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Species Description:
Gracilaria tikvahiae
is a highly opportunistic species common in estuaries and bays, especially
where nutrient loading leads to either seasonal or year-round eutrophication (Peckol and Rivers 1995a, 1995b). Its morphology is highly variable, with colors
ranging from dark green to shades of red and brown; with outer branches that can
be either somewhat flattened or cylindrical in shape (Littler and Littler 1989). It can be found
in protected, quiescent bays, as well as in high energy coastline habitats.
This species grows free or attached to rocks or other substrata, and can reach a
height of 30 cm (Littler and Littler 1989). G. tikvahiae grows to
depths of approximately 10 m, but is most common at depths less than 1 m.
II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
Gracilaria tikvahiae occurs from cold temperate regions along the
eastern Atlantic coast from Nova Scotia to warm subtropical regions around
the east and west coasts of Florida and into the Caribbean.
IRL Distribution:
Found lagoon-wide in the Indian River Lagoon
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Gracilaria tikvahiae can grow vegetatively over
an indefinite period of time and has been shown to have a high growth rate under
non-limiting light and nutrient conditions (Hanisak 1981, LaPointe, Dawes and
Tenore 1984, LaPointe and Duke 1984, Peckol and Rivers 1995a, 1995b). The
productivity of this species can be as high as any terrestrial crop on
earth. Consequently, it
has become the focus of several studies into its commercial value, primarily as
a producer of hydrocolloids such as agar and carrageenan (Silverthorne and
Sorenson 1971, Dawes 1987, Hanisak 1987).
Abundance:
Gracilaria tikvahiae is abundant throughout its
range. It can be especially dominant in areas of high eutrophication.
Locomotion:
Sessile
Reproduction:
Can be propagated vegetatively over long periods of time.
IV. PHYSICAL TOLERANCES
Temperature:
Optimum growth of G. tikvahiae occurs between 24 – 30 ° C
(Hanisak in Hwang, Williams and
Brinkhuis 1987). It can survive, but does not grow at temperatures below 12° C (LaPointe and Ryther 1981).
Temperature, more than light intensity, is the
critical factor which affects the seasonal variation in the amounts of proteins,
carbohydrates, and the R-Phycoerythrin:Chlorophyll a ratio (the ratio of
red photopigments to the primary green photopigment) in Gracilaria tikvahiae.
As long as nutrients are not limiting, protein and carbohydrate levels tend to
show an inverse relationship to both temperature and light, decreasing as
temperature and light increase.
Salinity:
euryhaline. Gracilaria tikvahiae is highly plastic in its responses
to changing salinity and temperature (Dawes 1994).
Other Physical Tolerances:
As an opportunistic species, Gracilaria
tikvahiae is better able to tolerate eutrophic conditions than some other
algae. Under eutrophic conditions, it accumulates as dense unattached mats
which may reach more than 0.5 m in thickness and account for greater than 90% of
the standing algal biomass (Peckol and Rivers 1995). Aggregation of this kind
often creates a highly reducing environment rich in ammonia and low in oxygen. Gracilaria
tikvahiae tolerates hypoxia relatively well, and tends to reduce its level
of cellular respiration in order to offset poor environmental conditions. (Peckol and Rivers 1995).
Under laboratory conditions, net
photosynthesis and growth of Gracilaria tikvahiae decreases when pH of
the culture medium increases above 8.0
V. COMMUNITY ECOLOGY
Trophic Mode:
Gracilaria tikvahiae is an
autotrophic species, capable of storing relatively large amounts of dissolved
nitrite (NO2-), nitrate (NO3-), and
amino acids as a nitrogen pool. This capacity suggests that growth in this
species can be somewhat uncoupled from nutrient uptake. a concept which has
several beneficial applications to the aquaculture industry, particularly if
optimal growth can continue after all available nitrogen has been removed from
the water (Hwang, Williams and Brinkhuis 1987).
Light intensity and temperature mediate the
uptake of nitrate and ammonium in macroalgae. LaPointe , Dawes and Tenore (1984)
showed that light intensity is also the major factor which influences seasonal
variation in the levels of Chlorophyll a, R-Phycoerythrin and % Nitrogen
in Gracilaria tikvahiae tissues, with reduced light intensity causing
increased levels of photopigments. Photosynthesis and growth rates for this
species are maximized when light intensity is high and temperature is in the
optimum range of 24 - 30° C. However, studies have suggested (LaPointe and Duke
1984) that in order to maximize growth, macroalgae have the ability to increase
their photosynthetic capacity by optimizing pigment levels based on lighting
conditions. In Gracilaria tikvahiae, acclimation to reduced light
intensity results in an increase in both pigment levels and photosynthetic
ability. Acclimation to light saturation results in decreased pigment levels and
increased photosynthetic capacity.
Competitors:
In a field experiment performed
in a eutrophic embayment in Massachusetts (Peckol and Rivers 1995), G.
tikvahiae had a growth rate up to 4 times faster in mixed-species plots
under saturating light conditions than did Cladophora vagabunda, a
primary competitor. However, under limiting lighting conditions, C. vagabunda
showed a higher growth rate. These results suggest that distributional patterns
of these species are influenced greatly by interspecific competition, with G.
tikvahiae being a better competitor under optimum lighting conditions, and C.
vagabunda being a better competitor under low-light conditions. Distribution
patterns in the bay appear to confirm this finding: Gracilaria tikvahiae
was restricted to shallower areas of the bay; and C. vagabunda
was found in nearly monospecific stands at deeper levels.
Gracilaria tikvahiae
may have another competitive advantage as well. It has rapid growth and nitrogen
uptake rates, with high nitrogen storage capacity in its tissues. Rivers and
Peckol (1995) found that while C. vagabunda was capable of utilizing only
dissolved CO2, Gracilaria tikvahiae was able to use several different
forms of dissolved inorganic carbon (DIC), a nutrient not normally considered
limiting to algae. Thus, the amount of DIC present could aid in indirectly
controlling some important aspects of photosynthesis.
Habitat:
Common in both high energy and low energy zones. It is often found in highly
eutrophic areas where it forms thick, unattached mats that can comprise over 95%
of the standing biomass (Peckol and Rivers 1995).
VI. SPECIAL STATUS
Special Status:
Economic
Economic Importance:
Large-scale cultivation of Gracilaria
tikvahiae for production of agar and other hydrocolloids is becoming more
feasible with the advancement of land-based aquaculture systems (Huguenin 1976,
Bird et al. 1981, Habig and Ryther 1983, DeBusk and Ryther 1984). Gracilaria
tikvahiae is also under consideration as a potential energy-producing plant.
When fermented, this plant is among one of the highest methane producers.
Additionally, because it is highly opportunistic, G. tikvahiae has
been shown useful as a tertiary treatment alternative for aquaculture and sewage effluent (Ryther 1979).
Report by: K. Hill,
Smithsonian Marine Station
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Page last updated: July 25, 2001
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