Biology Reference
In-Depth Information
1.2 Photosynthesis Under Limiting Light Conditions
Sublittoral species growing in deep water must be adapted to chronic low light. For
planktonic algae, generally a lower depth limit of 1% of the surface irradiance is
regarded as defining the euphotic zone (Steemann Nielsen
1975
), and this limit can
be even 0.1% for picoplankton in oceanic waters. Seaweeds of the order
Laminariales
reach their lower growth limit at about 0.6-1.2% of surface light,
whereas for some deep growing rhodophytes a minimum of 0.001-0.05% was
determined (L
uning
1981
,
1990
). The photosynthetic rate of different
Laminaria
species exceeds the respiratory rate and thus the compensation point (
E
c
) at about
5-8
€
mol m
2
s
1
, whereas in deep water red algae an irradiation of about
m
mol m
2
s
1
is already sufficient (L
2
uning
1981
). Deepest crustose macroalgae
seem to survive at an absolute light minimum of about 0.01
€
m
mol m
2
s
1
(Littler
et al.
1986
). These red crustose corallines show a large light absorptance and
employ light-harvesting pigments with a high energy-cost in their production per
unit light absorption rate in a given underwater spectrum (Raven and Geider
2003
).
Raven et al. (
2000
) outlined that it is difficult to explain growth of algae below
0.5
m
mol m
2
s
1
as there are energy-consuming reactions which use an increasing
fraction of energy input when photon flux density decreases. Among these pro-
cesses are redox back reactions of reaction center II, the leakage of H
+
through
thylakoid membranes and the turnover of photosynthetic proteins. The first of the
two processes limit the rate of linear electron transport and ADP phosphorylation,
while the latter consumes ATP. Thus, it is not yet clear how crustose red algae can
grow down to 274-m water depth where the average incident photon flux density for
12 h day
1
does not exceed 0.02
m
mol m
2
s
1
(Raven and Geider
2003
).
Low light adapted species are typically characterized by high photosynthetic
efficiencies and very low light compensation (
E
c
) and saturation points (
E
k
),
ranging between 1 and 15 and between 14 and 52
m
mol photons m
2
s
1
, respec-
tively (Dunton and Jodwalis
1988
; Wiencke et al.
1993
; Weykam et al.
1996
;
Brouwer
1996
; Eggert and Wiencke
2000
).
For a certain time period net photosynthetic rates need to be high enough that
seaweeds can promote growth, reproduction, as well as enough energy storage to
cope with reduced light availability also during the winter season. Using data on
daily changes of in situ irradiance and P-E derived parameters such as photosyn-
thetic capacity (
P
max
), dark respiration,
E
c
and
E
k
, it is possible to calculate the
daily periods over which carbon (C) assimilation exceeds C losses due to respira-
tory activity at different water depths. This metabolic C balance is regarded to
determine the lower distribution limit of algae. Due to low
E
k
values, species from
King George Island (Antarctica) are exposed during spring-summer for periods
m
12 h day
1
at about 20 m depths to irradiances above saturation (period of
saturation
H
sat
). Under these conditions, species such as
Palmaria decipiens
or
Desmarestia anceps
achieve positive C balances about 3 mg C g
1
FW d
1
.
At 30-m depth daily light availability decreases to values between 7 and 9 h in
summer, but C balance is still positive in 4 of 5 studied species (G´mez et al.
1997
).
