Richard W. Castenholz

CHARACTERIZATION AND BIOLOGICAL IMPLICATIONS OF SCYTONEMIN, A CYANOBACTERIAL SHEATH PIGMENT

Scytonemin, the yellow‐brown pigment of cyanobacterial (blue‐green algal) extracellular sheaths, was found in species thriving in habitats exposed to intense solar radiation. Scytonemin occurred predominantly in sheaths of the outermost parts or top layers of cyanobacterial mats, crusts, or colonies. Scytonemin appears to be a single compound identified in more than 30 species of cyanobacteria from cultures and natural populations. It is lipid soluble and has a prominent absorption maximum in the near‐ultraviolet region of the spectrum (384 nm in acetone; ca. 370 nm in vivo) with a long tail extending to the infrared region. Microspectrophotometric measurements of the transmittance of pigmented sheaths and the quenching of ultraviolet excitation of phycocyanin fluorescence demonstrate that the pigment was effective in shielding the cells from incoming near‐ultraviolet‐blue radiation, but not from green or red light. High light intensity (between 99 and 250 μmol photon · m−2· S−1, depending on species) promoted the synthesis of scytonemin in cultures of cyanobacteria. In cultures, high light intensity caused reduction in the specific content of Chl a and phycobilins, increase in the ratio of total carotenoids to Chl a, and scytonemin increase. UV‐A (320–400 nm) radiation was very effective in eliciting scytonemin synthesis. Scytonemin production was physiological and not due to a mere photochemical conversion. These results strongly suggest that scytonemin production constitutes an adaptive strategy of photoprotection against short‐wavelength solar irradiance.

A phototrophic gliding filamentous bacterium of hot springs, Chloroflexus aurantiacus, gen. and sp. nov.

Chloroflexus aurantiacus, gen. and sp. n., is a filamentous phototrophic bacterium of hot springs. On an agar surface, holotype strain J-10-fl glides at 0.01–0.04 μm/sec. The filaments are 0.6–0.7 μm in width and indeterminate in length. Pigments include bacteriochlorophyll c and bacteriochlorophyll a (identified by spectrophotometry) in addition to β and γ-carotene and glycosides of the latter. Chlorobium vesicles are present. Photoheterotrophic growth occurs under anaerobic conditions. Aerobic chemoheterotrophic growth also occurs in darkness or light. Bacteriochlorophyll syntheses cease under aerobic conditions but some types of carotenoids continue to be made. The filament coloration is orange under all except anaerobic conditions in low light intensity where it is dull green. The pH optimum is near 8, the temperature optimum between 52° and 60°C. The DNA base composition for strain J-10-fl is 54.9 ± 1.0 moles % guanine + cytosine. Chloroflexus is unique in that there have been no previous reports of filamentous or gliding phototrophic bacteria. The combinations of bacteriochlorophylls a and c and the presence of chlorobium vesicles in a photoheterotroph and in an organism capable of aerobic growth are also unique. This metabolically versatile organism extends the taxonomic and phylogenetic limits of the “green line” of phototrophic bacteria.

A thermophilic green sulfur bacterium from New Zealand hot springs, Chlorobium tepidum sp. nov.

Thermophilic green sulfur bacteria of the genus Chlorobium were isolated from certain acidic high sulfide New Zealand hot springs. Cells were Gram-negative nonmotile rods of variable length and contained bacteriochlorophyll c and chlorosomes. Cultures of thermophilic chlorobia grew only under anaerobic, phototrophic conditions, either photoautotrophically or photoheterotrophically. The optimum growth temperature for the strains of thermophilic green sulfur bacteria isolated was 47–48°C with generation times of about 2 h being observed. The upper temperature limit for growth was about 52°C. Thiosulfate was a major electron donor for photoautotrophic growth while sulfide alone was only poorly used. N2 fixation was observed at 48°C and cell suspensions readily reduced acetylene to ethylene. The G+C content of DNA from strains of thermophilic chlorobia was 56.5–58.2 mol% and the organisms positioned phylogenetically within the green sulfur bacterial branch of the domain Bacteria. The new phototrophs are described as a new species of the genus Chlorobium, Chlorobium tepidum.

Evidence for an ultraviolet sunscreen role of the extracellular pigment scytonemin in the terrestrial cyanobacterium Chlorogloeopsis sp.

