一类在厌氧、光照条件下进行不放氧光合作用，以光为能源、有机物为供氢体，不能以CO2作为主要或唯一碳源的
光合细菌.细胞内除细菌叶绿素外还含有较多类
胡萝卜素，故菌体不显绿色而为紫红色或褐色.有些菌在有光、无氧、利用光能同化CO2；在有氧、无光、存在有机物时，亦可利用有机物为碳源和能源，不合成光合色素，无光合作用；故属兼性光合细菌.光能异养细菌主要有两属，
红假单胞菌属（Rhodopseudomonas），细胞杆状；
红螺菌属（Rhodospirilum），细胞螺旋状.分布于海水或淡水中.Aclassofanaerobic,lightandholdundertheconditionsofoxygenphotosynthesistolightenergy,organicmatterasahydrogendonorbody,unabletoCO2asthemainorsolecarbonsourceforphotosyntheticbacteria.Inadditiontobacterialcellsalsocontainchlorophyllthingsmorecarotenoids,andthereforenosignificantbiomassforgreen,purpleorbrown.Somebacteriainthelight,oxygenandlowconcentrationsofH2Sconditions,canalsouseHydrogensulfideasabody,usinglightenergyassimilationofCO2;Inaerobics,nolight,thereisorganicmatter,mayalsouseorganiccompoundsascarbonandenergy,notsynthesisofphotosyntheticpigments,nophotosynthesis;wasthereforeconsideredfacultativephotosyntheticbacteria.lightheterotrophicbacteriamainlybelongtotwo,redpseudo-monas(Rhodopseudomonas),rod-shapedcells;Rhodospirillumsp(Rhodospirilum),cellspiral.foundinseawaterorfreshwater.