Economically valuable algae?
Who would have thought? In fact, the economically important
polysaccharide found in C. exasperatus, carageenan, was estimated
in 1991 to have generated $100 million dollars and involved the consumption of
250,000 tonne of algae per year (Lobban and Harrison, 1997). Carageenan, is one
of the two most economically valuable red algae products, the other being agar,
and the demand for both substances has risen to the point where it justifies
aquaculture endeavors. Carageenan has a gelling ability and is often used
in pharmaceutical applications, cosmetics and the food industry, both as a food
substitute and a food stabilizer. This last area may be the greatest source
of demand for carageenan, as it has a lesser stiffening ability than agar and
is thus more applicable for stabilizing food stuffs at a gel phase. For
example, carageenan is increasingly used to stabilize dairy products such as
milk and ice cream, as well as instant puddings, creams, and sauces which need
to gel without refrigeration (Lee, 1980).
The product: Carageenan
Carageenan is a phycocoloid comprised of
two components, kappa-carageenan and lambda-carageenan, both of which
are negatively charged polymers. Together the two principal components,
kappa and lambda-carageenan, form a gel when heated or cooled in the
presence of potassium ions. Kappa carageenan has been found to
be the sole gelling component, an important distinction as the relative
concentrations of the two components vary between species and life
history phases of the same species. For example, in C.
exasperatus, the relative kappa carageenan content of a gametophyte
is much higher than the relative concentration in a tetrasporophyte,
making the gametophyte the more valuable phase economically. Because C.
exasperatus is isomorphic, identifying the different life history
phases requires microscopic analysis or chemical tests, such as the resorcinol
test (Lee, 1980).
The development of field cultivation methods
Scientists at the University of Washington, specifically J.
Robert Waaland, have been especially active in exploring the possibilities for C.
exasperatus aquaculture. C. exasperatus has high aquaculture
potential because it is large, abundant, and has a high carageenan content. Attempts
at developing C. exasperatus as a commercial crop have had to start at
ground one, because there are no traditional aquaculture methods for this species. The
major limiting factor in C. exasperatus' s growth rates is the availability
of appropriate rock substrate, and thus efforts have focused on growing this
algae on artificial substrates. Not only would this procedure allow algae
to be cultivated more widely than the limitation of substrate availability allows,
but using artificial substrates would also simplify harvesting procedures.
Attempts have been made to grow C. exasperatus on a
number of artificial substrates with general success. The first trial
was to grow thalli transplants on plastic lines. These transplants
were successful and also indicated the maximum growth depth and season for C.
exasperatus, which were 3m below M.L.L.W. (mean low low water) and spring
through summer respectively. Frame transplant trials were also found
to be successful (Waaland, 1973).
The next step to investigate was spore colonization. This
was accomplished simultaneously by studying spore colonization in the field on
various artificial substrates (including nylon and polypropylene lines, nylon
webbing, and concrete blocks) and spore colonization in the laboratory under
controlled conditions with subsequent transplanting to the natural world. Both
techniques were successful at increasing understanding of small scale aquaculture
techniques for C. exasperatus, and opened the door for larger scale cultivation
Large Scale Cultivation
Large scale cultivation techniques for C. exasperatus have
since been designed. Overall, this algae has been found to be an
attractive species for aquaculture as it propagates readily vegetatively and
can probably produce two harvests if harvested at the beginning and end of its
growth season (early summer to late summer). Below is an outline of proposed
cultivation techniques in natural waters.
- Stock Selection: This step has led to the isolation of a fast-growing
strain of C. exasperatus, now commonly used.
- Site Selection: Growth in small test frames to test site suitability.
- Tank Growth: Laboratory germination and holding of germlings for
- Outplant: Transplant of germlings to nets, either surface
of bottom in natural waters.
- Harvest from Net: So far only accomplished successfully for surface
nets. Multiple harvests during one growing season (ie.
at beginning and end of summer) found to extend and increase growth.
Alternative cultivation techniques have also been proposed in tanks
or ponds, which would be filled with seawater and aerated. Aquaculture
efforts with C. exasperatus according to these techniques have
also been successful, negating concerns that year-round cultivation might
be unattractive because of difficulties in maintaining adequate winter
species density (a problem run up against with many other similar red
algae species) (Mumford and Waaland, 1980).
One of the benefits of cultivation is that
the harvest can be guaranteed to be uniform in life history phase (ie.
uniform in major carageenan type), which makes extraction and processing
easier. Once harvested, C. exasperatus is dried and then
washed with freshwater. This reduces the salt concentration
before the alga is boiled. Carageenan is readily soluble
in hot, but not cold, water, and thus must be separated from the water
and insoluble residue by a centrifuge. Finally, the product is
filtered and any remaining water is evaporated. At this point,
the carageenan goes to a baler, where it is shipped to the processor,
or to a mill, where it is converted to powdered form (Mumford and Waaland,
Ecological Problems and Solutions:
Ecological: Initial germination of C. exasperatus seedling was
found to be disrupted by diatom fouling. The diatoms would shade out the
new germlings, and efforts to dry the nets were ineffective as the diatoms
were more resistant to desiccation than the germlings. Small snails,
such as Lacuna, Callistoma, and Lirularia, were first considered to
be possible herbivores of the net cultures, however, they were actually found
to serve a positive function in initial germling growth. At this
stage, growth is too rapid to allow the snails to settle on the germlings,
however they are responsible for limiting diatom populations and thus reducing
fouling. The second major ecological problem facing cultivation efforts
is the shading by drifting kelp, specifically Laminaria. This problem
was dealt with by the replacement of submerge for floating frames, decreasing
the amount of shading by floating kelp and thus maintaining adequate light
for growth (Mumford and Waaland, 1980).
exasperatus Home | Marine Botany
Last modified: 3/18/99
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