In the Limelight: Photoreceptors in Cyanobacteria

ABSTRACT Certain cyanobacteria look green if grown in red light and vice versa. This dramatic color change, called complementary chromatic adaptation (CCA), is caused by alterations of the major colored light-harvesting proteins. A major controller of CCA is the cyanobacteriochrome (CBCR) RcaE, a red-green reversible photoreceptor that triggers a complex signal transduction pathway. Now, a new study demonstrates that CCA is also modulated by DpxA, a CBCR that senses yellow and teal (greenish blue) light. DpxA acts to expand the range of wavelengths that can impact CCA, by fine-tuning the process. This dual control of CCA might positively impact the fitness of cells growing in the shade of competing algae or in a water column where light levels and spectral quality change gradually with depth. This discovery adds to the growing number of light-responsive phenomena controlled by multiple CBCRs. Furthermore, the diverse CBCRs which are exclusively found in cyanobacteria have significant biotechnological potential.

Studies on phycobiliproteins in Algae. VI. Light-harvesting phycobiliprotein pigments in some Rhodophyta from the Adriatic Sea

The phycobiliprotein content in 5 species of red algae from the coast of the Adriatic Sea' was studied by chromatography on Sephadex G-100. The phycobiliproteins, R-phycoerythrin, C-phycocyanin and allophycocyanin were identified. The total content of phycobiliproteins ranged from 0.152 (<em>Phyllophora nervosa</em>) to 1.874 mg•g^-1 dry wt. (<em>Plocamium cartilagineum</em>). The dominant phycobiliproteins were found to belong to the phycocyanin group, this resulting from complementary chromatic adaptation.

Polyphasic Characterization of a Thermotolerant Siderophilic Filamentous Cyanobacterium That Produces Intracellular Iron Deposits

ABSTRACT Despite the high potential for oxidative stress stimulated by reduced iron, contemporary iron-depositing hot springs with circum-neutral pH are intensively populated with cyanobacteria. Therefore, studies of the physiology, diversity, and phylogeny of cyanobacteria inhabiting iron-depositing hot springs may provide insights into the contribution of cyanobacteria to iron redox cycling in these environments and new mechanisms of oxidative stress mitigation. In this study the morphology, ultrastructure, physiology, and phylogeny of a novel cyanobacterial taxon, JSC-1, isolated from an iron-depositing hot spring, were determined. The JSC-1 strain has been deposited in ATCC under the name Marsacia ferruginose, accession number BAA-2121. Strain JSC-1 represents a new operational taxonomical unit (OTU) within Leptolyngbya sensu lato. Strain JSC-1 exhibited an unusually high ratio between photosystem (PS) I and PS II, was capable of complementary chromatic adaptation, and is apparently capable of nitrogen fixation. Furthermore, it synthesized a unique set of carotenoids, but only chlorophyll a. Strain JSC-1 not only required high levels of Fe for growth (≥40 μM), but it also accumulated large amounts of extracellular iron in the form of ferrihydrite and intracellular iron in the form of ferric phosphates. Collectively, these observations provide insights into the physiological strategies that might have allowed cyanobacteria to develop and proliferate in Fe-rich, circum-neutral environments.

FdTonB is involved in the photoregulation of cellular morphology during complementary chromatic adaptation in Fremyella diplosiphon

We have characterized a Fremyella diplosiphon TonB protein (FdTonB) and investigated its function during complementary chromatic adaptation. Sequence similarity analysis of FdTonB (571 aa) led to identification of several conserved domains characteristic of TonB proteins, including an N-terminal transmembrane domain, a central proline-rich spacer and a C-terminal TonB-related domain (TBRD). We identified a novel glycine-rich domain containing (Gly-X) n repeats. To assess FdTonB function, we constructed a ΔtonB mutant through homologous recombination based upon truncation of the central proline-rich spacer, glycine-rich domain and TBRD. Our ΔtonB mutant exhibited an aberrant cellular morphology under green light, with expanded cell width compared to the parental wild-type (WT) strain. The cellular morphology of the ΔtonB mutant recovered upon WT tonB expression. Interestingly, tonB expression was found to be independent of RcaE. As ΔtonB and WT strains respond in the same way when grown under iron-replete versus iron-limited conditions, our results suggest that FdTonB is not involved in the classic TonB function of mediating cellular adaptation to iron limitation, but exhibits a novel function related to the photoregulation of cellular morphology in F. diplosiphon.

Shedding new light on the regulation of complementary chromatic adaptation

Complementary chromatic adaptation (CCA) is a light-dependent acclimation process that occurs in cyanobacteria and likely is related to increased fitness of these organisms in natural environments. Although CCA has been studied for over 40 years, significant advances in our understanding of the molecular foundations of CCA are still emerging. In this minireview, I explore recently reported developments that include novel insights into the molecular mechanisms utilized in the photoregulation of pigmentation and the molecular basis of light-dependent changes in cellular morphology, which are central elements of the process of CCA. I also discuss future avenues of study that are expected to lead to additional progress in our understanding of CCA and our general appreciation of light sensing and photomorphogenesis in cyanobacteria.