Seaweed Responses to Temperature - Seaweed Biology - page 56

determined by the August isotherm. Moreover, seaweed distribution is affected by
temperature-daylength interactions (Dring 1984 ; Molenaar 1996 ). Optimum
temperatures for growth and reproduction or lethal temperatures for the non-
hardiest life stage are typically irrelevant in explaining geographic distribution.
Suboptimal or sublethal temperatures often prevail at species distribution
boundaries. For instance, in case of the tropical to warm-temperate species
Cladophora albida , which has optimum growth temperatures as high as 25-30 C,
temperatures are even suboptimal for year-long growth over a large part of the
distribution range at the NW Atlantic coast (Cambridge et al. 1984 ). Also, many
seaweeds from the Arctic grow suboptimally during most of the year and they have
a wider distribution in cold-temperate regions (G´mez et al. 2009 ).
Temperature ranges for survival, growth, and reproduction have been deter-
mined experimentally for a large number of seaweed species during the last three
decades. Supporting the concepts described above, large-scale biogeographic dis-
tribution patterns of many seaweed species were explained by combining the
species' thermal traits with local seawater temperatures. Species with similar
thermal responses and the same types of distribution boundaries have been assigned
to phytogeographic distribution groups (c.f. van den Hoek 1982a , 1982b ; Breeman
1988 ; Wiencke et al. 1994 for designation of groups).
Ecotypic differentiation in thermal responses would be expected in broadly
distributed seaweed species, living in a strong thermal gradient. Genetically distinct
temperature ecotypes are locally adapted to the particular temperature regime
(ecotype definition sensu Turesson 1922 ) and may influence the location of geo-
graphic boundaries in a number of species. On the other hand, populations may not
be distinct from each other and temperature requirements change gradually over the
distribution area, i.e., variation is of an ecoclinal nature. The recognition of
ecotypes is intrinsically linked to the definition of species (e.g., Hey 2006 ), a
subject that requires molecular approaches as many seaweed morphospecies are
composed of biologically and genetically distinct lineages (Andreakis et al. 2007 ;
Wattier and Maggs 2001 ; Boedeker et al. 2008 ). Studies on local adaptation and
adaptive evolution in new environments of seaweeds that include molecular data
still remain few (Johansson et al. 2003 ; Bergstrom et al. 2005 ; Pereyra et al. 2009 ;
Verbruggen et al. 2009 ). Most studies on ecotypic differentiation in seaweeds were
done with biogeographical isolates, grown under the same laboratory conditions
in “common-environment experiments.” The temperature responses of thermal
traits (i.e., upper and lower survival temperatures, growth range, temperature
requirements for reproduction, and photosynthesis) are typically determined and
compared with the annual temperature regime of the respective habitats. Especially,
the annual growth yield (potential monthly growth rate) is a useful parameter to
evaluate local temperature adaptation as it allows to combine the seasonal temper-
ature range with the temperature dependence of growth (e.g., applied in Breeman
and Pakker 1994 ; Bischoff and Wiencke 1995 ; Eggert et al. 2003a ). It is concluded
from these “common-environment studies” that observed differences between
isolates have a genetic basis and the results provide evidence for intra-specific
differentiation. Thermal ecotypes or ecoclinal variation has been described for