244
12.3.2 The Effect of UV on Photosynthetic Carbon Fixation
UV can damage the DNA of phytoplankton (Häder and Gao 2015) and inhibit its
repair (Gao et al. 2008; Rastogi et al. 2014) as well as negatively affect its physiological metabolism. Moreover, UV can also produce active free oxygen radicals
indirectly which causes oxidative stress in the cells (Häder et al. 2014), which may
lower the photosynthesis rate (Wu et al. 2005). Solar UV also changes the morphology of filamentous cyanobacteria and causes their death (Wu et al. 2005). Although
in most areas UV-B accounts for less than 1% of the total solar energy, its harmful
effect is usually stronger than that of UV-A, which accounts for about 6–8% of the
total solar energy. However, when the water is mixed or solar radiation is fluctuating, the inhibition of photosynthetic carbon fixation by UV is significantly lower
(Li et al. 2013). During a long evolution process, phytoplankton has acquired
mechanisms to resist UV radiation, for example, the synthesis of the UV-shielding
pigments such as mycosporine-like amino acids (MAAs), the removal mechanism
of active oxygen free radicals, and the repair mechanisms of proteins and DNA
(Häder et al. 2014; Rastogi et al. 2014). However, in the surface layer, phytoplankton cells are inevitably affected by UV, resulting in a decrease of carbon fixation
(Helbling et al. 2003; Häder and Gao 2015). But in some cases, such as on cloudy
days or in a relatively deeper layer, where the solar radiation is reduced to moderate
or low levels, UV radiation may even stimulate carbon fixation (Fig. 12.3). It has
been demonstrated that phytoplankton assemblages in coastal waters can utilize
UV-A to drive their photosynthetic carbon fixation (Gao et al. 2007a, b). Further
investigations showed that, after filtering out the visible light, a diatom-dominated
phytoplankton community can still carry out photosynthetic carbon fixation, and the
fixation rate steadily rises with increased UV-A intensity. However, the presence of
UV-B decreases carbon fixation (Fig. 12.3a). The positive effect of UV-A is related
to the intensity of solar radiation received by the phytoplankton and the size of the
cells (Li et al. 2011a; Li and Gao 2013). Usually, phytoplankton species with larger
cell sizes show higher capacity to utilize UV-A to fix carbon.
12.4 The Combined Effects of OA and UV Radiation
The oceanic upper mixed layer is facing the pressure of OA, rising temperatures and
enhanced UV radiation, etc. Due to the lack of data related to effects of multiple
global change drivers (Riebesell and Gattuso 2015), it is difficult to predict the trend
of biological carbon fixation and the efficiency of the biological carbon pump.
Although there is a large body of papers on the effects of OA or UV on the physiology of marine primary producers, little knowledge has been gained in understanding the compounding/coupling effects of OA and UV (Beardall et al. 2014).
The photosystems of plants show rapid responses to UV radiation. For example,
the high radiation at noon lowers the PSII photochemical efficiency within a few minutes. Combined effects of OA and UVR (280–400 nm) significantly lower the PSII
K. Gao and D.-P. Häder
12.3.2 The Effect of UV on Photosynthetic Carbon Fixation
UV can damage the DNA of phytoplankton (Häder and Gao 2015) and inhibit its
repair (Gao et al. 2008; Rastogi et al. 2014) as well as negatively affect its physiological metabolism. Moreover, UV can also produce active free oxygen radicals
indirectly which causes oxidative stress in the cells (Häder et al. 2014), which may
lower the photosynthesis rate (Wu et al. 2005). Solar UV also changes the morphology of filamentous cyanobacteria and causes their death (Wu et al. 2005). Although
in most areas UV-B accounts for less than 1% of the total solar energy, its harmful
effect is usually stronger than that of UV-A, which accounts for about 6–8% of the
total solar energy. However, when the water is mixed or solar radiation is fluctuating, the inhibition of photosynthetic carbon fixation by UV is significantly lower
(Li et al. 2013). During a long evolution process, phytoplankton has acquired
mechanisms to resist UV radiation, for example, the synthesis of the UV-shielding
pigments such as mycosporine-like amino acids (MAAs), the removal mechanism
of active oxygen free radicals, and the repair mechanisms of proteins and DNA
(Häder et al. 2014; Rastogi et al. 2014). However, in the surface layer, phytoplankton cells are inevitably affected by UV, resulting in a decrease of carbon fixation
(Helbling et al. 2003; Häder and Gao 2015). But in some cases, such as on cloudy
days or in a relatively deeper layer, where the solar radiation is reduced to moderate
or low levels, UV radiation may even stimulate carbon fixation (Fig. 12.3). It has
been demonstrated that phytoplankton assemblages in coastal waters can utilize
UV-A to drive their photosynthetic carbon fixation (Gao et al. 2007a, b). Further
investigations showed that, after filtering out the visible light, a diatom-dominated
phytoplankton community can still carry out photosynthetic carbon fixation, and the
fixation rate steadily rises with increased UV-A intensity. However, the presence of
UV-B decreases carbon fixation (Fig. 12.3a). The positive effect of UV-A is related
to the intensity of solar radiation received by the phytoplankton and the size of the
cells (Li et al. 2011a; Li and Gao 2013). Usually, phytoplankton species with larger
cell sizes show higher capacity to utilize UV-A to fix carbon.
12.4 The Combined Effects of OA and UV Radiation
The oceanic upper mixed layer is facing the pressure of OA, rising temperatures and
enhanced UV radiation, etc. Due to the lack of data related to effects of multiple
global change drivers (Riebesell and Gattuso 2015), it is difficult to predict the trend
of biological carbon fixation and the efficiency of the biological carbon pump.
Although there is a large body of papers on the effects of OA or UV on the physiology of marine primary producers, little knowledge has been gained in understanding the compounding/coupling effects of OA and UV (Beardall et al. 2014).
The photosystems of plants show rapid responses to UV radiation. For example,
the high radiation at noon lowers the PSII photochemical efficiency within a few minutes. Combined effects of OA and UVR (280–400 nm) significantly lower the PSII
K. Gao and D.-P. Häder