Biology Reference
In-Depth Information
artificial UVR reduces photosynthetic rates in deep water plants significantly while
photosynthesis in the same species collected from shallow waters is unaffected. To
cope with higher irradiance levels in shallow waters a capability for fast recovery
from light stress is one prerequisite (Hanelt
1998
).
The acclimation potential of maximal quantum yield of photosynthesis to chang-
ing radiation conditions was studied in detail in the Arctic/cold temperate brown alga
Alaria esculenta
(Bischof et al.
1999
). In this species, acclimation to changing
radiation conditions occurs within very few days. This is of great ecological impor-
tance as algae, subjected to 6 months of darkness during the Arctic winter and sea ice
with snow cover shielding the algae from solar radiation in spring, become suddenly
exposed to high radiation, and thus also to UV, as soon as sea ice breaks up.
A. esculenta
shows two different responses involved in the acclimation of maximal
quantum yield of photosynthesis. At first, after a few days of exposure to artificial
UVR, the recovery from induced photoinhibitory processes proceeds significantly
faster; later, the degree of photoinhibition decreases. This implies again that different
molecular mechanisms are involved in photoacclimation. In conclusion, photo-
synthesis of macroalgae from the intertidal zone is rather resistant to natural UV-B
radiation. Algae from the upper sublittoral seem to be able to acclimate rapidly to fast
changes of solar irradiance, consequently reducing the adverse effects of UVR
exposure or even need UV-B as induction for repair processes (Hanelt and Roleda
2009
). Deep water algae react highly sensitive to UVR (Bischof et al.
2000b
),
but, due to the absorption of the water body, UV-B is no natural component of the
ambient light at higher depths. However, it is important to note that UVR may also
exert adverse effects on the algae, e.g., growth rate reduction and/or reproductive
success which is not reflected by a reduced photosynthetic activity.
Two aspects have received little attention so far. In many Polar species growth
rates are highest in Spring (Wiencke
1990a
,
b
; see also Chap.
13
by Wiencke and
Amsler), which are also affected by UVR, and not only photosynthesis of young
thallus parts (Dring et al.
1996
; Wood
1987
).
Moreover, some species from the
Arctic partly reproduce in spring. During this time, algal spores were found to be the
most light-sensitive life history stage of the studied brown algae and are strongly
affected by increased UV-B radiation, both in respect to their photosynthetic
performance and their susceptibility to DNA damage (Wiencke et al.
2000
). As
has been widely publicized, increased UV-B due to ozone depletion occurs mainly
in the Polar Spring due to atmospherical and geographical reasons. Therefore, Polar
species will be most exposed to the anthropogenic increase of UV-B radiation.
1.5 Light Absorption and Light Spectrum
The color of the algae is mainly based on the accessory photosynthetic pigments of
the LHC. Engulfment of a cyanobacterium or eukaryotic microalga by a process
called endosymbiosis is a cause of physiological, structural and genetic adaptation
of the different types of chloroplasts in evolution. The brown color typical for the
