Introduction
Algae are of significant environmental and commercial importance.
The environmental impact of algal blooms can have significant negative
economic consequences, however these are outweighed by the positive
roles of algae. Algae are not only sources of food for humans and
animals, but are also the sources of a wide range of chemical compounds
such as the phycocolloids used in industry, food technology and as
pharmaceuticals. In the last few decades the emphasis has moved from
‘wild' harvests to the farming and controlled cultivation and
to the production of valuable new products. Continued technical innovation
and market demand will result in further major advances and an expansion
of the commercially available species and products. Genetic engineering
methods are also beginning to be used for strain improvement, and
algal genes are being used for the improvement of other plants such
as crop plants. Australia has a great algal diversity and this, combined
with optimal conditions for commercial-scale algal production, provides
great opportunities for new industries.
Algae, especially
toxic microalgae, are also having an increased impact on humans
and this is leading to advances in the management of aquatic ecosystems
to manage and control algal blooms.
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History of
Microalgae Culture
Microalgae culture
is one of the modern biotechnologies. The first unialgal cultures
were achieved by Beijerinck in 1890 with Chlorella vulgaris,
and the use of such cultures for studying plant physiology was developed
by Warburg in the early 1900's. Mass culture of microalgae really
began to be a focus of research after 1948 at Stanford (USA), Essen
(Germany) and Tokyo and the classic book edited by Burlew (1953)
summarises many of these early studies. Interest in applied algal
culture continued, especially with studies on the use of algae as
photosynthetic gas exchangers for space travel and as microbial
protein sources.
Commercial large-scale
culture of microalgae commenced in the early 1960's in Japan with
the culture of Chlorella followed in the early 1970's with
the establishment of a Spirulina harvesting and culturing
facility in Lake Texcoco, Mexico by Sosa Texcoco S.A. In 1977 Dai
Nippon Ink and Chemicals Inc. established a commercial Spirulina
plant in Thailand, and by 1980 there were 46 large-scale factories
in Asia producing more than 1000 kg of microalgae (mainly Chlorella)
per month (Kawaguchi, 1980) and in 1996 about 2000t of Chlorella
were traded in Japan alone. Other Spirulina plants were eastablished
in the USA (eg. Microbio in California and Cyanotech in Hawaii).
Commercial production of Dunaliella salina, as a source of
ß-carotene, became the third major microalgae industry when
production facilities were established by Western Biotechnology
Ltd and Betatene Ltd (now Cognis Nutrition & Health) in Australia
in 1986. These were soon followed by other commercial plants in
Israel and the USA. As well as these algae, the large-scale production
of cyanobacteria (blue-green algae) commenced in India at about
the same time. More recently several plants producing Haematococcus
pluvialis as a source of astaxanthin have been established in
the USA and India. Thus in a short period of about 30 years the
industry of microalgal biotechnology has grown and diversified significantly.
Products
from Microalgae
Carotenoids
Several species
of microalgae, especially green algae, accumulate high concentrations
of carotenoids such as ß-carotene, astaxanthin and canthaxanthin.
These carotenoids have wide application as natural colourants and
antioxidants. The first of these carotenoids to be commercialised
was ß-carotene from the green halophilic flagellate, Dunaliella
salina and Australia is now the major producer of natural ß-carotene
from Dunaliella. Research on Dunaliella production
began in Australia in the mid 1970's at the Roche Research Institute
of Marine Pharmacology and also at CSIRO and eventually resulted
in the construction of two production plants in South Australia
and Western Australia. Other Dunaliella plants are currently
being developed at Dampier in Western Australia.
The ability
to grow at very high salt concentrations where few other organisms
can survive, its high temperature tolerance (up to about 40oC),
and the high cell content of ß-carotene (up to 14% of dry
wt.) made this alga an attractive candidate for commercial production
of this carotenoid. The extreme conditions under which this alga
grows means that relatively simple cultivation systems can be used.
The two commercial production plants in Australia, at Hutt Lagoon
in Western Australia and at Whyalla in South Australia use large
shallow ponds of several hundred hectares in area to grow the alga.
These ponds have a depth of between 30 and 60 cm and are only mixed
by wind and thermal convection. The harvested biomass is extracted
and pure ß-carotene or mixed carotenoids are sold as a nutritional
supplement and natural food colouring. Dried Dunaliella powder
is also sold as a feed additive for aquaculture to pigment crustaceans
such as prawns.
Another carotenoid
of great interest is astaxanthin. Astaxanthin is used as a pigmenter
for farmed salmonid fish as well as a dietary antioxidant. The production
of algal astaxanthin from Haematococcus pluvialis is quite
a different process from that used for ß-carotene production
from D. salina. The chlorophyte, Haematococcus pluvialis
is a freshwater alga which normally grown in temporary water bodies
such as depressions in rock, puddles, flowerpots and birdbaths.
The optimum temperature for growth is about 22-25oC. Furthermore,
the astaxanthin is produced in a thick walled resting stage, the
aplanospore, whereas maximum growth occurs in a green thin-walled
flagellated stage. This necessitates a two-stage culture process,
one optimised for biomass production and the other for astaxanthin
production. Being a freshwater alga, open air culture as used for
Dunaliella, Chlorella and Spirulina is not
feasible, and Haematococcus must be grown in a closed photobioreactor
to avoid contamination. The large-scale culture systems proposed
for this alga involve a growth stage in a closed, temperature controlled
photobioreactor to achieve maximum biomass, followed by a astaxanthin-accumulating
stage under high light conditions, preferably in nutrient poor medium.
Commercial production is under way in Hawaii and Israel, and other
ventures have been proposed elsewhere.
Research on
algal production of other carotenoids such as lutein and canthaxanthin
from other algae such as Chlorella spp. and Chlamydomonas
spp. is under way.
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