Summary• As iron (Fe) deficiency is a main limiting factor of ocean productivity, its effects were investigated on interactions between photosynthesis and nitrogen fixation in the marine nonheterocystous diazotrophic cyanobacterium Trichodesmium IMS101.• Biophysical methods such as fluorescence kinetic microscopy, fast repetition rate (FRR) fluorimetry, and in vivo and in vitro spectroscopy of pigment composition were used, and nitrogenase activity and the abundance of key proteins were measured.• Fe limitation caused a fast down-regulation of nitrogenase activity and protein levels. By contrast, the abundance of Fe-requiring photosystem I (PSI) components remained constant. Total levels of phycobiliproteins remained unchanged according to single-cell in vivo spectra. However, the regular 16-kDa phycoerythrin band decreased and finally disappeared 16-20 d after initiation of Fe limitation, concomitant with the accumulation of a 20-kDa protein cross-reacting with the phycoerythrin antibody. Concurrently, nitrogenase expression and activity increased. Fe limitation dampened the daily cycle of photosystem II (PSII) activity characteristic of diazotrophic Trichodesmium cells. Further, it increased the number and prolonged the time period of occurrence of cells with elevated basic fluorescence (F 0 ). Additionally, it increased the effective cross-section of PSII, probably as a result of enhanced coupling of phycobilisomes to PSII, and led to up-regulation of the Fe stress protein IsiA.• Trichodesmium survives short-term Fe limitation by selectively down-regulating nitrogen fixation while maintaining but re-arranging the photosynthetic apparatus. Abbreviations:Chl, chlorophyll; FKM, fluorescence kinetic microscopy/microscope (for twodimensional (imaging) measurements of fluorescence kinetics); FRRf, fast repetition rate fluorimeter; F 0 , basic fluorescence yield of a dark-adapted sample, fluorescence in nonactinic measuring light; F m , maximum fluorescence yield of a dark-adapted sample; , maximum fluorescence yield of a sample during exposure to actinic light, i.e. diminished by nonphotochemical quenching; , maximum fluorescence yield of a fully light-adapted sample at the end of the actinic light period of the measurement, diminished by nonphotochemical quenching; , fluorescence yield under actinic irradiance immediately before the measurement of ; F v , variable fluorescence yield; F v = F m − F 0 , i.e. response to a supersaturating flash in the dark-adapted state of photosystem II (PSII); F v /F m , maximal efficiency of dark-adapted PSII (In this study,

The extended superfamily of chlorophyll a/b binding proteins comprises the Light-Harvesting Complex Proteins (LHCs), the Early Light-Induced Proteins (ELIPs) and the Photosystem II Subunit S (PSBS). The proteins of the ELIP family were proposed to function in photoprotection or assembly of thylakoid pigment-protein complexes and are further divided into subgroups with one to three transmembrane helices. Two small One-Helix Proteins (OHPs) are expressed constitutively in green plant tissues and their levels increase in response to light stress. In this study, we show that OHP1 and OHP2 are highly conserved in photosynthetic eukaryotes, but have probably evolved independently and have distinct functions in Arabidopsis. Mutations in OHP1 or OHP2 caused severe growth deficits, reduced pigmentation and disturbed thylakoid architecture. Surprisingly, the expression of OHP2 was severely reduced in ohp1 T-DNA insertion mutants and vice versa. In both ohp1 and ohp2 mutants, the levels of numerous photosystem components were strongly reduced and photosynthetic electron transport was almost undetectable. Accordingly, ohp1 and ohp2 mutants were dependent on external organic carbon sources for growth and did not produce seeds. Interestingly, the induction of ELIP1 expression and Cu/Zn superoxide dismutase activity in low light conditions indicated that ohp1 mutants constantly suffer from photo-oxidative stress. Based on these data, we propose that OHP1 and OHP2 play an essential role in the assembly or stabilization of photosynthetic pigment-protein complexes, especially photosystem reaction centers, in the thylakoid membrane.

