THE POSSIBLE PHOTOSYNTHETIC USE OF SULFIDE BY THE FILAMENTOUS PHOTOTROPHIC BACTERIA OF HOT SPRINGS1

Filamentous gliding bacteria containing bacteriochlorophylls a and c occur abundantly in hot springs of alkaline pH and usually arc in direct contact with blue-green photoautotrophs even though the bacteria may have a higher growth temperature limit. In contrast, hot springs with at least 0.2 mg liter-l of soluble sulfide in water of 60-70°C support orange to green mats of these bacteria upstream from and independent of the presumed organic donation of the blue-green algae. In these cases sulfide seemed the only plausible source of rcductant in phototrophism and CO, the only likely carbon source. In the field an d in culture, the stimulatory effect of sulfide on COz photoincorporation was only a small ( 1.2-3.4-fold) increase over the nonsulfide rate. Anaerobic illuminated cultures showed an increase in dry weight and pigment in basal autotrophic medium only when sulfide was added. However, small amounts of yeast extract resulted in far greater yields. The combined observations and results suggest that photoautotrophy, with sulfide as electron donor, is a possible alternative to photoheterotrophy in this newly discovered group of phototrophic bacteria. Bacteriochlorophylls a and c in gliding filamentous microorganisms were first rcported by Pierson and Castcnholz ( 1971). Thcsc organisms constitute a previously undescribcd group of prokaryotcs subsequently rcfcrred to as “photosynthetic flexibacteria” (Castenholz 1973). However, only superficial appearance, dimensions, and gliding behavior rclatc them at this point to the nonphotosynthetic flexibactcria as characterized by Pringshcim ( 1951.) or Lewin ( 1969). Isolations of this type of organism in pure culture have originated primarily from algal-bacterial mats of hot springs with pH values above about 6.0. The characteristic orange gelatinous mat in hot springs, which underlies the top cover of blue-green algae, appears to bc composed almost entirely of photosynthetic flexibacteria (see Castenholz 1973). These mats occur at steady temperatures of from near 70°C to below the thermal range of 40-45”C. The type designated F-2 (Pierson and Castenholz 1971)) and subsequently by the generic l This work was supported by U.S. National Science Foundation Grants GB-18799 and GB31945X and by a fellowship from the John Simon Guggenheim Memorial Foundation. name ChZorofZexis2 (Pierson and Castenholz, in prep. ), occurs in hot springs the world over. It is characterized by gliding filaments 0.5 to 1.2 pm wide with a ratio of bacteriochlorophyll c to bacteriochlorophyll a ranging from about 1: 1 to 10: 1. The carotenoids of one strain (Culture J-lo-fl) are mainly p-carotene (28% ), y-carotene (23%)) variants and glycosidcs of y-carotcnc (440/o), and cchinenonc (4 s) (Halfen ct al. 1972). In the laboratory Chloroflexis grows well as an anaerobic photo-heterotroph (Pierson and Castcnholz 1971)) and it appears that the most rapid growth occurs under these conditions (Pierson and Castcnholz, in prep. ) . Aerobic hetcro trophic growth also occurs in darkness; in lighted aerobic conditions growth also occurs but it is not known to what cxtcnt this is phototrophic. Synthesis of bacteriochlorophyll apparently ccascs under aerobic conditions ( Pierson 1973) l In the early stages of isolating this organism it became apparent that it could not bc sustained alone on the defined medium used for the growth of blue-green algae. 2 This name has already been printed elsewhere in advance of proper publication. En. LIMNOLOGY AND OCEANOGRAPHY 863 NOVEMBER 1973, V. 18(6) 864 RICHARD We CASTENHOLZ This medium is inorganic except for the chelator nitrilotriacetic acid and, at times, the buffer glycylglycinc ( Castenholz 1970). However, the addition of either yeast extract or a donor blue-green algal population rcsultcd in measurable growth (Pierson and Castenholz 1971) . My observations over the past few years have been that the ChZorofZexis populations in hot spring streams are located in direct association wifth blue-green photoautotrophs except whcrc inorganic sulfide is obviously available from upstream sources. In the latter cast, mats of Chloroflexis may occur upstream of the blue-green algae. The purpose of this report is to dcmonstratc this correlation in New Zealand, Iceland, and North American hot springs and to show that sulfide does stimulate 14C-HC03 pho toincorporation and growth in these organismsthus, implying that sulfide may act as the electron donor in a photoautotrophic mode of growth. I would like to thank the Botany Dcpartment, University of Auckland, New Zealand, for providing office and laboratory space and technical assistance during part of 1970-1971. I would also like to thank the National Rcscarch Council of Iceland and the Chief Naturalist, Yellowstone National Park, for permission to collect and conduct research. MATERIALS AND METHODS