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Aureococcus anophagefferens is a picoplanktonic microalga that is very well adapted to growth at low nutrient and low light levels, causing devastating blooms ("brown tides") in estuarine waters. To study the factors involved in long-term acclimation to different light intensities, cells were acclimated for a number of generations to growth under low light (20μmolphotonsm(-2)s(-1)), medium light (60 or 90μmolphotonsm(-2)s(-1)) and high light (200μmolphotonsm(-2)s(-1)), and were analyzed for their contents of xanthophyll cycle carotenoids (the D pool), fucoxanthin and its derivatives (the F pool), Chls c(2) and c(3), and fucoxanthin Chl a/c polypeptides (FCPs). Higher growth light intensities resulted in increased steady state levels of both diadinoxanthin and diatoxanthin. However, it also resulted in the conversion of a significant fraction of fucoxanthin to 19'-butanoyloxyfucoxanthin without a change in the total F pool. The increase in 19'-butanoyloxyfucoxanthin was paralleled by a decrease in the effective antenna size, determined from the slope of the change in F(0) as a function of increasing light intensity. Transfer of acclimated cultures to a higher light intensity showed that the conversion of fucoxanthin to its derivative was a relatively slow process (time-frame of hours). We suggest the replacement of fucoxanthin with the bulkier 19'-butanoyloxyfucoxanthin results in a decrease in the light-harvesting efficiency of the FCP antenna and is part of the long-term acclimative response to growth at higher light intensities., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Cryptophytes like the cryptomonad Guillardia theta are part of the marine phytoplankton and therefore major players in global carbon and biogeochemical cycles. Despite the importance for the cell in being able to cope with large changes in illumination on a daily basis, very little is known about photoprotection mechanisms in cryptophytes. Here, we show that Guillardia theta is able to perform non-photochemical quenching, although none of the usual xanthophyll cycle pigments (e.g., zeaxanthin, diadinoxanthin, diatoxanthin) are present at detectable levels. Instead, acclimation to high light intensity seems to involve an increase of alloxanthin. Guillardia theta has genes for 2 one-helix "light-harvesting-like" proteins, related to some cyanobacterial genes which are induced in response to high light stress. Both the plastid-encoded gene (hlipP) and the nucleomorph-encoded gene (HlipNm) are expressed, but transcript levels decrease rather than increase during high light exposure, suggesting that they are not involved in a high light stress response. The HlipNm protein was detected with a specific antibody; expression was constant, independent of the light exposure., (Copyright © 2011 Elsevier B.V. All rights reserved.)

As an important component of marine phytoplankton, diatoms must be able to cope with large changes in illumination on a daily basis. They have an active xanthophyll cycle and non-photochemical quenching (NPQ), but no homolog has been detected for the gene encoding the PsbS protein required for NPQ in plants. However, diatoms do have a branch of the light-harvesting complex superfamily, the Lhcx clade, which is most closely related to the LI818 (LhcSR) genes of the green alga Chlamydomonas, known to be upregulated in response to a variety of stresses. When cultures of the diatom T. pseudonana grown under low light (40 micromol photons m(-2) s(-1)) were exposed to high light stress (HL, 700 micromol photons m(-2) s(-1)), transcripts of three of these genes (Lhcx1, Lhcx4, Lhcx6) were transiently accumulated. The amount of Lhcx6 protein was low under low light, but increased continuously during 10h of HL exposure, then slowly dropped to background levels in the dark. However, HL had little effect on the Lhcx1 protein, which was present under low light and only doubled after HL exposure. Diatoxanthin levels increased throughout the HL period with no change in diadinoxanthin. The fraction of NPQ attributable to photoinhibitory quenching (qI) also increased throughout the HL exposure. Taken together, the Lhcx6 protein could be associated with diatoxanthin binding and play a direct role in excess energy dissipation via sustained quenching during acclimation to prolonged HL stress, while the Lhcx1 protein may play a more structural role in thylakoid membrane organization under all conditions., (Copyright (c) 2010 Elsevier B.V. All rights reserved.)

