Biology Reference
In-Depth Information
e.g., by an increased activity of DNA photolyase, or elevated expression of
psbA-gene and consequently enhanced turnover of D1. Therefore, the ecological
implications of UVB exposure can only be validated in approaches mimicking
natural radiation conditions, particularly reflecting the naturally occurring relative
ratios of the different wavelength ranges. In particular, algae and aquatic plants
from high irradiance environments (e.g., tropical shallow water ecosystems) show
distinct (sometimes even promoting) interactions of UVB- and PAR-induced
photoinhibition (Hanelt et al.
2006
; Hanelt and Roleda
2009
).
The importance of temperature for the respective UV susceptibility observed in
seaweeds has also been demonstrated; however, the level of knowledge is still far
behind compared to that existing for higher plants, in which, e.g., a chilling-induced
increase in optical UV screening has been observed, even in the absence of UVB
(Bilger et al.
2007
). This observation formed the base for intense studies on the
interactive effects of temperature and UVB exposure. Low temperatures generally
pose the problem of slowed down enzyme reactions and consequently generally
reduced metabolic activity.
In concreto
, this also applies to enzymatic repair
processes in response to UVB exposure. At low temperatures, the activity of repair
enzymes (DNA-photolyase, excision repair), as well as synthetic pathways (e.g.,
D1-turnover) might be not operative at the velocity required to keep up with rate of
damage occurring at a certain irradiance (of UVB and/or PAR); thus, accumulation
of damage will be the consequence. In turn, under conditions of reduced activity of
repair enzymes, organisms may rely more strongly on optical protection strategies.
However, studies on the photoprotective potential of phlorotannins in Arctic brown
algal zoospores and juvenile gametophytes revealed that changes in phlorotannin
content were neither affected by low nor by high temperatures (M
uller et al.
2009
;
Steinhoff et al. unpublished). Taking the high antioxidative potential of
phlorotannins (Connan et al.
2006
; Zubia et al.
2007
) into consideration,
phlorotannins might be produced and rapidly oxidized again (Steinhoff et al.
2012
). Therefore, photoprotective substances might also help to protect brown
macroalgal cells by scavenging ROS within a wide temperature range. Nevertheless,
several studies investigating the interactive effects of temperature and UV exposure
on macroalgal spores (Wiencke et al.
2006
;Muller et al.
2008
,
2009
; Steinhoff et al.
2011a
,
b
) lead to the conclusion that at low temperatures, solar radiation effects
might be better compensated than at increased water temperatures indicating the
potential impact of rising water temperatures on early macroalgal life stages.
Still, a moderately increased temperature has been found to compensate for
UVB-induced damage due to the higher activity of repair pathways. The impact of
temperature increase on UV susceptibility of photosynthetic activity was studied in
two
Ulva
species from Antarctic and subantarctic regions (Rautenberger and
Bischof
2006
). An isolate of the Antarctic/cold-temperate
Ulva bulbosa
(now
also referred to as
U. hookeriana
) was compared to the cosmopolitan
U. clathrata
by exposing them to identical conditions of UV radiation at 0
and 10
C. In both
species, exposure to 10
C almost completely compensated for the UV-induced
inhibition of photosynthetic quantum yield observed at 0
C (see Fig.
20.1
).
Observed results were striking for two reasons: (1) in
U. bulbosa
UV-induced
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