Biology Reference
In-Depth Information
its reason of maintenance along the evolution of algae are not well understood, its
expression has been demonstrated and apparently, under certain environmental
conditions, it can have consequences for the whole carbon metabolism of algae
(Raven
1997
).
2.4 Light-Independent Carbon Fixation
Carboxylation is not an exclusive feature of RUBISCO; seaweeds are equipped with
a suite of diverse nonphotosynthetic enzymes that, like C4 and CAM in plants, are
able to carboxylate and decarboxylate various C3 and C4 compounds. Light-
independent carbon fixation (LICF) is also called “dark carbon fixation” or
“
-site of acceptors such
as phosphoenolpyruvate (PEP) or pyruvate. Two enzymes, phosphoenolpyruvate
carboxylase (PEPC) and phosphoenolpyruvate carboxykinase (PEP-CK), are espe-
cially important in seaweeds. The role of PEP-CK, which uses CO
2
as inorganic
carbon source, in LICF has been demonstrated for various species of seaweeds, in
particular large brown algae (K
b
-carboxylation,” since inorganic carbon is fixed into the
b
uppers and Kremer
1978
; Johnston and Raven
1986
;
Cabello-Pasini et al.
2000
). In contrast to PEPC, during the PEP-CK catalysis the
energy of the phosphorylated group of PEP is saved by phosphorylation of nucleo-
side diphosphates. The first studies using radiocarbon (
14
C) in different groups of
seaweeds revealed that amino acids such as aspartate, glutamate, citrate, and alanine
were primarily
14
C labeled (Akagawa et al.
1972
; Kremer
1981
; Kerby and Evans
1983
). The formation of oxalacetic acid (OAA) as a key intermediate of the Krebs
cycle, suggested a link with anabolic processes (Kremer
1981
). In fact, an apparent
function of LICF is the replenishing of carbon via “anaplerotic” reactions, especially
when pyruvate is degraded to acetyl-CoA during glycolysis (Kremer
1981
). How-
ever, LICF reactions do not increase the net fixed carbon but are essential for cell
metabolism, i.e., the pathway provides indispensable C4 acids that are not
synthesized in the Calvin-Benson cycle.
Like photosynthetic C-fixation rates, LICF rates show considerable variation
among different seaweeds; however, there is a tendency of higher values in brown
algae compared to Chlorophytes and Rhodophytes (Table
2.1
). In Chlorophytes and
Rhodophytes, LICF rarely exceeds 1
€
mol
14
Cg
1
FW h
1
, which in terms of their
contribution to the photosynthetic carbon is normally
m
1% (Cabello-Pasini and
Alberte
1997
). In the case of brown algae, values can be considerably higher (up to
9.6
<
mol
14
Cg
1
FW h
1
), accounting for up to 48% of the photosynthetic fixation
(Kremer
1981
). Especially high LICF rates have been reported in growing thallus
areas of Laminariales (e.g.,
Laminaria
and
Lessonia
) and during the spring/summer
season for temperate and cold-temperate species. In the case of temperate red and
brown algae, values of LICF can also be important (Cabello-Pasini and Alberte
1997
). Carboxylation measured as the activity of PEP-CK is also linked to growth
requirements, especially in species with marked seasonality in growth and photo-
synthetic carbon fixation, e.g., polar seaweeds (Weykam
1996
; Weykam et al.
m
