239
12.2.2 Responses of Photosynthetic Organisms to Ocean
Acidification
Chemical changes as a result of OA can influence physiological processes of organisms as well as their heredity and evolution. Marine phytoplankton (prokaryotic species and microalgae) as well as macroalgae can convert CO 2 into organic materials,
which accounts for 50% of the global primary production (Field et al. 1998). Analysis
of the ecological effects of OA on photosynthetic carbon fixation is key to understanding the relationship between OA and marine ecosystem and estimating the amount of
CO 2 absorbed by the oceans.
A number of previous studies have shown that OA may either promote growth
and photosynthesis of phytoplankton (Reibesell and Tortell 2011) or have no effect
(Tortell et al. 2000; Kim et al. 2006; Gao and Campbell 2014) or have a negative
effect on it, such as promoting mitochondrial respiration and photorespiration (Wu
et al. 2010; Gao et al. 2012b). Shipboard studies in the South China Sea during
several research cruises showed that the effect of OA on the photosynthetic carbon
fixation by phytoplankton relies on the intensity of solar radiation and the depth
distribution of phytoplankton. The increased concentration of CO 2 lowers the carbon fixation of phytoplankton communities and inhibits the growth of diatoms in
shallow waters or under high irradiances while accelerating the growth of diatoms
or augmenting carbon fixation in deeper layers or at low irradiances (Fig. 12.1).
The biological mechanism behind this phenomenon is that the increased concentration of CO 2 downregulates the active uptake capacity of inorganic carbon (CO 2 ,
HCO 3
− ) of the cells, saving energy for CO 2 -concentrating mechanisms, which promotes the growth of phytoplankton at low irradiances. Such energy saving interacts
with increased acidity to stimulate light stress under intense levels of solar radiation,
so that photorespiration can be enhanced (Gao et al. 2012a, b). On the other hand,
in order to resist OA, phytoplankton cells increase synthesis of toxic phenolics and
gain extra energy by degrading them, with mitochondrial respiration being enhanced.
The underlying mechanism is that OA promotes several metabolic pathways such as
β-oxidation, the tricarboxylic acid (TCA) cycle, glycolysis, etc. (Fig. 12.2), leading
to a higher concentration of phenolics (toxic compounds) and a higher respiratory
rate (Jin et al. 2015).
Nutrients also influence the physiological responses of phytoplankton to OA (Li
and Gao 2012; Verspagen et al. 2014). The substrate of cyanobacterial and algal
carboxylation is CO 2 , while more than 90% of inorganic carbon in the surface ocean
is HCO 3
− . Thus, if seawater carbonate chemistry changes (pCO 2 rises, pH decreases
etc.), utilization of inorganic carbon and the energy to maintain an intracellular acidbase balance will be affected (Mackey et al. 2015). Furthermore, OA can influence
some metabolic pathways of harmful red tide species with regard to toxin production. For example, under OA conditions, more toxic compounds were accumulated
in the cells of Pseudo-nitzschia (Sun et al. 2011) and of toxic dinoflagellates
(Hattenrath-Lehmann et al. 2015).
12 Effects of Ocean Acidification and UV Radiation on Marine Photosynthetic…
12.2.2 Responses of Photosynthetic Organisms to Ocean
Acidification
Chemical changes as a result of OA can influence physiological processes of organisms as well as their heredity and evolution. Marine phytoplankton (prokaryotic species and microalgae) as well as macroalgae can convert CO 2 into organic materials,
which accounts for 50% of the global primary production (Field et al. 1998). Analysis
of the ecological effects of OA on photosynthetic carbon fixation is key to understanding the relationship between OA and marine ecosystem and estimating the amount of
CO 2 absorbed by the oceans.
A number of previous studies have shown that OA may either promote growth
and photosynthesis of phytoplankton (Reibesell and Tortell 2011) or have no effect
(Tortell et al. 2000; Kim et al. 2006; Gao and Campbell 2014) or have a negative
effect on it, such as promoting mitochondrial respiration and photorespiration (Wu
et al. 2010; Gao et al. 2012b). Shipboard studies in the South China Sea during
several research cruises showed that the effect of OA on the photosynthetic carbon
fixation by phytoplankton relies on the intensity of solar radiation and the depth
distribution of phytoplankton. The increased concentration of CO 2 lowers the carbon fixation of phytoplankton communities and inhibits the growth of diatoms in
shallow waters or under high irradiances while accelerating the growth of diatoms
or augmenting carbon fixation in deeper layers or at low irradiances (Fig. 12.1).
The biological mechanism behind this phenomenon is that the increased concentration of CO 2 downregulates the active uptake capacity of inorganic carbon (CO 2 ,
HCO 3
− ) of the cells, saving energy for CO 2 -concentrating mechanisms, which promotes the growth of phytoplankton at low irradiances. Such energy saving interacts
with increased acidity to stimulate light stress under intense levels of solar radiation,
so that photorespiration can be enhanced (Gao et al. 2012a, b). On the other hand,
in order to resist OA, phytoplankton cells increase synthesis of toxic phenolics and
gain extra energy by degrading them, with mitochondrial respiration being enhanced.
The underlying mechanism is that OA promotes several metabolic pathways such as
β-oxidation, the tricarboxylic acid (TCA) cycle, glycolysis, etc. (Fig. 12.2), leading
to a higher concentration of phenolics (toxic compounds) and a higher respiratory
rate (Jin et al. 2015).
Nutrients also influence the physiological responses of phytoplankton to OA (Li
and Gao 2012; Verspagen et al. 2014). The substrate of cyanobacterial and algal
carboxylation is CO 2 , while more than 90% of inorganic carbon in the surface ocean
is HCO 3
− . Thus, if seawater carbonate chemistry changes (pCO 2 rises, pH decreases
etc.), utilization of inorganic carbon and the energy to maintain an intracellular acidbase balance will be affected (Mackey et al. 2015). Furthermore, OA can influence
some metabolic pathways of harmful red tide species with regard to toxin production. For example, under OA conditions, more toxic compounds were accumulated
in the cells of Pseudo-nitzschia (Sun et al. 2011) and of toxic dinoflagellates
(Hattenrath-Lehmann et al. 2015).
12 Effects of Ocean Acidification and UV Radiation on Marine Photosynthetic…