Biology Reference
In-Depth Information
5.4 Salinity Ecotypes
Habitats with strong horizontal salinity gradients such as estuaries or the Baltic Sea
promote genetic adaptation and eventually speciation. Indeed, the geographic semi-
isolation from the Atlantic and the steep horizontal salinity gradient of the Baltic
Sea have caused local genetic adaptation in several seaweeds and contributed to the
formation of so-called ecotypes. Ecotypes (
sensu
Turesson,
1922
) are genetically
distinct populations of a species that are locally adapted to particular environmental
conditions. The ability of populations to adapt to a novel habitat is a key component
of the speciation process. Ecotypes are recognized as physiologically distinct
variants of a species. The concept of ecotypes is widely accepted in relation to
ecophysiological diversity in seaweeds when considering salinity stress responses.
Several Baltic seaweeds were physiologically and morphologically compared with
their marine counterparts and a number of salinity ecotypes have been described
such as
Ceramium strictum
(Rueness and Kornfeldt
1992
),
Delesseria sanguinea
and
Membranoptera alata
(Rietema
1993
), and
Fucus vesiculosus
(Nygard and
Dring
2008
). But data from other regions also clearly point to strong ecotypic
differentiation between populations of the same seaweed species such as estuarine
and marine isolates of
Pylaiella littoralis
(Bolton
1979
),
Polysiphonia lanosa
(Reed
1984
),
Bostrychia radicans
, and
B. moritziana
(Karsten et al.
1993b
,
1994
).
Although the kelp genus
Laminaria
is stenohaline (Bartsch et al.
2008
), ecotypic
differentiation in terms of growth under different salinities has been reported in
North Atlantic populations of
Saccharina latissima
originating from Long Island
Sound, New York, and Cape Neddick, Maine (Gerard et al.
1987
), and might be
considered as a mechanism to adapt to locally environmentally unfavorable
conditions. However, in most seaweeds the underlying molecular mechanisms of
ecotypic differentiation are unstudied.
One exception is
Bostrychia tenuissima
, which is restricted to Southern Australia
and New Zealand. Previous studies have revealed two distinct patterns in the
presence of osmotically active polyols. Southern Australian populations only have
sorbitol whereas northern Australian populations have in addition to sorbitol also
dulcitol. These polyol patterns led to speculation on ecotypic differentiation in both
population types (Karsten et al.
1995
). Using molecular approaches, a 100% con-
gruence was found between polyol patterns and three plastid haplotypes observed
among all isolates studied, which experimentally proved for the first time a genetic
basis for different ecotypes of the same seaweed species (Zuccarello et al.
1999
).
5.5 Outlook
Since the review of Kirst (
1990
) on salinity tolerance in algae not much scientific
progress has been made in seaweeds. In strong contrast, in cyanobacteria many
molecular mechanisms involved in osmotic acclimation have been successfully