Abstract— The proposed photoprotective role of the UV‐A absorbing, extracellular pigment scytonemin was studied in the terrestrial cyanobacterium Chlorogloeopsis sp. strain O‐89‐Cgs(1). UV‐A (315–400 nm) caused growth delay, cell growth restarting only when scytonemin had accumulated in the extracellular envelopes. Cultures with scytonemin were more resistant to photoinhibition of photosynthesis than cultures without scytonemin, the differential resistance being much greater to UV‐A‐caused photoinhibition than to photoinhibition caused by visible light. The presence of scytonemin in the extracellular envelopes was correlated with the inability of UV‐A radiation to induce strong photopigment fluorescence (685 nm emission), regardless of the specific content os photosynthetic pigments. The physical removal of the scytonemin containing extracellular envelopes brought about the loss of UV‐A resistance as measured by photobleaching rates of chlorophyll a under conditions of physiological inactivity (desiccation). These observations provide strong evidence for the proposed protective role of scytonemin, as a passive UV‐A sunscreen, in cyanobacteria.

Growth and photosynthesis in an extreme thermophile, Synechococcus lividus (Cyanophyta)

SummaryA high temperature strain of the blue-green alga, Synechococcus lividus has been cultured and cloned in defined medium. 1.S. lividus (Culture OH-68-s, Clone H-Xf) is an obligate thermophile with a temperature range of growth from 54 to 72°C. The optimal conditions for growth were at 63 to 67°C and a light intensity greater than 700 ft-c, resulting in a reproducible minimum generation time of 11 hours. Growth was depressed in the supra-optimal range from 68 to 72°C. The temperature characteristic or coefficient (μ) of growth was calculated as 13,750 cal mole-1. This value would not distinguish this organism from mesophilic and psychrophilic yeasts and bacteria.2.Clone H-Xf photosynthesized from as low as 33 to 75°C during short exposure times (20 min) without prior acclimation to the incubation temperatures. Longer exposures to the higher temperatures indicated that the “stable” upper limit for photosynthesis was 73°C when cells were grown above 60°C, but was only 70°C for cultures grown at 55 and 57°C.3.Abrupt shifts of exponential cultures between optimal (65°C) and near minimal (55°C) growth temperatures showed that lag periods occurred before normal growth commenced at the new temperatures. However, these lag periods on downward temperature shifts followed only after a period of residual growth. Similar shifts from optimal to subminimal (45°C) temperatures indicated that growth continued for a period of time before entering an extended stationary phase prior to senescence. Both of the latter types of experiments may indicate that products synthesized at 65°C are consumed by residual growth after the shifts to lower temperatures, but that these are replaced after a long delay (acclimation) at 55°C and not at all at 45°C.4.Photoincorporation of 14C−NaHCO3 was highly sensitive to subminimal temperatures after the first hour of exposure. The data suggest that the photosynthetic system could be involved in determining both the upper and lower limits of growth in this organism.

Cyanobacterial Responses to UV-Radiation

The influence of ultraviolet radiation (UVR) on populations of microorganisms has been the subject of serious investigation for at least the past 20–25 years. UVR that is applicable to the Earth’s surface (past or present) is arbitrarily divided into UVA (400–320 or 315 nm), UVB (280–320 or 315 nm), UVC (∼180–280 nm). Although essentially all organisms are affected by UVR, microorganisms show more rapid, immediate and measurable effects than macro-organisms. This chapter is mainly relegated to UVR and cyanobacteria, although UV effects on other phototrophs and microorganisms, when relevant, will be included. Some ancestors of living cyanobacteria, the oldest oxygenic organisms, may have evolved in the Archean or early Proterozoic Eons, from 3.5 to 2.5 Gyr, respectively, in a time when UV radiation fluxes reaching the surface, particularly UVB and UVC, were much higher than at present. The latter wavelength region (UVC) does not reach the Earth’s surface at present. Thus, cyanobacteria and other microorganisms in that distant age had to have evolved a strategy to tolerate these greater levels of UV radiation, and at present this strategy may demonstrably involve multiple devices, even within one organism. The best understood in the past several years for numerous organisms has been the active metabolic strategies that compensate for the destruction of vital genetic components, such as the development of efficient metabolic DNA repair systems. The implementation of gliding motility system for escaping the effects of high visible and UV radiation has been better described and understood. Some of the most revealing results in the last 10 years have been an almost complete understanding of the regulation of the UV-protective compounds, scytonemin and mycosporine-like compounds, that partially or completely avoid the damage caused by UV radiation.

Phylum BX. Cyanobacteria

The oxygenic photosynthetic procaryotes comprise a single taxonomic and phylogenetic group (see master phylogenetic tree of the Bacteria). In the last edition of the Manual, two separate groups were described, but it is now apparent that members of the Prochlorales simply represent different, unrelated genera which fall into the main cluster of the Cyanobacteria (see Oxygenic Photosynthetic Bacteria, below). The principal character that defines all of these oxygenic photosynthetic procaryotes is the presence of two photosystems (PSII and PSI) and the use of H2O as the photoreductant in photosynthesis. Although facultative photo- or chemo-heterotrophy may occur in some species or strains, all known members are capable of photoautotrophy (using CO2 as the primary source of cell carbon).

