Biology Reference
In-Depth Information
1. INTRODUCTION
The conversion of solar into chemical energy by plants and cyano-
bacteria is essential to life on earth. However, when the amount of light
energy exceeds the capacity of these organisms' photosynthetic apparatus
to harness it, the light poses a threat to life. Cyanobacteria, like plants, carry
out oxygenic photosynthesis using two macromolecular assemblies, Photo-
system I (PSI) and Photosystem II (PSII), linked by an electron transport
chain. In conditions of excessive light, the photosynthetic electron transport
chain becomes stalled in a reduced state and reactive oxygen species (ROS)
are formed which leads to severe cell damage. Nutrient starvation and low
CO
2
conditions predispose photosynthetic organisms to this threat at even
relatively low irradiance.
Cyanobacteria have evolved at least two key photoprotective mecha-
nisms to cope with abrupt and fluctuating changes in the quality and
intensity of light: State transitions (reviews, e.g.
Minagawa, 2010
;
Rochaix,
2010
;
Wollman, 2001a
) and the orange carotenoid protein (OCP)-related
nonphotochemical quenching (NPQ) (previous reviews:
Bailey & Gross-
man, 2008
;
Karapetyan, 2007
;
Kerfeld, Alexandre, et al., 2009
;
Kerfeld &
Kirilovsky, 2010
;
Kirilovsky, 2007
;
Kirilovsky & Kerfeld, 2012
). Both
involve rapid changes (seconds to minutes) in the photosynthetic apparatus
and a decrease in the effective size of the PSII antenna, but by very differ-
ent means.
To understand these photoprotective mechanisms, it is necessary to
place them in the context of the distinctive light-harvesting antenna of
cyanobacteria. In cyanobacteria, the major light-harvesting antenna is an
extramembranous complex known as the phycobilisome. Phycobilisomes
are composed of several types of chromophorylated phycobiliproteins and
linker peptides needed for structural organization and function (for reviews,
see
Adir, 2005
;
Glazer, 1984
;
Grossman, Schaefer, et al., 1993
;
MacColl, 1998
;
Tandeau de Marsac, 2003
). Phycobilisomes have a trimeric core from which
rods radiate. The major core protein is allophycocyanin (APC). The rods are
more variable: in most freshwater cyanobacterial species, the rods contain
only phycocyanin (PC), whereas in many marine cyanobacteria, phycoery-
thrin (PE) or phycoerythrocyanin (PEC) are found in the distal end of the
rods. These complexes are attached to the outer surface of the thylakoid
membranes (
Gantt & Conti, 1966
) via the large, chromophorylated, core
membrane linker protein L
CM
(ApcE) (
Redlinger & Gantt, 1982
), which
