Fróðskaparrit - 01.01.1999, Side 168

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HIGHINORGANIC CARBON EXTRACTION CAPACITY OF SUBMERGED
MACROPHYTES FROM SOFTWATER FAROESE LAKES
duces the ecological significance of HC03"
use, in particular in soft-water and acid
lakes and streams.
C-4 photosynthesis has so far been found
in Hydrilla verticillata only (Reiskind et
al., 1997). Hydrilla lacks Krantz anatomy
to separate the two carboxylation systems
of C-4, as known from terrestrial C-4
plants. Instead, the two systems appear to
be separated at the cell level, as revealed by
immunocytochemical gold-labelling and
fluorescence techniques (Reiskind et al.,
1989). C-4 carboxylation and phospho-
enolpyruvate carboxylase (PEPcase) is
found in the cytosol and C-3 carboxylation
and ribulose biphosphate carboxylase-oxy-
genase is confined to the chloroplasts. The
ecophysiological benefit of C-4 is the sup-
pression of photorespiration, through
which 50% of the fixed carbon may be lost
as respiratory C02 in C-3 plants.
A number of submerged macrophytes,
most belonging to the isoetids (Keeley,
1996), have developed an inorganic carbon
uptake system that resembles the terrestrial
CAM plant system and allows the plants to
take up inorganic carbon throughout most
of the light/dark cycle ((Madsen, 1987a).
The main benefit of CAM for isoetids in
soft-water lakes with low inorganic carbon
concentrations is by far the extended period
available for carbon uptake. However, it ap-
pears that CAM may also have a stimulat-
ing effect on net photosynthesis, due to an
inhibitory effect on photorespiration
caused by the high intemal C02 concentra-
tions encountered during decarboxylation
(Madsen, 1987a).
The physiological and biochemical
adaptations in submerged macrophytes as
described above are believed to be directed
towards maximising inorganic carbon up-
take efficiency. The trade-off is enhanced
mnning costs for inorganic carbon uptake
and the need for investments in uptake sys-
tems. The ability to use CCM is not a con-
stant feature, however, but appears to be
regulated in response to growth conditions
and only shows high activity when needed,
i.e. when the availability of inorganic car-
bon is low relative to the demand. Thus, bi-
carbonate-users often down-regulate their
affinity and capacity for HC03-uptake
when grown at high C02 concentration
(Sand-Jensen and Gordon, 1993). Similar-
ly, CAM activity has been found to be sup-
pressed for plants growing at high C02 or
in low light, where the inorganic carbon re-
quirement is low (Madsen, 1987b).
In the present study, we measured the
carbon extraction capacity for a range of
submerged macrophyte species from lakes
of the Faroe Islands. The lakes are all soft-
water and low in inorganic carbon (H.-H.
Schierup, pers. comm.) and support a sub-
merged macrophyte flora that includes nu-
merous species known from the literature
to have the potential for using bicarbonate
in photosynthesis (Hansen, 1966; Maberly
and Spence, 1983). The specific objectives
of the study were 1) to examine if species
known to be bicarbonate users in alkaline
lakes maintain the ability in the more soft-
water, Faroese lakes, where the low affinity
for HC03' may restrict the ecological ad-
vantage of HC03" use, and 2) to test
whether the extraction capacity of the
plants was related to lake water alkalinity.