Microorganisms are of major important in industrial wastewater
treatment, agricultural and aquaculture. They reside in the
sediment and other substrates, and in the water of aquaculture
facilities, as well as in and on the cultured species. Microorganisms
may have positive or negative effects on the outcome of aquaculture
operations. Positive microbial activities include elimination
of toxic materials such as ammonia, nitrite, and hydrogen
sulfide, degradation of uneaten feed, and nutrition of aquatic
animals such as shrimp, fish; production of aqua-farmer. These
and other functions make microorganisms key players in the
health and sustainability of aquaculture. Yet, microorganisms
are among the least known and understood elements in aquaculture.
Like other areas in aquaculture, microorganisms require management
The world of microorganisms is made of bacteria, fungi, algae,
protozoa, and viruses. They are group together only because
of their small size, and not by their function. If, for example,
the same taxonomical rules were applied to larger animals,
some fish, shrimp, green plants, birds and mammals would be
grouped together. Some microorganisms such as viruses, bacteria,
and protozoa are notoriously small, under one mm. Others,
like algae and fungi, have large size relatives (such as the
brown algae that is among the largest living organisms). Unlike
larger organisms, the morphology of microorganisms is relatively
poor and is confined to few shapes and colors. However, their
poor morphology is compensated by great physiological versatility.
Viruses are very small, ranging between 0.01 and 0.03 microns,
and only visible by using an electron microscope. They cannot
live independently, and only multiply inside the cells of
other organisms. However, their demand for a host is fairly
specific. For example, it is unlikely that a crustacean virus
will attack humans or fish. Viruses are also the simplest
of all organisms and are made of nucleic acid (either DNA
or RNA), frequently coated with a protein layer.
Algae are photosynthetic organisms (contain chlorophyll) and
obtain their energy from the sun and their carbon from carbon
dioxide. Their size ranges from one micron to many meters.
All organisms that use carbon dioxide for their carbon requirement
are called autotrophs. Algae are generally beneficial in aquaculture
by supplying oxygen and a natural food base for the cultured
animals, such as dinoflagellates that cause the red ties.
Fungi are similar to algae, but they do not contain chlorophyll
and require pre-formed organic matter as energy and carbon
sources (e.g., sugars, fat, protein, and other carbohydrates).
Such organisms are called heterotrophs. Fungi, ranging in
size from a few microns to several centimeters, grow either
independently by feeding on decaying matter, or in association
with plants and animals.
Protozoa are heterotrophs, mostly free-living, feeding mainly
by devouring smaller microorganisms. Their size ranges between
two and 200 micron meters. A large group of
protozoa, the Sporozoa, are parasites. Small numbers of protozoa
contain chlorophyll and can switch between autotrophic and
heterotrophic modes of feeding, based on light conditions.
Bacteria range in size from 0.1 to 15 micron, with some "
giants "that may reach half a millimeter. They make up
the most metabolically diverse group of living organisms.
Although some are parasitic to animals and plants, the majority
of bacteria are free-living, having either a neutral or beneficial
relationship with humans and other animals and plants. Their
metabolic versatility is incredible: while most are heterotrophs,
using either light or chemical energy. One of their most remarkable
characteristics is their ability to multiply rapidly, with
generation times usually ranging between minutes to hours.
Bacteria and other microorganisms, most notably fungi, are
able to metabolize and transform numerous organic and inorganic
compounds. Therefore, man has used them for thousands of years
for making yogurt, pickles, bread, cheese, wine, and more
recently for waste purification and wastewater purification.
Process controlled by microorganisms can occur aerobically
(in the presence of oxygen) or anaerobically (with no oxygen
present). The starting materials and the end products of such
processes vary based on the microorganisms' capabilities (as
reflected in their genetic makeup), and the environment in
which these processes occur (e.g., availability of oxygen,
temperature, salinity, pH, etc.
MICROBIAL PROCESS IN AQUACULTUTRE
Generally, aerobic microbial processes yield compounds which
can be beneficial, and are either not toxic or have lox toxicity
levels in aquaculture ponds or tanks. Oxidation of organic
matter to carbon dioxide, a process which is the main consumer
of oxygen in aquaculture ponds or tanks.
of ammonia to nitrate via nitrite, which also consumes large
quantity of oxygen. xidation of reduced sulfur compounds (such
as hydrogen sulfide and elemental sulfur) to sulfate, a process
that generally has low oxygen demand in aquaculture. Conversion
of carbon dioxide to biomass by autotrophic bacteria (such
as the nitrifying bacteria) with a relatively mall amount
of biomass produced in aquaculture facilities, when compared
to the conversion of carbon dioxide to biomass by algae.
of carbon dioxide to biomass by algae depending on the availability
of light. Excluding feeding, the photosynthetic process in
aquaculture is the main input of carbon source and natural
food for aquatic animals.
MICROBIAL PROCESSES IN AQUACULTURE
Microbial anaerobic processes, if not controlled, can produce
compounds that are highly toxic to cultured animals.
Consumption of organic matter, without the utilization of
free oxygen, resulting in products which are usually not fully
oxidized (such as alcohols, organic acids). Reduction of nitrate
and nitrite, which can yield either nitrogen gas or ammonia.
In aquaculture, due to the toxicity of ammonia and nitrite,
ammonia production is not welcomed, while nitrogen gas production
is beneficial. However, in agriculture, the opposite is true
- the conversion of nitrate and nitrite to nitrogen das result
in a loss of fertilizers. Reduction of sulfur compounds to
hydrogen sulfide as a final product, a compound, which is
toxic to most animals at even very low concentrations.