Biology Reference
In-Depth Information
(Bischof et al.
1999
; see also Chap.
1
by Hanelt and Figueroa). The potential for
acclimation is not only the precondition to endure stress caused by exposure to
harmful radiation, but is a prerequisite to establish over wide depth ranges and to
endure the seasonal variation of radiation conditions (Bischof et al.
1998b
,
1999
).
However, the process of acclimation of photosynthetic activity in Arctic brown
seaweeds to changing radiation conditions showed a distinct sequence of events,
which may be indicative for the different molecular mechanisms involved:
in
A. esculenta
under repeated UV exposure, the competence of recovery from
UV-induced photoinhibition increased after just a few exposure/recovery cycles.
This might indicate an activation of different repair mechanisms, counteracting the
impact of UV exposure by a faster replacement of damaged molecules. Moreover,
the degree of inhibition became smaller (Bischof et al.
1998b
,
1999
), which might
also be related to an activated ROS defense system counteracting UVB-mediated
oxidative stress (see Chap.
6
by Bischof and Rautenberger) or to the formation of
UV-screening compounds, like phlorotannins (Schoenwaelder
2002a
,
b
).
In fact, biosynthesis and accumulation of UV-screening substances has been
described as one of the most important physiochemical acclimation mechanisms
against biologically harmful UV radiation. In red algae, mycosporine-like amino
acids (MAAs) have been extensively studied as potent UV-screening substances
(Karsten et al.
1998
; Conde et al.
2000
). While MAAs have been mainly observed
in the Rhodophyta (Hoyer et al.
2001
; Huovinen et al.
2004
), Phaeophyta and most
Chlorophyta typically lack these compounds, with the green alga
Prasiola crispa
ssp.
antarctica
being an interesting exception and containing high concentrations of
a unique MAA with an absorption maximum at 324 nm (Hoyer et al.
2001
; Karsten
et al.
2005
). The role of MAA accumulation as an acclimatory response toward the
respective radiation environment becomes apparent from the observed decrease in
cellular concentration with increasing depth (Hoyer et al.
2001
). In general, cellular
MAA concentrations in red algae have been shown to be positively correlated with
the natural UV doses (Karsten et al.
1998
; Huovinen et al.
2004
). The flexibility of
acclimation with respect to the synthesis of UV-screening compounds is reflected
by strong seasonal as well as microscale variation in MAA contents: thus,
specimens of the red alga
Palmaria decipiens
collected in Antarctic winter
contained low concentrations of UV-absorbing compounds and exhibited signifi-
cantly higher values in summer (Post and Larkum
1993
). Furthermore, in individual
red algal specimens cross-sectional and longitudinal MAA concentration gradients
have been described depending on the respective microenvironment of radiation,
the age, or the tissue type (Karsten and Wiencke
1999
; Hoyer et al.
2001
). Based on
their ability and flexibility of MAA synthesis, red algae have been classified into
three categories (Hoyer et al.
2001
): Type I—species which completely lack the
ability to synthesize MAAs, as e.g., most of the deep-water algae; Type II—species
which synthesize MAAs in variable concentrations in response to the variation of
the respective environmental conditions, and Type III—species which always
contain high concentrations of MAAs, typically these are algal species populating
very shallow waters or even the intertidal zone and are, thus, exposed to strong
irradiances and large amplitudes of environmental variation.
