A new type of complementary chromatic adaptation exemplified byPhormidium sp. C86: Changes in the number of peripheral rods and in the stoichiometry of core complexes in phycobilisomes

Abstract

The marine cyanobacteriumPhormidium sp. strain C86 changes the phycobilisome type depending on light quality. Red-light-adapted cells contained hemidiscoidal phycobilisomes with a photosystem II:phycobilisome ratio of 2.2, while green-light-adapted cells exhibited hemiellipsoidal phycobilisomes with a photosystem II:phycobilisome ratio of 4.4, as determined by a combined analysis of freeze-fractured thylakoid membranes and ultrathin sections and by photochemical determinations of photosystems and phycobilisomes. Core complexes of phycobilisomes of red- and green-light-adapted cells were isolated by affinity chromatography and were subsequently separated into two allophycocyanin-containing fractions. The high-molecular-weight fraction, with a sedimentation coefficient of 24 S and a calculated mol. wt. of 860,000, contained complexes of the quaternary structure (α 9 AP β 8 AP β19.5AP)2·(LCM)2 and tricylindrical shape, previously designated APCM. This fraction was similar in size in red- and green-light-adapted cells; however, differences were detected in the low-molecular-weight allophycocyanin fraction containing the “trimeric” complexes with a sedimentation coefficient of 6 S. As shown by comparison of spectral and stoichiometric data of intact phycobilisomes and isolated core complexes, the amount of the αAPB-containing core complex (α 2 AP αAPBβ 3 AP ·L C 10 ) was greater in core fractions of green-light phycobilisomes, whereas the amount of the core complexes (α 3 AP β 3 AP ·L C 10 ) designated AP·L C 10 , was higher in cores of red-light phycobilisomes.Phormidium sp. is the first organism examined that exhibits a new type of complementary chromatic adaptation by altering the composition of the phycobilisome core and the number and composition of peripheral rods and by changing the ratio of photosystem II to phycobilisomes. A model summarizing the structural consequences of the results is presented.