Raceway pond
used to cultivate microalgae.
[1]
The water is kept in constant motion with a powered
paddle wheel
.
Microalgae
or microscopic algae grow in either marine or freshwater systems. They are
primary producers
in the oceans that convert water and carbon dioxide to
biomass
and oxygen in the presence of sunlight.
[2]
The oldest documented use of microalgae was 2000 years ago, when the Chinese used the
cyanobacteria
Nostoc
as a food source during a famine.
[3]
Another type of microalgae, the cyanobacteria
Arthrospira
(
Spirulina
), was a common food source among populations in Chad and Aztecs in Mexico as far back as the 16th century.
[4]
Today cultured microalgae is used as direct feed for humans and land-based farm animals, and as feed for cultured aquatic species such as molluscs and the early larval stages of fish and crustaceans.
[5]
It is a potential candidate for
biofuel
production.
[6]
Microalgae can grow 20 or 30 times faster than traditional food crops, and has no need to compete for arable land.
[6]
[7]
Since microalgal production is central to so many commercial applications, there is a need for production techniques which increase productivity and are economically profitable.
Commonly cultivated microalgae species
[
edit
]
Microalgae are microscopic forms of
algae
, like this
coccolithophore
which are between 5 and 100 micrometres across
Hatchery production techniques
[
edit
]
A range of microalgae species are produced in hatcheries and are used in a variety of ways for commercial purposes. Studies have estimated main factors in the success of a microalgae hatchery system as the dimensions of the container/bioreactor where microalgae is cultured, exposure to light/
irradiation
and concentration of cells within the reactor.
[13]
Open pond system
[
edit
]
This method has been employed since the 1950s across the CONUS.
[14]
There are two main advantages of culturing microalgae using the
open pond
system.
[15]
Firstly, an open pond system is easier to build and operate.
[15]
Secondly, open ponds are cheaper than closed bioreactors because closed
bioreactors
require a cooling system.
[15]
However, a downside to using open pond systems is decreased productivity of certain commercially important strains such as
Arthrospira sp.
, where optimal growth is limited by temperature.
[13]
That said, it is possible to use waste heat and CO
2
from industrial sources to compensate this.
[16]
[17]
[18]
[19]
[20]
Air-lift method
[
edit
]
This method is used in outdoor cultivation and production of microalgae; where air is moved within a system in order to circulate water where microalgae is growing.
[15]
The culture is grown in transparent tubes that lie horizontally on the ground and are connected by a network of pipes.
[15]
Air is passed through the tube such that air escapes from the end that rests inside the reactor that contains the culture and creates an effect like stirring.
[15]
Closed reactors
[
edit
]
The biggest advantage of culturing microalgae within a closed system provides control over the physical, chemical and biological environment of the culture.
[13]
This means factors that are difficult to control in open pond systems such as evaporation, temperature
gradients
and protection from ambient contamination make closed reactors favoured over open systems.
[13]
Photobioreactors are the primary example of a closed system where abiotic factors can be controlled for. Several closed systems have been tested to date for the purposes of culturing microalgae, few important ones are mentioned below:
Horizontal photobioreactors
[
edit
]
This system includes tubes laid on the ground to form a network of loops. Mixing of microalgal suspended culture occurs through a pump that raises the culture vertically at timed intervals into a
photobioreactor
. Studies have found pulsed mixing at intervals produces better results than the use of continuous mixing. Photobioreactors have also been associated with better production than open pond systems as they can maintain better temperature gradients.
[13]
An example noted in higher production of
Arthrospira sp.
used as a dietary supplement was attributed to higher productivity because of a better suited temperature range and an extended cultivation period over summer months.
[13]
Vertical systems
[
edit
]
These reactors use vertical
polyethylene
sleeves hung from an iron frame. Glass tubes can also be used alternatively.
Microalgae are also cultured in vertical alveolar panels (VAP) that are a type of
photobioreactor
.
