Biology Reference
In-Depth Information
yellowish color to the water body around and thus may increase UVB absorption in
the water, before it may reach algal specimens (P
´
rez-Rodr
´
guez et al.
1998
,
2001
).
However, an extended screening of 71 green macroalgal species revealed that in
fact optical UV screening in green seaweeds is rather exceptional (Pescheck et al.
2010
), and thus green algae largely need to rely on alternative mechanisms to resist
the harmful effects of UV exposure. An alternative strategy to achieve optical
shielding on a cellular base is the arrangement of specimens in dense assemblages,
e.g., macroalgal mats. In these systems which are usually formed by highly
opportunistic species (e.g.,
Ulva, Chaetomorpha
) under high nutrient loads and
low wave exposure, physiologically healthy and productive algal material is
shielded by the algal layers above, generating a steep small-scale gradient of
UVB and PAR, but also chlorophyll content and photosynthetic performance in
those assemblages (Bischof et al.
2002
,
2006b
). A prominent example for the
effectiveness of this strategy is found in mats of
Ulva rotundata
from southern
Spain (Bischof et al.
2002
,
2003
). Thus, this protection by canopy arrangement does
not require the synthesis of specific UV-screening compounds; however, it is at the
expense of the top algal layers.
20.4
Interaction with Other Abiotic Factors
Seaweeds and particularly those inhabiting the intertidal and shallow subtidal are
exposed to (inter-) dependently changing abiotic parameters; thus the interactive
effects of UV radiation in concert with the change of other abiotic parameters are
important to address. In the early studies on UV effects on seaweed physiology
conducted under laboratory conditions, the proper ratio of radiative power of the
different wavelength ranges was largely neglected and the significance to the
effects observed was largely underestimated. Among others, this was shown in a
study conducted on the green macroalga
Ulva lactuca
isolated from Greenland
(Fredersdorf and Bischof
2007
). Here algal material was exposed to a constant
irradiance of UVB radiation, however, at different irradiances of background
photosynthetically active radiation (PAR). The different effects on algal photosyn-
thesis of the respective wavelengths ranges (PAR, UVA, UVB) were identified by
the combination of various cutoff filters. In this study, the importance of back-
ground PAR for the impact of UVB effects observed became evident: The extent of
UVB-induced inhibition of photosynthetic quantum yield was highly dependent on
the irradiance of PAR in the setup and was diminishing with increasing PAR
irradiance. However, due to the different mechanisms of PAR- and UVB-induced
inhibition of photosynthesis, the additional effect of UVB rather became apparent
in the recovery kinetics from photoinhibition, resulting in a delay of recovery under
UVB exposure, presumably due to the increased defragmentation of photosynthetic
units, i.e., the D1 reaction centre protein. In contrast, increased levels of PAR as
well as UVA may provide an additional energy input to fuel simultaneously
operating repair processes counteracting the damaging impact of UV exposure,
