Biomedical subjects

Anthony W D Larkum

Publications and source records attributed to Anthony W D Larkum.

15 recordsLinked to original sources

PubMed2006

Chromatic photoacclimation, photosynthetic electron transport and oxygen evolution in the chlorophyll d-containing oxyphotobacterium Acaryochloris marina.

Changes in photosynthetic pigment ratios showed that the Chlorophyll d-dominated oxyphotobacterium Acaryochloris marina was able to photoacclimate to different light regimes. Chl d per cell were higher in cultures grown under low irradiance and red or green light compared to those found when grown under high white light, but phycocyanin/Chl d and carotenoid/Chl d indices under the corresponding conditions were lower. Chl a, considered an accessory pigment in this organism, decreased respective to Chl d in low irradiance and low intensity non-white light sources. Blue diode PAM (Pulse Amplitude Modulation) fluorometry was able to be used to measure photosynthesis in Acaryochloris. Light response curves for Acaryochloris were created using both PAM and O(2) electrode. A linear relationship was found between electron transport rate (ETR), measured using a PAM fluorometer, and oxygen evolution (net and gross photosynthesis). Gross photosynthesis and ETR were directly proportional to one another. The optimum light for white light (quartz halogen) was about 206+/-51 micromol m(-2) s(-1) (PAR) (Photosynthetically Active Radiation), whereas for red light (red diodes) the optimum light was lower (109+/-27 micromol m(-2) s(-1) (PAR)). The maximum mean gross photosynthetic rate of Acaryochloris was 73+/-7 micromol mg Chl d(-1) h(-1). The gross photosynthesis/respiration ratio (P(g)/R) of Acaryochloris under optimum conditions was about 4.02+/-1.69. The implications of our findings will be discussed in relation to how photosynthesis is regulated in Acaryochloris.

Chlorophyll↗

PubMed2005

The nature of the photosystem II reaction centre in the chlorophyll d-containing prokaryote, Acaryochloris marina.

Pigment-protein complexes enriched in photosystem II (PS II) have been isolated from the chlorophyll (Chl) d containing cyanobacterium, Acaryochloris marina. A small PS II-enriched particle, we call 'crude reaction centre', contained 20 Chl d, 0.5 Chl a and 1 redox active cytochrome b-559 per 2 pheophytin a, plus the D1 and D2 proteins. A larger PS II-enriched particle, we call 'core', additionally bound the antenna complexes, CP47 and CP43, and had a higher chlorophyll per pheophytin ratio. Pheophytin a could be photoreduced in the presence of a strong reductant, indicating that it is the primary electron acceptor in photosystem II of A. marina. A substoichiometric amount of Chl a (less than one chlorophyll a per 2 pheophytin a) strongly suggests that Chl a does not have an essential role in the photochemistry of PS II in this organism. We conclude that PS II, in A. marina, utilizes Chl d and not Chl a as primary electron donor and that the primary electron acceptor is one of two molecules of pheophytin a.

Chlorophyll↗

PubMed2005

Ecology: a niche for cyanobacteria containing chlorophyll d.

The cyanobacterium known as Acaryochloris marina is a unique phototroph that uses chlorophyll d as its principal light-harvesting pigment instead of chlorophyll a, the form commonly found in plants, algae and other cyanobacteria; this means that it depends on far-red light for photosynthesis. Here we demonstrate photosynthetic activity in Acaryochloris-like phototrophs that live underneath minute coral-reef invertebrates (didemnid ascidians) in a shaded niche enriched in near-infrared light. This discovery clarifies how these cyanobacteria are able to thrive as free-living organisms in their natural habitat.

Animals↗

PubMed2005

Influence of structure on binding of chlorophylls to peptide ligands.

Four classes of chlorophyll (Chl), a, b, c, and d, are involved in photosynthesis within cyanobacteria, algae, and plants. These classes have different evolutionary origins, chemical properties, and biological functions. Our results demonstrate that peptide-bound ligands provided by the imidazole group of histidine and the charge-compensated glutamate-arginine ion pair readily form coordination bonds with Chls a and d but do not interact significantly with Chls b and c. These ligands are apparently not sufficiently strong Lewis bases to displace strongly coordinated water from Chls b and c. These differences determine specificity of binding of Chls in light-harvesting complexes and play an important role in assembly of stable Chl-protein complexes, which has had a profound impact on the evolution of photosynthetic organisms.

