Thesis subject

Light harvesting, light adaptation and photprotection in aquatic photosynthesis studied by time-resolved fluorescence spectroscopy

PhD thesis Volha Chukhutsina, Februari 25, 2015

Photosynthesis is the biological conversion of light energy to chemical bond energy that
is stored in the form of organic carbon compounds. Photosynthetic organisms generate
about 2×1011 tons of global biomass annually (Field et al., 1998). Extensive knowledge
of this specific and highly efficient light energy conversion process is of fundamental
Although plants deliver the major contribution to global carbon fixation, the
contribution of aquatic photosynthetic organisms is sometimes overlooked:
Approximately 45% of photosynthesis occurs in aquatic environments (Falkowski,
1994; Field et al., 1998). Aquatic photosynthetic organisms often live in turbulent
waters, which leads to fluctuation light conditions as they move between different
depths. Fluctuations in the incident light intensity and spectrum can pose major
challenges to the photosynthetic apparatus. In particularly, suddenly increasing light
intensities can cause damage to the photosynthetic apparatus. Photosynthetic organisms
have photoprotection and photo-acclimation mechanisms to respond to changes in light
intensity on the short (minutes) and long (days to months) time scales, respectively.
Aquatic photosynthetic organisms, thus, provide an excellent example of the capacity of
the photosynthetic apparatus to adapt to fluctuating light conditions (Ruban et al.,
2004). The scope of this thesis is to describe light-adaptation strategies of two aquatic
model organisms: the diatom Cyclotella meneghiniana and the cyanobacterium
Synechocystis (sp. PCC 6803). Diatoms, a group of algae, are among the most abundant of
aquatic photosynthetic organisms (Mann, 1999; Yool and Tyrrell, 2003), whereas
cyanobacteria or “blue green algae” are known for their capacity to survive extreme
environmental conditions, from hypersaline bays up to Antarctic ice basins.
In this introduction, the general mechanism of photosynthesis is described first,
together with an overview of unique characteristics of photosynthesis in aquatic
organisms (section 1.1). Then some aspects of the physical process of light absorption
common to all photosynthetic pigments are discussed (section 1.2). Sections 1.3 and 1.4
describe photosynthetic pigments and pigment-protein complexes present in the
organisms discussed further on in this thesis. Various light adaptation strategies of the
photosynthetic apparatus are summarized in section 1.5. Section 1.6 is d