Biology Reference
In-Depth Information
or geographical region, carbon fixation reaches maxima of close to 40-50
mol
14
Cg
1
FW h
1
. However, age and thallus part can be relevant components of
variability. For example, in complex morphs (e.g., some red algae and large brown
algae)
14
C fixation can considerably increase in mature thallus regions (which attain
a well-developed photosynthetic apparatus) compared to meristematic (growing)
zones (Kuppers and Kremer
1978
;G
´
mez et al.
2007
). At a molecular level, the
number of active sites (~4-8 mM), the concentrations of CO
2
, levels of O
2
(which
competes with CO
2
), and RuBP are key factors determining the in vivo kinetics of
RUBISCO (Woodrow and Berry
1988
). In contrast to the terrestrial C4 plants
enriching the concentration of CO
2
via decarboxylation of C4-acids (van
Caemmerer and Furbank
2003
), seaweeds increase the availability of inorganic
CO
2
to RUBISCO (and in parallel inhibits the oxygenase activity of the enzyme)
through the action of CCMs (Raven
2010
).
m
2.3.3 Photorespiration
The oxygenase property of RUBISCO, mainly of organisms with diffusive entry of
CO
2
, is a relevant topic in photosynthetic physiology. In fact, RUBISCO catalyzes
the competitive oxidation of RuBP by fixation of O
2
to RuBP to form glycolate and
PGA, a pathway-denominated C2 oxidative photosynthetic carbon cycle, which
coexists with the Calvin-Benson cycle. In strict sense, the term “photorespiration”
implies the consumption of O
2
and release of CO
2
in the light and thus the process
depends on the CO
2
/O
2
balance, the so-called CO
2
compensation. At low partial
pressure of CO
2
and high O
2
, photosynthetic carbon fixation is competitively
inhibited by the oxygenase activity of RUBISCO with formation of CO
2
from the
metabolism of glycolate (Raven et al.
2005
). Thus, photorespiration is integrated in
the whole photosynthetic carbon metabolism (Tolbert
1997
).
Photorespiration in seaweeds has been less studied than in other photosynthetic
organisms, probably because seaweeds exhibit CCMs. However, an effect of O
2
on
carbon fixation has been demonstrated for some representative seaweeds, which
exhibit ratios of oxygenase to carboxylase activities between 0.1 and 0.25 (Raven
1997
; Giordano et al.
2005
). In seaweeds physiologically resembling C3 plants, e.g.,
understory red algae that acquire carbon via diffusive CO
2
entry, the effects of
photorespiration on photosynthetic carbon fixation are higher than in other groups
(Raven
2010
). In addition, the detection of various enzymes involved in the
glycolate metabolism (e.g., P-glycolate phosphatase, glycolate oxidase, and
glycolate dehydrogenase) (Gross
1990
; Suzuki et al.
1991
) as well as some of their
products in different seaweeds (Reiskind et al.
1988
) suggests that photorespiratory
carbon oxidation is widespread in these organisms and in many ways similar to
terrestrial plants (Raven
1997
). The fate of glycolate in the cell, which includes its
oxidation to glyoxylate in peroxisomes and further conversion to amino acids and
CO
2
, has been studied only in some seaweeds (Iwamoto and Ikawa
1997
). Overall,
although the implications of the photorespiratory pathway for seaweed ecology and