Amino Acid Sequence↗

PubMed2005

Structure of a large photosystem II supercomplex from Acaryochloris marina.

Acaryochloris marina is a prochlorophyte-like cyanobacterium containing both phycobilins and chlorophyll d as light harvesting pigments. We show that the chlorophyll d light harvesting system, composed of Pcb proteins, functionally associates with the photosystem II (PSII) reaction center (RC) core to form a giant supercomplex. This supercomplex has a molecular mass of about 2300 kDa and dimensions of 385 A x 240 A. It is composed of two PSII-RC core dimers arranged end-to-end, flanked by eight symmetrically related Pcb proteins on each side. Thus each PSII-RC monomer has four Pcb subunits acting as a light harvesting system which increases the absorption cross section of the PSII-RC core by almost 200%.

Cyanobacteria↗

PubMed2005

Chlorophyll d: the puzzle resolved.

Chlorophyll a (Chl a) has always been regarded as the sole chlorophyll with a role in photochemical conversion in oxygen-evolving phototrophs, whereas chlorophyll d (Chl d), discovered in small quantities in red algae in 1943, was often regarded as an artefact of isolation. Now, as a result of discoveries over the past year, it has become clear that Chl d is the major chlorophyll of a free-living and widely distributed cyanobacterium that lives in light environments depleted in visible light and enhanced in infrared radiation. Moreover, Chl d not only has a light-harvesting role but might also replace Chl a in the special pair of chlorophylls in both reactions centers of photosynthesis.

Animals↗

PubMed2004

Unique origin and lateral transfer of prokaryotic chlorophyll-b and chlorophyll-d light-harvesting systems.

pcb genes, encoding proteins binding light-harvesting chlorophylls, were cloned and sequenced from the Chl d-containing cyanobacterium, Acaryochloris marina, and the Chl b-containing cyanobacterium, Prochloron didemni. Both organisms contained two tandem pcb genes. Peptide fingerprinting confirmed the expression of one of the A. marina pcb genes. Phylogenetic tree reconstruction using distance-matrix and maximum-likelihood methods indicated a single origin of the pcb gene family, whether occurring in Chl b-containing or Chl d-containing organisms. This may indicate widespread lateral transfer of the Pcb protein-based light-harvesting system.

Biological Evolution↗

PubMed2004

Raman properties of chlorophyll d, the major pigment of Acaryochloris marina: studies using both Raman spectroscopy and density functional theory.

The Raman spectroscopy of purified chlorophyll (Chl) d extracted from Acaryochloris marina has been measured over the wide region of 250-3200 cm(-1) at 77 K following excitation of its Soret band at 488 nm and analyzed with the aid of hybrid density-functional vibrational analyses. A Raman peak specific to Chl d, which arises from the formyl group 3(1) C=O stretching, was clearly observed at 1659 cm(-1) with medium intensity. Peaks due to other C=O stretching vibrations of the 13(1) keto-, 13(3) ester- and 17(3) groups were also observed. Four very strong peaks were observed in the range of 1000-1600 cm(-1), assigned to the CC stretching and mixtures of the CH3 bend and CN stretching. CCC and NCC bending contribute to medium intensity peaks at 986 and 915 cm(-1). Out-of-plane CH bending at Chl d methine sites 10, 5 and 20 contribute to observed peaks at 885, 864 and 853 cm(-1), respectively. A few modes involving the MgN stretching and MgNC bending motions were observed in the very low frequency range. Density functional theory (DFT) calculations have been used to make assignments on the observed Raman spectrum and the DFT results have been found to be in good agreement with the experimental results.

Bacteria↗

PubMed2003

Structure of a photosystem II supercomplex isolated from Prochloron didemni retaining its chlorophyll a/b light-harvesting system.