Oxidative and nitrosative stress leaves footprints in the plant chloroplast in the form of oxidatively modified proteins. Using a mass spectrometric approach, we identified 126 tyrosine and 12 tryptophan nitration sites in 164 nitrated proteolytic peptides, mainly from photosystem I (PSI), photosystem II (PSII), cytochrome b(6) /f and ATP-synthase complexes and 140 oxidation products of tyrosine, tryptophan, proline, phenylalanine and histidine residues. While a high number of nitration sites were found in proteins from four photosynthetic complexes indicating that the nitration belongs to one of the prominent posttranslational protein modifications in photosynthetic apparatus, amino acid oxidation products were determined mostly in PSII and to a lower extent in PSI. Exposure of plants to light stress resulted in an increased level of tyrosine and tryptophan nitration and tryptophan oxidation in proteins of PSII reaction center and the oxygen-evolving complex, as compared to low light conditions. In contrast, the level of nitration and oxidation of these amino acid residues strongly decreased for all light-harvesting proteins of PSII under the same conditions. Based on these data, we propose that oxidative modifications of proteins by reactive oxygen and nitrogen species might represent an important regulatory mechanism of protein turnover under light stress conditions, especially for PSII and its antenna proteins.

Using a label-free mass spectrometric approach, we investigated light-induced changes in the distribution of phosphorylated and nitrated proteins within subpopulations of native photosynthetic complexes in the thylakoid membrane of Arabidopsis thaliana leaves adapted to growth light (GL) and subsequently exposed to high light (HL). Eight protein phosphorylation sites were identified in photosystem II (PSII) and the phosphorylation level of seven was regulated by HL as determined based on peak areas from ion chromatograms of phosphorylated and non-phosphorylated peptides. Although the phosphorylation of PSII proteins was reported in the past, we demonstrated for the first time that two minor antenna LHCB4 isoforms are alternately phosphorylated under GL and HL conditions in PSII monomers, dimers and supercomplexes. A role of LHCB4 phosphorylation in state transition and monomerization of PSII under HL conditions is proposed. We determined changes in the nitration level of 23 tyrosine residues in five photosystem I (PSI) and nine PSII proteins and demonstrated for the majority of them a lower nitration level in PSI and PSII complexes and supercomplexes under HL conditions, as compared to GL. In contrast, the nitration level significantly increased in assembled/disassembled PSI and PSII subcomplexes under HL conditions. A possible role of nitration in (1) monomerization of LHCB1-3 trimers under HL conditions (2) binding properties of ferredoxin-NADP+ oxidoreductase to photosystem I, and (3) PSII photodamage and repair cycle, is discussed. Based on these data, we propose that the conversely regulated phosphorylation and nitration levels regulate the stability and turnover of photosynthetic complexes under HL conditions.

Summary• As the nonheterocystous diazotrophic cyanobacterium Trichodesmium lives both at the ocean surface and deep in the water column, it has to acclimate to vastly different irradiances. Here, we investigate its strategy of light acclimation in several ways.• In this study, we used spectrally resolved fluorescence kinetic microscopy to investigate the biophysics of photosynthesis in individual cells, analysed cell extracts for pigment and phycobiliprotein composition, measured nitrogenase activity and the abundance of key proteins, and assayed protein synthesis ⁄ degradation by radioactive labelling.• After acclimation to high light, Trichodesmium grew faster at 1000 lmol m )2 s )1 than at 100 lmol m )2 s )1 . This acclimation was associated with decreasing cell diameter, faster protein turnover, the down-regulation of light-harvesting pigments and the outer part of the phycobiliprotein antenna, the up-regulation of light-protective carotenoids, changes in the coupling of phycobilisomes to the reaction centres and in the coupling of individual phycobiliproteins to the phycobilisomes. The latter was particularly interesting, as it represents an as yet unreported light acclimation strategy.• Only in the low light-acclimated culture and only after the onset of actinic light did phycourobilin and phycoerythrin contribute to photochemical fluorescence quenching, showing that these phycobiliproteins may become quickly (in seconds) very closely coupled to photosystem II. This fast reversible coupling also became visible in the nonphotochemical changes of the fluorescence quantum yield.