[13]
This photobioreactor is characterised by low productivity. However, this problem can be overcome by modifying the
surface area
to
volume
ratio; where a higher ratio can increase productivity.
[13]
Mixing and
deoxygenation
are drawbacks of this system and can be addressed by bubbling air continuously at a mean flow rate. The two main types of vertical photobioreactors are the Flow-through VAP and the Bubble Column VAP.
[13]
In darkness
[
edit
]
By using an electrocatalytic process to produce
acetate
from water, electricity and carbon dioxide, which is then used by the algae as food source, sunlight and photosynthesis is no longer required. The method is still at an early stage, but experiments with algae like
Chlamydomonas reinhardtii
have turned out to be promising.
[21]
[22]
Flat plate reactors
[
edit
]
Flat plate reactors(FPR) are built using narrow panels and are placed horizontally to maximise sunlight input to the system.
[23]
The concept behind FPR is to increase the surface area to volume ratio such that sunlight is efficiently used.
[15]
[23]
This system of microalgae culture was originally thought to be expensive and incapable of circulating the culture.
[23]
Therefore, FPRs were considered to be unfeasible overall for the commercial production of microalgae. However, an experimental FPR system in the 1980s used
circulation
within the culture from a gas exchange unit across horizontal panels.
[23]
This overcomes issues of circulation and provides an advantage of an open gas transfer unit that reduces oxygen build up.
[23]
Examples of successful use of FPRs can be seen in the production of
Nannochloropsis sp.
used for its high levels of
astaxanthin
.
[24]
Fermentor-type reactors
[
edit
]
Fermentor-type reactors (FTR) are bioreactors where
fermentation
is carried out. FTRs have not developed hugely in the cultivation of microalgae and pose a disadvantage in the surface area to volume ratio and a decreased efficiency in utilizing sunlight.
[15]
[23]
FTR have been developed using a combination of sun and artificial light have led to lowering production costs.
[23]
However, information available on large scale counterparts to the laboratory-scale systems being developed is very limited.
[23]
The main advantage is that extrinsic factors i.e. light can be controlled for and productivity can be enhanced so that FTR can become an alternative for products for the
pharmaceutical
industry.
[23]
Commercial applications
[
edit
]
Use in aquaculture
[
edit
]
Microalgae is used to culture
brine shrimp
, which produce dormant eggs (pictured). The eggs can then be hatched on demand and feed to cultured fish larvae and crustaceans.
Microalgae is an important source of nutrition and is used widely in the
aquaculture
of other organisms, either directly or as an added source of basic nutrients. Aquaculture farms rearing larvae of
molluscs
,
echinoderms
,
crustaceans
and
fish
use microalgae as a source of nutrition. Low bacteria and high microalgal biomass is a crucial food source for shellfish aquaculture.
[25]
Microalgae can form the start of a chain of further aquaculture processes. For example, microalgae is an important food source in the
aquaculture of brine shrimp
. Brine shrimp produce dormant eggs, called
cysts
, which can be stored for long periods and then hatched on demand to provide a convenient form of live feed for the aquaculture of
larval fish
and crustaceans.
[26]
[27]
Other applications of microalgae within aquaculture include increasing the
aesthetic
appeal of finfish bred in captivity.
[25]
One such example can be noted in the
aquaculture of salmon
, where microalgae is used to make salmon flesh pinker.
[25]
This is achieved by the addition of natural pigments containing
carotenoids
such as
astaxanthin
produced from the microalgae
Haematococcus
to the diet of farmed animals.
[28]
Two microalgae species,
I. galbana
and
C. calcitrans
are mostly composed of proteins, which are used to brighten the color of salmon and related species.
[29]
Human nutrition
[
edit
]
The main species of microalgae grown as health foods are
Chlorella
and
Spirulina
(
Arthrospira platensis
). The main forms of production occur in small scale ponds with artificial mixers.
