Biology Reference
In-Depth Information
19.5.1 Noncalcifying Seaweeds
CCM activity in algae is in most cases associated with the ability to use, indirectly
or directly, HCO 3 ions (e.g., Smith and Bidwell 1989 ;Bj
ork et al. 1992 , 1993 ;
Axelsson et al. 1999 , 2000 ; Mercado et al. 2006 ). Since the shift in DIC species due
to increased atmospheric CO 2 will result in a small proportional change in HCO 3
compared to CO 2 and CO 3 2 concentrations, this will confer no/little advantage on
species relying on HCO 3 use for carbon acquisition (Beardall et al. 1998 ).
Consequently, species with an active CCM are unlikely to show stimulation of
photosynthesis or growth with an increase in atmospheric CO 2 levels. Conversely,
when modern day terrestrial C 3 plants were grown at Glacial CO 2 concentrations
(180-200 ppm), they exhibited a reduction in photosynthesis and growth, and
delayed reproduction (Ward 2005 ; Gerhart and Ward 2010 ). Further long-term
studies on seaweed's response to high CO 2 of the future and low CO 2 of the past
may provide crucial understanding on their adaptive capability to changing CO 2
over geological and evolutionary timescales.
Macroalgae grown at elevated CO 2 showed downregulation of CCM activity
(e.g., Fucus serratus , Johnston and Raven 1991 ; Ulva lactuca , Magnusson et al.
1996 ) and the energy savings from downregulating CCM are hypothesized to
increase growth, but no increase in growth rate was observed in brown seaweed
species with known active CCMs, e.g., Laminaria digitata and Saccharina
latissima , grown under Glacial, Preindustrial, and 750 ppm CO 2 concentrations
(Roleda MY, Stecher A, Gutow L, Bartsch I and Wiencke C, unpublished data).
Among red seaweeds, photosynthesis and growth rates of Porphyra yezoensis
in culture were enhanced under high (1,000 and 1,600 ppm) CO 2 concentrations
(Gao et al. 1991 ). Enhancement of growth with increasing CO 2 is also reported in
Gracilaria species (Gao et al. 1993b ). For the CO 2 -user Lomentaria articulata ,
higher relative carbon growth (52%) was measured under 2
of the current ambient
CO 2 compared to its growth (23%) under 5
ubler et al. 1999 ).
At a high photon flux density (PFD), the seawater concentration of DIC becomes
insufficient to saturate photosynthesis of intertidal algae (Mercado et al. 2001 ).
Therefore, when photosynthetic rate exceeds the rate at which CO 2 is supplied,
photosynthesis becomes inhibited (Murru and Sandgren 2004 ). Under such carbon-
limiting conditions, employing the energetically expensive CCM becomes
ecologically advantageous making most intertidal macroalgal species DIC
saturated (e.g., Surif and Raven 1989 , 1990 , Beer 1994 ). Carbon acquisition in 38
species of red macroalgae showed that intertidal species used both dissolved CO 2
and HCO 3 while subtidal algae are typically restricted to the use of DIC in the
form of dissolved CO 2 (Murru and Sandgren 2004 ). Another study showed that
three species of intertidal Gelidiales have a low affinity for HCO 3 in their natural
habitat (Mercado et al. 2001 ).
Because DIC availability is relatively high and replenished from carbonates in
shells and rocks, DIC-limited aquatic photosynthesis is considered rare; however,
there is evidence that some subtidal species are DIC limited, e.g., Dilophus
CO 2 (K
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