Environmental Engineering Reference
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result in an increase in photosynthesis rates of seaweeds, and consequently
increased growth rates. This response to acidifi cation of the seawater would
be positive in the case of CO 2 -limited seaweeds; however the response may
be very different depending on the species.
According to the results of various experiments, the response of seaweed
photosynthesis against high concentrations of CO 2 in water is highly
variable and species-specifi c. Some species have lower photosynthetic rates
under culture conditions with high concentrations of CO 2 (Johnston and
Raven 1990, Garcia-Sanchez et al. 1994). Other studies have demonstrated
higher photosynthesis rates and increases in growth rate of red algae in
presence of double levels of CO 2 than today's atmospheric concentrations
(Gao et al. 1991, Andría et al. 1999, Kübler et al. 1999, Zou 2005); however,
the opposite effect has been found in other species of red seaweeds (Garcia-
Sanchez et al. 1994, Mercado et al. 1999, Israel et al. 1999). Furthermore,
studies on species from the three main groups of seaweed have shown that
growth rates in the presence of CO 2 enriched seawater could be similar to
those obtained in seawater without enrichment (Israel and Hophy 2002).
Many seaweeds have carbon concentration mechanisms, allowing them to
use bicarbonate for photosynthesis (Gao and McKinley 1994, Beardall et al.
1998). The general consensus is that seaweeds having these mechanisms
would not increase their productivity in the future conditions of high CO 2
(Beardall et al. 1998). Some experiments have also found that seaweeds can
acclimate to achieve high concentrations of CO 2 decreasing the content of
their pigments (bleaching), similarly to the response of seaweeds to high
irradiance conditions (Garcia-Sanchez et al. 1994, Mercado et al. 1999,
Andría et al. 1999, 2001).
Intertidal seaweeds experience the daily exposure to air and during this
emersion period they must use the atmospheric CO 2 in the photosynthesis
process which diffuses much faster than in water (Raven 1999). For these
species living in the intertidal, the increase in atmospheric CO 2 levels may
have advantages over other species by increasing their photosynthetic
production during the periods of emersion (Gao et al. 1999, Zou and Gao
2002, 2010, Zou et al. 2007). In these cases, growth responses to increases in
the availability of CO 2 could be much lower compared to subtidal species
(Beardall et al. 1998).
There is also evidence that increased CO 2 concentrations in the seawater
may increase nitrogen assimilation rates in the form of nitrate in red, brown
and green algae, which could be related to higher nitrogen requirement to
withstand high growth rates (Gao et al. 1993b, Gordillo et al. 2001, Zou et
al. 2001, Zou 2005).
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