[10]
Arthrospira platensis
is a blue-green microalga with a long history as a food source in East Africa and pre-colonial Mexico. Spirulina is high in protein and other nutrients, finding use as a
food supplement
and for malnutrition. It thrives in open systems and commercial growers have found it well-suited to cultivation. One of the largest production sites is
Lake Texcoco
in central Mexico.
[30]
The plants produce a variety of nutrients and high amounts of
protein
, and is often used commercially as a nutritional supplement.
[31]
[32]
Chlorella
has similar nutrition to spirulina, and is very popular in
Japan
. It is also used as a
nutritional supplement
, with possible effects on
metabolic rate
.
[33]
Production of
long chain omega-3 fatty acids
important for human diet can also be cultured through microalgal
hatchery
systems.
[34]
Australian scientists at
Flinders University
in
Adelaide
have been experimenting with using marine microalgae to produce proteins for human consumption, creating products like "
caviar
",
vegan
burgers,
fake meat
,
jams
and other
food spreads
. By manipulating microalgae in a
laboratory
, the
protein
and other
nutrient
contents could be increased, and flavours changed to make them more palatable. These foods leave a much lighter
carbon footprint
than other forms of protein, as the microalgae absorb rather than produce
carbon dioxide
, which contributes to the
greenhouse gases
.
[35]
Biofuel production
[
edit
]
In order to meet the demands of
fossil fuels
, alternative means of fuels are being investigated.
Biodiesel
and
bioethanol
are renewable
biofuels
with much potential that are important in current research. However,
agriculture
based
renewable fuels
may not be completely sustainable and thus may not be able to replace fossil fuels. Microalgae can be remarkably rich in oils (up to 80% dry weight of
biomass
) suitable for conversion to fuel. Furthermore, microalgae are more productive than land based agricultural crops and could therefore be more sustainable in the long run. Microalgae for
biofuel
production is mainly produced using tubular
photobioreactors
.
[2]
Pharmaceuticals and cosmetics
[
edit
]
Novel
bioactive
chemical compounds can be isolated from microalgae like sulphated
polysaccharides
. These compounds include
fucoidans
,
carrageenans
and
ulvans
that are used for their beneficial properties. These properties are
anticoagulants
,
antioxidants
,
anticancer
agents that are being tested medical research.
[36]
Red microalgae are characterised by pigments called
phycobiliproteins
that contain natural colourants used in
pharmaceuticals
and/or
cosmetics
.
[37]
Biofertilizer
[
edit
]
Blue green alga was first used as a means of fixing nitrogen by allowing
cyanobacteria
to multiply in the soil, acting as a
biofertilizer
.
Nitrogen fixation
is important as a means of allowing
inorganic compounds
such as
nitrogen
to be converted to
organic
forms which can then be used by plants.
[38]
The use of cyanobacteria is an economically sound and environmentally friendly method of increasing productivity.
[39]
This method has been use for
rice
production in India and Iran, using the nitrogen fixing properties of free living cyanobacteria to supplement nitrogen content in soils.
[38]
[39]
Other uses
[
edit
]
Microalgae are a source of valuable molecules such as
isotopes
i.e. chemical variants of an element that contain different neutrons. Microalgae can effectively incorporate isotopes of
carbon
(
13
C),
nitrogen
(
15
N) and
hydrogen
(
2
H) into their biomass.
[40]
13
C and
15
N are used to track the flow of carbon between different trophic levels/food webs.
[41]
Carbon, nitrogen and
sulphur
isotopes can also be used to determine disturbances to bottom dwelling communities that are otherwise difficult to study.
[41]
Issues
[
edit
]
Cell fragility is the biggest issue that limits the productivity from closed
photobioreactors
.
[42]
Damage to cells can be attributed to the turbulent flow within the
bioreactor
which is required to create mixing so light is available to all cells.
[42]
See also
[
edit
]
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[
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