Prochlorophytes are a class of cyanobacteria that do not use phycobiliproteins as light-harvesting systems, but contain chlorophyll (Chl) a/b-binding Pcb proteins. Recently it was shown that Pcb proteins form an 18-subunit light-harvesting antenna ring around the photosystem I (PSI) trimeric reaction center complex of the prochlorophyte Prochlorococcus marinus SS120. Here we have investigated whether the symbiotic prochlorophyte Prochloron didemni also contains the same supermolecular complex. Using cells isolated directly from its ascidian host, we found no evidence for the presence of the Pcb-PSI supercomplex. Instead we have identified and characterized a supercomplex composed of photosystem II (PSII) and Pcb proteins. We show that 10-Pcb subunits associate with the PSII dimeric reaction center core to form a giant complex having an estimated Mr of 1,500 kDa with dimensions of 210 x 290 A. Five-Pcb subunits flank each long side of the dimer and assuming each binds 13 Chl molecules, increase the antenna size of PSII by approximately 200%. Fluorescence emission studies indicate that energy transfer occurs efficiently from the Pcb antenna. Modeling using the x-ray structure of cyanobacterial PSII suggests that energy transfer to the PSII reaction center is via the Chls bound to the CP47 and CP43 proteins.

Light-Harvesting Protein Complexes↗

PubMed2003

Simultaneous time resolution of the emission spectra of fluorescent proteins and zooxanthellar chlorophyll in reef-building corals.

Light is absorbed by photosynthetic algal symbionts (i.e. zooxanthellae) and by chromophoric fluorescent proteins (FP) in reef-building coral tissue. We used a streak-camera spectrograph equipped with a pulsed, blue laser diode (50 ps, 405 nm) to simultaneously resolve the fluorescence spectra and kinetics for both the FP and the zooxanthellae. Shallow water (<9 m)-dwelling Acropora spp. and Plesiastrea versipora specimens were collected from Okinawa, Japan, and Sydney, Australia, respectively. The main FP emitted light in the blue, blue-green and green emission regions with each species exhibiting distinct color morphs and spectra. All corals showed rapidly decaying species and reciprocal rises in greener emission components indicating Förster resonance energy transfer (FRET) between FP populations. The energy transfer modes were around 250 ps, and the main decay modes of the acceptor FP were typically 1900-2800 ps. All zooxanthellae emitted similar spectra and kinetics with peak emission (approximately 683 nm) mainly from photosystem II (PSII) chlorophyll (chl) a. Compared with the FP, the PSII emission exhibited similar rise times but much faster decay times, typically around 640-760 ps. The fluorescence kinetics and excitation versus emission mapping indicated that the FP emission played only a minor role, if any, in chl excitation. We thus suggest the FP could only indirectly act to absorb, screen and scatter light to protect PSII and underlying and surrounding animal tissue from excess visible and UV light. We conclude that our time-resolved spectral analysis and simulation revealed new FP emission components that would not be easily resolved at steady state because of their relatively rapid decays due to efficient FRET. We believe the methods show promise for future studies of coral bleaching and for potentially identifying FP species for use as genetic markers and FRET partners, like the related green FP from Aequorea spp.

Animals↗

PubMed2003

Examination of the photophysical processes of chlorophyll d leading to a clarification of proposed uphill energy transfer processes in cells of Acaryochloris marinas.

A comprehensive study of the photophysical properties of chlorophyll (Chl) d in 1:40 acetonitrile-methanol solution is performed over the temperature range 170-295 K. From comparison of absorption and emission spectra, time-dependent density-functional calculations and homologies with those of Chl a, we assign the key features of the absorption and fluorescence spectra. Possible photophysical energy relaxation mechanisms are summarized, and thermal equilibration processes are studied in detail by monitoring the observed emission profiles and quantum yields as a function of excitation energy. In particular, we concentrate on emission subsequent to excitation in the extreme far-red tail of the Qy absorption spectrum, with this emission partitioned into contributions from hot-band absorptions as well as uphill energy transfer processes that occur subsequent to absorption. No unusual photophysical processes are detected for Chl d; it appears that all intramolecular relaxation processes reach thermal equilibration on shorter timescales than the fluorescence lifetime even at 170 K. The results from these studies are used to reinterpret a previous study of photochemical processes observed in intact cells and their acetone extracts of the photosynthetic system of Acaryochloris marina. In the study of Mimuro et al., light absorbed by Chl d at 736 nm is found to give rise to emission by another species, believed to also be Chl d, at 703 nm; this uphill energy transfer process is easily rationalized in terms of the thermal equilibration processes that we deduced for Chl d. However, no evidence is found in the experimental results of Mimuro et al. to support claims that (nonequilibrium) uphill energy transfer is additionally observed to Chl a species that emit at 670-680 nm. This finding is relevant to broader issues concerning the nature of the special pair in photosystem II of A. marina because suggestions that it is comprised of Chl a can only be correct if nonthermal uphill energy transfer processes from Chl d are operative.

