (LHCII). One xanthophyll, violaxanthin (Vio), is loosely bound at a site close to a chlorophyll b (Chl). No final answer has yet been found for the role of this specific xanthophyll. We study the electronic structure of Vio in the presence of Chl and under the influence of the LHCII environment, represented by a point charge field (PCF). We compare the capability of the long range corrected density functional theory (DFT) functional CAM-B3LYP to B3LYP for the modeling of the UV/vis spectrum of the Vio + Chl pair. CAM-B3LYP was reported to allow for a very realistic reproduction of bond length alternation of linear polyenes, which has considerable impact on the carotenoid structure and spectrum. To account for the influence of the LHCII environment, the chromophore geometries are optimized using an ONIOM(DFT/6-31G(d):PM6) scheme. Our calculations show that the energies of the locally excited states are almost unaffected by the presence of the partner chromophore or the PCF. There are, however, indications for excitonic coupling of the Chl Soret band and Vio. We propose that Vio may accept energy from blue-light excited Chl. KW - CAM-B3LYP KW - Density functional theory KW - Light-harvesting complex II KW - Photosynthesis KW - Violaxanthin KW - Chlorophyll b Y1 - 2012 U6 - https://doi.org/10.1016/j.jphotobiol.2011.12.007 SN - 1011-1344 VL - 109 IS - 2 SP - 12 EP - 19 PB - Elsevier CY - Lausanne ER - TY - JOUR A1 - Sakuraba, Yasuhito A1 - Balazadeh, Salma A1 - Tanaka, Ryouichi A1 - Müller-Röber, Bernd A1 - Tanaka, Ayumi T1 - Overproduction of Chl b retards senescence through transcriptional reprogramming in arabidopsis JF - Plant & cell physiology N2 - Leaf senescence is a developmentally and environmentally regulated process which includes global changes in gene expression. Using Arabidopsis as a model, we modified Chl arrangement in photosystems by overexpressing the catalytic domain (the C domain) of chlorophyllide a oxygenase (CAO) fused with the linker domain (the B domain) of CAO and green fluorescent protein (GFP). In these plants (referred to as the BCG plants for the B and C domains of CAO and GFP), the Chl a/b ratio was drastically decreased and Chl b was incorporated into core antenna complexes. The BCG plants exhibited a significant delay of both developmental and dark-induced leaf senescence. The photosynthetic apparatus, CO2 fixation enzymes and the chloroplast structure were lost in wild-type plants during senescence, while BCG plants retained them longer than the wild type. Large-scale quantitative real-time PCR analyses of 1,880 transcription factor (TF) genes showed that 241 TFs are differentially expressed between BCG plants and wild-type plants at senescence, similar to 40% of which are known senescence-associated genes (SAGs). Expression profiling also revealed the down-regulation of a large number of additional non-TF SAGs. In contrast, genes involved in photosynthesis were up-regulated, while those encoding Chl degradation enzymes were down-regulated in BCG plants. These results demonstrate that alteration of pigment composition in the photosynthetic apparatus retards senescence through transcriptional reprogramming. KW - Arabidopsis KW - Chloroplast KW - Chlorophyllide a oxygenase KW - Photosynthesis KW - Senescence Y1 - 2012 U6 - https://doi.org/10.1093/pcp/pcs006 SN - 0032-0781 VL - 53 IS - 3 SP - 505 EP - 517 PB - Oxford Univ. Press CY - Oxford ER -