Investigation of diversity of unusual pigments and novel photosynthetic apparatus in aerobic anoxygenic phototrophic bacteria

Abstract

Photosynthetic complexes produced by aerobic anoxygenic phototrophs (AAP) Roseicyclus mahoneyensis, strain ML6, Chromocurvus halotolerans, EG19 and Charonomicrobium ambiphototrophicum, EG17 were studied to examine unusual pigments, proteins and photosynthetic function. We found ML6 spontaneously mutates to lose its light harvesting (LH) complex 2, or reaction centre (RC) and LH1, resulting in mutants ML6(BN9O), ML6(B) and ML6(DB). EG19 and EG17 contain a red-shifted LH1 complex, and EG17 is the first AAP known to photosynthesize both aerobically and anaerobically. From all organisms, membranes and complexes were purified. Flash induced difference spectra revealed ML6, ML6(BN9O), EG19 and EG17 were photosynthetically active aerobically, though only EG17 was capable of rapid electron transport without oxygen. ML6(B) and ML6(DB) were not capable of photosynthesis. Action spectra universally showed that bacteriochlorophyll and bacteriopheophytin were the primary LH pigments and carotenoids were not involved in photosynthesis. Redox midpoint potentials of the QA, P+ and RC-bound cytochrome were typical of AAP, except for the QA of EG17 (+5mV, between what is expected for AAP and anaerobic purple non-sulfur bacteria). RC, LH1 and LH2 subunits were of typical sizes. The exception was the LH1 of EG19 and EG17, which was slightly larger than previously published. Strain EG19 excretes an orange-brown compound into the growth medium, previously hypothesized to be a siderophore. We identified it as a catechol siderophore, measuring approximately 800Da. It is hydrophilic and may contain the amino acids valine, proline, aspartate, 2-aminobutenoic acid, and N-acetyl-L-glutamate. Lastly, a sampling trip to Central Gold Mine, Nopiming Provincial Park, Canada was undertaken to study the presence of AAP in this extreme environment. Abundant AAP were found and five were chosen for study: strains NM4.16, NM4.18, C4, C9, and C11. All are mesophiles, grow on complex media, and tolerate a wide range of pH, temperature and salinity. Isolates were highly resistant to the toxic metalloid oxides tellurite, tellurate, selenite, selenate, metavanadate and orthometavanadate and reduced tellurite to tellurium. All are α-Proteobacteria, with C4 and NM4.16 closely related to Porphyrobacter colymbi (99.4% and 99.7% sequence similarity, respectively), C9 to Brevundimonas variabilis (99.1%), C11 to Brevundimonas bacteroides (98.6%), and NM4.18 to Erythromonas ursincola (98.5%).