Biology Reference
In-Depth Information
Similar traits of optical protection against UVB exposure can also be found in
brown seaweeds, however, based on another class of chemical compounds, the
so-called phlorotannins (Ragan and Glombitza
1986
). Phlorotannins have been
invoked as multifunctional compounds acting in deterring herbivores and microbes,
in adhesion and in strengthening algal cell walls (Schoenwaelder
2002b
), and in
absorbing UV radiation (Ragan and Glombitza
1986
; Pavia et al.
1997
; Henry and
Van Alstyne
2004
). Phlorotannins do also possess a high antioxidant activity (e.g.,
Ahn et al.
2007
) and are, thus, important for scavenging ROS. With respect to the
acclimation of brown seaweeds toward exposure to detrimental UV radiation
several strategies have been described in which phlorotannins do play a vital role:
(1) a generally high tissue concentration of phlorotannins acting as UV screen
(Schoenwaelder
2002a
,
b
), (2) an induction of phlorotannins in response to the
environmental radiation conditions (Halm et al.
2010
), (3) an exudation of
phlorotannins and other phenolic compounds into the surrounding medium creating
an UV-absorbing microenvironment (P´rez-Rodr´guez et al.
1998
; Roleda et al.
2010
; Steinhoff et al.
2011b
), or (4) an excess inclusion of phlorotannins in cell
walls shielding harmful radiation (Schoenwaelder
2002b
). An induction of
phlorotannins after exposure to UVB radiation was first described in the brown
alga
Ascophyllum nodosum
(Pavia et al.
1997
). An induction of phlorotannins due
to UVB and UVA radiation was also described for
Macrocycstis integrifolia
(Swanson and Druehl
2002
). An increase in the size of phlorotannin containing
vesicles, the so-called physodes, was observed in various Laminariales from
Spitsbergen after UVB exposure indicating an induction of phlorotannin synthesis
(Wiencke et al.
2006
). This has recently been verified in the UV-tolerant species
Alaria esculenta
and
Saccorhiza dermatodea
(Steinhoff unpublished data,
Steinhoff et al.
2011b
). An exudation of phlorotannins as response to artificial
UVB radiation was observed in
Macrocystis integrifolia
(Swanson and Druehl
2002
) creating so-called UV-refugia. Again, high phlorotannin contents and high
exudation rates might reflect an adaptation of seaweeds to the radiation climate and
overall habitat requirements. In contrast, induction and variable exudation rates of
phlorotannins reflect the acclimation potential to environmental changes. Interest-
ingly, the precursors of MAAs and phlorotannins are both synthesized by the
Shikimate pathway, with phenylalanine being an important intermediate. The
reason why finally different specific UV screens are applied in different algal
divisions represents an interesting aspect to study in terms of algal evolution.
In contrast to the specific UV-absorbing compounds found in red and brown
algae, there is inconsistency about the role of optical UV protection in the group of
green algae. The occurrence of MAAs in supralittoral green algae such as
Prasiola
sp. (Karsten et al.
2005
) seems to be quite exceptional. Similar as phlorotannin
exudation in brown algae, some green seaweeds have also been found to generate
microenvironments depleted from UVB, e.g., by excreting UVB-absorbing
substances. The green
Dasycladus vermicularis
is commonly found in the shallow
waters of the Mediterranean as well as in rock pools. Under conditions of high
irradiance and increased water temperature and salinity, the alga excretes high
concentration of hydroxycumarine, a phenolic compound which may provide a