Chlorophyll↗

PubMed2003

Hetero: a program to simulate the evolution of DNA on a four-taxon tree.

We present a computer program to simulate the evolution of a nucleotide sequence on a phylogenetic tree with four tips. The program, Hetero, allows users to assign lineage-specific differences in the rate matrices used to describe the evolutionary process. It has a simple user interface and output, making it equally useful in the teaching and research of phylogenetics.

Algorithms↗

PubMed2002

Raman spectroscopy of chlorophyll d from Acaryochloris marina.

The Raman spectroscopy of chlorophyll (Chl) d isolated from Acaryochloris marina has been measured in the range of 250-3200 cm(-1) at 77 K following excitation of its B(x) band at 488 nm. A peak at 1659 cm(-1) of medium intensity arising from Cz=O stretching vibration in the formyl group 3(1) specific to Chl d was observed clearly. Peaks due to other Cz=O stretching vibrations of the 13(1) keto-, 13(3) ester- and 17(3) groups have also been observed with much weaker intensities. Intense Raman peaks in the range of 1000-1800 cm(-1) are reported and homologous comparison with corresponding Raman shifts of Chl a, Chl b and BChl a are presented.

Bacterial Proteins↗

PubMed2002

The major light-harvesting pigment protein of Acaryochloris marina.

The major light-harvesting protein complex containing chlorophyll (Chl) d was isolated from Acaryochloris marina thylakoid membranes. Isolation was achieved by detergent solubilisation followed by separation on 6-40% sucrose gradients using ultracentrifugation. The best Chl d yield (70%) used 0.3% dodecyl maltoside, 0.15% octyl glucoside, 0.05% zwittergent 3-14 with the detergent:total Chl d ratio around 10:1 (w/w). Characterisation of the light-harvesting pigment protein complex (lhc) involved non-denaturing electrophoresis, SDS-PAGE, absorbance and fluorescence spectroscopy. The main polypeptide in the lhc was shown to be ca. 34 kDa and to contain Chl d and Chl a, indicating that the Acaryochloris lhc is similar to that of prochlorophytes. The Chl a level varied with the culture conditions, which is consistent with previous findings.

Bacterial Proteins↗

PubMed2002

An in situ study of photosynthetic oxygen exchange and electron transport rate in the marine macroalga Ulva lactuca (Chlorophyta).

Direct comparisons between photosynthetic O(2) evolution rate and electron transport rate (ETR) were made in situ over 24 h using the benthic macroalga Ulva lactuca (Chlorophyta), growing and measured at a depth of 1.8 m, where the midday irradiance rose to 400-600 mumol photons m(-2) s(-1). O(2) exchange was measured with a 5-chamber data-logging apparatus and ETR with a submersible pulse amplitude modulated (PAM) fluorometer (Diving-PAM). Steady-state quantum yield ((F(m)'-F(t))/F(m)') decreased from 0.7 during the morning to 0.45 at midday, followed by some recovery in the late afternoon. At low to medium irradiances (0-300 mumol photons m(-2) s(-1)), there was a significant correlation between O(2) evolution and ETR, but at higher irradiances, ETR continued to increase steadily, while O(2) evolution tended towards an asymptote. However at high irradiance levels (600-1200 mumol photons m(-2) s(-1)) ETR was significantly lowered. Two methods of measuring ETR, based on either diel ambient light levels and fluorescence yields or rapid light curves, gave similar results at low to moderate irradiance levels. Nutrient enrichment (increases in [NO(3) (-)], [NH(4) (+)] and [HPO(4) (2-)] of 5- to 15-fold over ambient concentrations) resulted in an increase, within hours, in photosynthetic rates measured by both ETR and O(2) evolution techniques. At low irradiances, approximately 6.5 to 8.2 electrons passed through PS II during the evolution of one molecule of O(2), i.e., up to twice the theoretical minimum number of four. However, in nutrient-enriched treatments this ratio dropped to 5.1. The results indicate that PAM fluorescence can be used as a good indication of the photosynthetic rate only at low to medium irradiances.

Journal Article↗