We have previously reported that large unilamellar liposomes made from the neutral galactolipid digalactosyldiacylglycerol (DGDG) will aggregate in the presence of monovalent or divalent cations, behavior that would not have been predicted for an uncharged lipid (Webb et al. (1988) Biochim. Biophys. Acta 938, 323-333). In this paper, the effects of including the other major thylakoid lipids on the Mg2+ concentration required for aggregation of DGDG vesicles has been examined. Addition of the neutral, hexagonal-II phase preferring lipid monogalactosyldiacylglycerol (MGDG) to DGDG up to 50 mol% had no effect, suggesting that the MGDG head group is as effective in causing aggregation as the DGDG head group. Addition of 0.5 to 5.0 mol% of either of the two anionic lipids phosphatidylglycerol (PG) or sulfoquinovosyldiacylglycerol (SQDG) inhibited the aggregation of DGDG vesicles, probably by the development of an electrostatic potential. Phosphatidylcholine (PC) in amounts up to 25 mol% did not inhibit or promote aggregation. Vesicles with a composition similar to that of thylakoids (DGDG/MGDG/SQDG/PG, 1:2:0.5:0.5) required 65 mM MgCl2 in the presence of 200 mM KCl, i.e., higher concentrations than are present in the chloroplast stroma. If MGDG made up more than 25 mol% of any combination of lipids, vesicle aggregation could not be reversed by dilution. These results are consistent with cations playing a role in mediating the close approach of bilayers via an effect on head-group hydration and head-group interaction between bilayers.

Acetabularia chloroplasts can incorporate radioactive amino acids for up to several hours in vitro. The incorporation is sensitive to chloramphenicol and lincomycin, insensitive to cycloheximide, and completely light-dependent. At least 35 discrete labelled bands can be separated by SDS-polyacrylamide gel electrophoresis: 20--24 in the soluble fraction and 13--15 in the membrane fraction. Most of the label (80--85%) is in the membrane fraction, and 90% of that is in a polypeptide of 32 000 daltons. Chlorophyll-protein complexes were purified from in vitro labelled chloroplasts by SDS-polyacrylamide gel electrophoresis. CP I (P-700-chlorophyll alpha-protein complex) and its apoprotein were both labelled. This shows that the apoprotein is synthesized on chloroplast ribosomes, and can be integrated correctly into the thylakoid membrane in the absence of any cytoplasmic contribution. In contrast, no label was incorporated into the two polypeptides of CP II, the light-harvesting chlorophyll a/b complex.

The kinetic complexity of Acetabularia cliftonii chloroplast DNA is 1.52 +/- 0.26 . 10(9) daltons, compared to 0.2 .10(9) daltons for Chlamydomonas chloroplast DNA. There is an average of three genomes per chloroplast. The unusually large size of the Acetabularia genome may reflect the ancient evolutionary history of this organism.

When Acetabularia cliftonii chloroplast DNA (p = 1.706 g/cm3) is centrifuged in an ethidium bromide-CsCl gradient, the lower band is enriched for DNA with a buoyant density of 1.712 g/cm3 containing small covalently closed circular molecules. The minicircles measure 4.15 +/- 0.30 mum in the closed conformation and 4.35 +/- 0.20 mum in the open conformation. They are not of nuclear or bacterial origin, and appear to exist as independent entities within the chloroplast, although a mitochondrial origin cannot be completely ruled out. No 40-45 mum circles, as found in other chloroplasts, were found in either ethidium bromide-CsCl fraction. None were found in total chloroplast DNA by any of a number of methods tried. Linear molecules up to 200 mum were measured in chloroplast lysates. The main chloroplast genome may consist of very large circular molecules which are broken by even small shear forces.

Saprolegnia diclina DNA has been fractionated using preparative AgNO3/Cs2SO4 and CsCl density gradients. In addition to the previously identified major satellite DNA, there are two minor DNA components banding at 1.682 and 1.701 g - cm(-3) in CsCl. Purified major satellite DNA bands at 1.707 g - cm(-3) giving a base composition of 48% G + C in good agreement with 47% G + C calculated from its Tm value. The nuclear DNA base composition is 58% G + C by both methods. The base composition of the major satellite DNA suggests that it may represent ribosomal DNA cistrons.