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The impact of different intensities of green light on the bacteriochlorophyll homologue composition of the chlorobiaceae Prosthecochloris aestuarii and Chlorobium phaeobacteroides

Rutger de Wit^1,

¹ Laboratoire d'Océanographie Biologique, CNRS-UMR 5805 Université Bordeaux 1, 2 rue du Professeur Jolyet, F-33120 Arcachon, France
² Department of Chemistry, University of York, Heslington, York YO10 5DD, UK

Correspondence
Rutger de Wit
rde-wit{at}univ-montp2.fr

Members of the Chlorobiaceae and Chloroflexaceae are unique among the phototrophic micro-organisms in having a remarkably rich chlorophyll pigment diversity. The physiological regulation of this diversity and its ecological implications are still enigmatic. The bacteriochlorophyll composition of the chlorobiaceae Prosthecochloris aestuarii strain CE 2404 and Chlorobium phaeobacteroides strain UdG 6030 was therefore studied by both HPLC with photodiode array (PDA) detection and liquid chromatography-mass spectrometry (LC-MS). These strains were grown in liquid cultures under green light (480–615 nm) at different light intensities (0·2–55·7 µmol photons m^–2 s^–1), simulating the irradiance regime at different depths of the water column of deep lakes. The specific growth rates of Ptc. aestuarii under green light achieved a maximum of 0·06 h^–1 at light intensities exceeding 6 µmol photons m^–2 s^–1, lower than the maximum observed under white light (approx. 0·1 h^–1). The maximal growth rates of Chl. phaeobacteroides under green light were slightly higher (0·07 h^–1) than observed for Ptc. aestuarii and were achieved at 3·5 and 4·3 µmol photons m^–2 s^–1. LC-MS/MS analysis of pigment extracts revealed most (>90 %) BChl c homologues of Ptc. aestuarii to be esterified with farnesol. The homologues differed in mass by multiples of 14 Da, reflecting different alkyl subsituents at positions C-8 and C-12 on the tetrapyrrole macrocycle. The relative proportions of the individual homologues varied only slightly among different light intensities. The specific content of BChl c was maximal at 3–5 µmol photons m^–2 s^–1 [400±150 nmol BChl c (mg protein)^–1]. In the case of Chl. phaeobacteroides, the specific content of BChl e was maximal at 4·3 µmol photons m^–2 s^–1 [115 nmol BChl e (mg protein)^–1], and this species was characterized by high carotenoid (isorenieratene) contents. The major BChl e forms were esterified with a range of isoprenoid and straight-chain alcohols. The major isoprenoid alcohols comprised mainly farnesol and to a lesser extent geranylgeraniol. The straight-chain alcohols included C₁₅, C_{15 : 1}, C₁₆, C_{16 : 1} and C₁₇. Interestingly, the proportion of straight alkyl chains over isoprenoid esterified side chains shifted markedly with increasing light intensity: the isoprenoid side chains dominated at low light intensities, while the straight-chain alkyl substituents dominated at higher light intensities. The authors propose that this phenomenon may be explained as a result of changing availability of reducing power, i.e. the highly reduced straight-chain alcohols have a higher biosynthetic demand for NADPH₂ than the polyunsaturated isoprenoid with the same number of carbon atoms.

Present address: UMR 5119 CNRS-Université Montpellier II ‘Ecosystèmes lagunaires’, Université Montpellier II, Case 093, 34095 Montpellier Cedex 05, France.