In recent years there has been widespread interest in rearing aquatic
organisms of nutritional and commercial value (Calaprice, 1976). The
most hopeful prospect for marine prawn culture in the United Kingdom
(Wickins, 1976), the Americas (Hanson & Goodwin, 1977) and South
Africa probably lies in intensive culture under controlled conditions.
A closed system approach, in which a captive body of water is circulated,
provides the scope for water quality management which results ~n
maximum water utilization and minimal discharge. On the other hand,
direct utilization of sea-water in open systems presents problems for
aquaculture since this water is subjected to diurnal and seasonal fluctuations
in temperature, salinity and turbidity, as well as contamination
from industrial, agricultural and maritime sources. Furthermore,
large mariculture farms release enormous amounts of organic wastes which
result in eutrophication and could lead to environmental deterioration
of coastal waters (Gerhardt, 1978).
It is well established that circulated sea-water develops an unusual
~on~c composition as a result of the metabolic activity of the prawns
and of the nitrifying bacteria in the biological filter. The changes
include elevated levels of ammonia, nitrite and nitrate and reduced pH.
The presence of even sublethal levels of these nitrogenous compounds ~n
closed systems have been found to affect growth of penaeid spec~es
(Wickins, 1976). Ammonia and nitrite, which rapidly accumulate in the
water, are usually maintained at nontoxic levels by nitrification in
the biological filters (Spotte, 1974; Johnson & Sieburth, 1974). The
chemolithotrophic bacteria responsible for nitrification are presently
classified by their ยท cellular morphology and by the oxidation of either
ammonia and nitrite (Watson, 1974). The predominant ammonia- and
nitrite-oxidizing bacteria isolated from natural environments are
Nitrosomonas europaea and Nitrobacter winogradskyi, respectively (Watson
et aZ., 1981). Direct observation of nitrifying bacteria in natural
environments, however, has been limited to studies involving light
microscopy with immunofluorescent techniques (Fliermans et aZ., 1974;
Fliermans & Schmidt, 1975).
The electron microscopic observation of nitrifying bacteria ~sdifficult
in natural microcosms with low levels of nitrification and with the
presence of sunlight and anaerobic conditions conducive to the enrichment
of other bacteria with a similar ultrastructure. However, in
closed systems with extremely active nitrification but poor light conditions,
the occurrence of morphologically similar forms in numbers
that could be easily detected by electron microscopy is unlikely
(Johnsort & Sieburth, 1976). Furthermore, the cyst-like colonies of the
nitrifiers are unique and are not found with the methane-oxidizing
bacteria with a similar ultrastructure (Davies & Whittenbury, 1970;
Smith & Ribbons, 1970), whereas the thick cell wall of the cyanobacteria
(Carr & Whitton, 1973) and the distinctive cell morphologies of the
purple sulphur and purple nonsulphur bacteria (Pfennig, 1967) separate
them from the nitrifiers. Therefore, closed systems with active nitrification
provide the ideal environment to study the activities of nitrifiers
in conjunction with their relative abundance, nature and diversity.
In spite of the opportunity offered by closed systems, previous studies
(Kawai et aZ., 1965; Wickins, 1976; Gerhardt, 1978; Mevel & Chamroux,
1981) on nitrification have been primarily indirect observations on
rates of ammonia and nitrite oxidation to nitrate (Johnson & Sieburth,
1976). Studies on the enumeration and identification of nitrifiers ~n
closed systems have been seriously neglected. Kawai et aZ. (1964)
included the enumeration of nitrifiers in their study on nitrification
while,in a qualitative study, an attempt to identify the in situ nitrifiers
1n closed systems (Johnson & Sieburth, 1976) was not very successful.
This study was undertaken to investigate the three basic aspects of
nitrification necessary for the understanding of such a process in closed
systems, viz., the oxidation of ammonia and nitrite to nitrate, and the
enumeration and identification of the nitrifying bacteria. Prior to
determining the concentrations of the nitrogenous compounds in the
culture water, various methods were evaluated for their accuracy and
reproducibility with both sea-water and culture water samples. This
approach is necessary in order to gauge the accuracy of results obtained
by such methods. Enumeration of nitrifying bacteria was preceded by an
investigation on the effect of incubation time on the maximum most
probable number , estimate. Such an investigation was necessary
because of the inconsistent approach to the enumeration of nitrifiers in
previous studies (Wilson, 1927; Walker et al., 1937; Lewis & Pramer,
1958; Molina & Rovira, 1964; Meiklejohn, 1965; Smith et al., 1968).
Incubation periods appear to have been chosen arbitrarily in previous
investigations. Identifi~ation of nitrifying bacteria necessitates the
isolation and purification of these organisms. Isolation of nitrifiers
1S a difficult and time-consuming task (Watson et al., 1981) and could
be the main reason for not being included in previous studies on nitrification.
Since the success of this study depended upon the isolation
and purification of these chemolithotrophs, this aspect is de~lt with
in detail.
The changes most likely to be associated with nitrification in a closed
system were also monitored 1n the culture water. These included pH,
dissolved oxygen and biochemical oxygen demand. Apart from a biological
sand filter, no other form of culture water treatment was effected
during the investigation. The effect of growing the "sugpo" or jumbo
tiger prawn, Penaeus monodon (Kinne, 1977) for 22 weeks in a captive
body of sea-water was evaluated by comparing the survival and wet mass
with those reported by other workers.
This study differs greatly from previous reports on nitrification in
closed systems because both the "causes" and "symptoms" of this important
detoxifying process are investigated. It is intended that the
findings of such a study would aid culturists in exploiting the nitrifying
potential of closed systems to its utmost. / Thesis (M.Sc.)-University of Durban-Westville, 1984.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/9791 |
| Date | 21 October 2013 |
| Creators | Pillay, Balakrishna. |
| Contributors | Roth, G. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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