Characterization of a novel chlorophyll b-containing Prochlorothrix species (Prochlorophyta) and its photosynthetic apparatus

A filamentous, nondifferentiated, unbranched bacterium designated strain NI VA-8/90 originating from Lake Malaren, Central Sweden, contained both Chl a and Chl b and was classified as Prochlorothrix sp. The ultrastructural sim ilarity with Prochlorothrix hollandica was evident from (1) the honeycomb-l ike subterminal aggregates of gas vesicles; (2) the Sayer-type junctions be tween the cytoplasmic membrane and the outer membrane of the cell envelope; and (3) the thylakoid centre-type orientation of intracytoplasmic lamellae . Among the ultrastructural aspects hitherto not reported in prochlorophyte s were (4) 10 nm viridosomes localized with 20 nm periodicity on the outer surface of the thylakoids (the identity of these structures with mini-phyco bilisomes was disproved by the absence of detectable phycobiliproteins; and (5) RuBisCO-immunopositive inclusions located in proximity to the thylakoi ds (the apparent absence of a membrane shell makes them analogous to pyreno ids). Despite close similarity of molecular details (Chl a:Chl b and PS1:PS 2 ratios; the set of carotenoids; the uncommon position of the long wavelen gth-maximum of total Chl), and in accordance with the above microscopic dis tinctions, this novel prochlorophyte can be distinguished from P. hollandic a under the provisional name Prochlorothrix scandica.