Chromatic acclimation and population dynamics of green sulfur bacteria grown with spectrally tailored light

Living organisms have to adjust to their surrounding in order to survive in stressful conditions. We study this mechanism in one of most primitive creatures – photosynthetic green sulfur bacteria. These bacteria absorb photons very efficiently using the chlorosome antenna complexes and perform photosynthesis in extreme low-light environments. How the chlorosomes in green sulfur bacteria are acclimated to the stressful light conditions, for instance, if the spectrum of light is not optimal for absorption, is unknown. Studying Chlorobaculum tepidum cultures with far-red to near-infrared light-emitting diodes, we found that these bacteria react to changes in energy flow by regulating the amount of light-absorbing pigments and the size of the chlorosomes. Surprisingly, our results indicate that the bacteria can survive in near-infrared lights capturing low-frequency photons by the intermediate units of the light-harvesting complex. The latter strategy may be used by the species recently found near hydrothermal vents in the Pacific Ocean

Temperature and Carbon Assimilation Regulate the Chlorosome Biogenesis in Green Sulfur Bacteria

Green photosynthetic bacteria adjust the structure and functionality of the chlorosome—the light-absorbing antenna complex—in response to environmental stress factors. The chlorosome is a natural self-assembled aggregate of bacteriochlorophyll (BChl) molecules. In this study, we report the regulation of the biogenesis of the Chlorobaculum tepidum chlorosome by carbon assimilation in conjunction with temperature changes. Our studies indicate that the carbon source and thermal stress culture of C. tepidum grows slower and incorporates fewer BChl c in the chlorosome. Compared with the chlorosome from other cultural conditions we investigated, the chlorosome from the carbon source and thermal stress culture displays (a) smaller cross-sectional radius and overall size, (b) simplified BChl c homologs with smaller side chains, (c) blue-shifted $Q_y$ absorption maxima, and (d) a sigmoid-shaped circular dichroism spectra. Using a theoretical model, we analyze how the observed spectral modifications can be associated with structural changes of BChl aggregates inside the chlorosome. Our report suggests a mechanism of metabolic regulation for chlorosome biogenesis.Chemistry and Chemical Biolog

Strong coupling between chlorosomes of photosynthetic bacteria and a confined optical cavity mode

Strong exciton–photon coupling is the result of a reversible exchange of energy between an excited state and a confined optical field. This results in the formation of polariton states that have energies different from the exciton and photon. We demonstrate strong exciton–photon coupling between light-harvesting complexes and a confined optical mode within a metallic optical microcavity. The energetic anti-crossing between the exciton and photon dispersions characteristic of strong coupling is observed in reflectivity and transmission with a Rabi splitting energy on the order of 150 meV, which corresponds to about 1,000 chlorosomes coherently coupled to the cavity mode. We believe that the strong coupling regime presents an opportunity to modify the energy transfer pathways within photosynthetic organisms without modification of the molecular structure.Chemistry and Chemical Biolog

Peroxidase activity and involvement in the oxidative stress response of roseobacter denitrificans truncated hemoglobin.

Roseobacter denitrificans is a member of the widespread marine Roseobacter genus. We report the first characterization of a truncated hemoglobin from R. denitrificans (Rd. trHb) that was purified in the heme-bound form from heterologous expression of the protein in Escherichia coli. Rd. trHb exhibits predominantly alpha-helical secondary structure and absorbs light at 412, 538 and 572 nm. The phylogenetic classification suggests that Rd. trHb falls into group II trHbs, whereas sequence alignments indicate that it shares certain important heme pocket residues with group I trHbs in addition to those of group II trHbs. The resonance Raman spectra indicate that the isolated Rd. trHb contains a ferric heme that is mostly 6-coordinate low-spin and that the heme of the ferrous form displays a mixture of 5- and 6-coordinate states. Two Fe-His stretching modes were detected, notably one at 248 cm-1, which has been reported in peroxidases and some flavohemoglobins that contain an Fe-His-Asp (or Glu) catalytic triad, but was never reported before in a trHb. We show that Rd. trHb exhibits a significant peroxidase activity with a (kcat/Km) value three orders of magnitude higher than that of bovine Hb and only one order lower than that of horseradish peroxidase. This enzymatic activity is pH-dependent with a pKa value ~6.8. Homology modeling suggests that residues known to be important for interactions with heme-bound ligands in group II trHbs from Mycobacterium tuberculosis and Bacillus subtilis are pointing toward to heme in Rd. trHb. Genomic organization and gene expression profiles imply possible functions for detoxification of reactive oxygen and nitrogen species in vivo. Altogether, Rd. trHb exhibits some distinctive features and appears equipped to help the bacterium to cope with reactive oxygen/nitrogen species and/or to operate redox biochemistry

Resonance Raman spectra of <i>Rd</i>. trHb Fe(II)CO complex in the low-frequency region.

<p>An excitation wavelength of 413.1 nm was used. The difference spectrum (diff) is the spectrum obtained by subtracting the ¹³C¹⁸O spectrum from the ¹²C¹⁶O spectrum. The Fe-CO stretching mode (ν_Fe-CO) and other heme modes are identified. The lines at 378 and 412 cm^-1 correspond to a bending mode of the propionate and of the vinyl groups, respectively.</p

Primer sequences used in this report.

<p>Primer sequences used in this report.</p

Transcript level profiles of some <i>R. denitrificans genes</i>.

<p>Probe of the transcript expression levels of <i>gpo</i> (glutathione peroxidase), <i>katG</i> (bi-functional catalase-peroxidase) and <i>glbO</i> (truncated hemoglobin) in cultures grown chemotrophically in a minimal medium supplied with 10 mM acetate and with or without 1 mM H₂O₂ (A), and probe of the transcript expression level of genes involved in carbon and nitrogen metabolism in cultures grown in a minimal medium supplied with 10 mM glucose and subjected to cycles of 12-h light followed by 12-h dark periods (B): <b>1</b>. RD1_2870 (phosphoglucomutase); <b>2</b>. RD1_2720 (glucose-6-phosphate isomerase); <b>3</b>. RD1_2879 (6-phosphogluconate dehydrase); <b>4</b>. RD1_2878 (KDPG aldolase); <b>5</b>. RD1_3376 (pyruvate carboxylase); <b>6</b>. RD1_0421 (malic enzyme); <b>7</b>. RD1_1609 (2-ketoglutarate dehydrogenase); <b>8</b>. RD1_2204 (isocitrate dehydrogenase); <b>9</b>. RD1_4240 (<i>glbO</i>, truncated hemoglobin); <b>10</b>. RD1_1561 (NO reductase); <b>11</b>. RD1_1562 (NO reductase).</p

A cell culture of <i>R. denitrificans</i> (A) and its UV-visible absorption spectrum with baseline subtracted (B).

<p>A cell culture of <i>R. denitrificans</i> (A) and its UV-visible absorption spectrum with baseline subtracted (B).</p

The phylogenetic tree of trHbs.

<p>The evolutionary history was inferred using the Neighbor-Joining method. Sperm whale myoglobin was used as the out-group. The analysis involved 46 amino acid sequences. All positions containing gaps and missing data were eliminated. Evolutionary analyses were conducted using the software MEGA6 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0117768#pone.0117768.ref051" target="_blank">51</a>]. Three groups (groups I, II and III) of trHbs are identified.</p