Studies of pigments and growth in Chloroflexus aurantiacus, a phototrophic filamentous bacterium

Abstract1.The chlorophyll pigments of Chloroflexus aurantiacus were separated by column chromatography on powdered sugar and were identified by spectrophotometry in various solvents as BChl a and BChl c (chlorobium chlorophyll 660). The bacteriopheophytins were also prepared and characterized-spectrophotometrically. The identity of the BChl a is tentative because of its anomalous phase test behavior and because of changes in its absorption spectrum observed under different conditions of preparation.2.Growth rates of Chloroflexus at 55°C and synthesis of the 2 chlorophylls were compared in cells growing under anaerobic conditions at different light intensities. Growth rates increased with increasing light intensity to a saturation level of about 0.30 doublings/hr at 20 000 lux and above during the second exponential phase of growth. The rate of the first exponential phase continued to increase, at least up to 50 000 lux. The specific content of both chlorophylls decreased with increasing light intensity but to different extents. A linear relationship between specific chlorophyll content and growth rate for either chlorophyll was only observed over a limited range of growth rates. The ratio of BChl c/BChl a decreased with increasing light intensity. The greatest change occurred between 300 and 5000 lux. The differential responses of BChl c and BChl a to light intensity were also demonstrated by shifting highly pigmented cells (grown at low light intensity) to high light intensity. In these cases different rates of synthesis of the 2 pigments followed initial adjustments. Carotenoid synthesis did not decrease with increasing light intensity under anaerobic conditions.3.Chlorophyll synthesis was suppressed under fully aerobic conditions in dark-ness and light. In either case the pigments were diluted out by continued cell growth. At least 1 carotenoid pigment, however, was synthesized under aerobic conditions. Other carotenoids characteristic of anaerobic growth were not observed.4.The chemoheterotrophic aerobic growth rate at 55°C in darkness (0.14 to 0.22 d/hr) was less than the maximum second phase phototrophic rate under anaerobic conditions (0.30 d/hr). Aerobic growth rate in the light was the same as in darkness if chlorophylls were lacking, but was enhanced if these pigments were still present. The oxygen consumption rate was partially suppressed in the light only when chlorophylls were present in the cells.5.A light-minus-dark diffrence spectrum revealed the presence of a light-induced reversible decrease in absorbance of BChl a with a maximum effect at 860 nm, tentatively identifying a reaction center complex.

Community structure and pigment organisation of cyanobacteria-dominated microbial mats in Antarctica

Benthic microbial mat communities were sampled from 20 lakes, ponds and streams of the McMurdo Sound region, Antarctica. At least five distinct assemblages could be differentiated by their cyanobacterial species composition, pigment content and vertical structure. The most widely occurring freshwater communities were dominated by thin-trichome (0·5–3 µm) oscillatoriacean species that formed benthic films up to several millimetres thick. ‘Lift-off mats’ produced mucilaginous mats 1–5 cm thick at the surface and edge of certain ponds. Another group of oscillatoriacean communities was characteristic of hypersaline pond environments; these communities were dominated by species with thicker trichomes such as Oscillatoria priestleyi. Black mucilaginous layers of Nostoc commune were widely distributed in aquatic and semi-aquatic habitats. Dark brown sheath pigmentation was also characteristic of less cohesive mats and crusts dominated by Pleurocapsa, Gloeocapsa and Calothrix. High performance liquid chromatograp...

Effect of environmental factors on the synthesis of scytonemin, a UV-screening pigment, in a cyanobacterium ( Chroococcidiopsis sp.)

The UV-screening pigment scytonemin is found in many species of ensheathed cyanobacteria. Past work has shown that the pigment is synthesized in response to exposure to UV-A irradiance. This study investigated the effect of other correlated stress factors including heat, osmotic and oxidative stress on the synthesis of scytonemin in a clonal cyanobacterial isolate (Chroococcidiopsis sp.) from an epilithic desert crust. Stress experiments were carried out both in conjunction with UV-A irradiance and in isolation. Increases in both temperature and photooxidative conditions in conjunction with UV-A caused a synergistic increase in the rate of scytonemin production. In contrast, increased salt concentration under UV-A irradiance inhibited scytonemin synthesis. However, unlike the responses to temperature and oxidative stress, cells synthesized low levels of scytonemin under osmotic stress in the absence of scytonemin-inducing irradiance. These results suggest that scytonemin induction may be regulated as a part of a complex stress response pathway in which multiple environmental signals affect its synthesis.