Scientists discover early warnings of abalone death-
Good news for farmers and abalone lovers.
Kasturi Reddy-Lopata, Andreas Lopata and PA Cook
University of Cape Town, South Africa.
The highly lucrative abalone industry stands to benefit from recent research
by UCT scientists who have identified sublethal doses of ammonia as early
warning signals for abalone death. The team have also identified what may be the
first evidence of heat shock proteins in abalone.
Water quality is the single most important factor affecting an abalone. It is not only a source of essential nutrients (such as
dissolved oxygen and nutrients but also acts as a waste disposal system). Poor water quality resulting from toxic substances or insufficient amounts of essential substances may retard growth and may cause death
of the abalone. Ammonia is the major end-product of protein metabolism in the
abalone, but high levels of ammonia can be damaging to abalone survival. It is thus important to determine the accurate levels of toxicity and tolerance of ammonia in
the abalone to help improve the water quality in South African farms.
In order to understand the toxicity of ammonia in aqueous media, it is important to understand
its chemical equilibrium in water. In aqueous solutions, ammonia exists in two chemical forms, namely free or unionised ammonia (NH3) and the ionised form (NH4). The toxicity of ammonia is due to the un-ionized form, while the ionised form
presents little toxicity. Wet chemical tests used on farms measure the sum of the un-ionised and ionised ammonia or total ammonia nitrogen
(TAN). The concentration of un-ionised ammonia is strongly dependent on pH,
temperature and salinity. It is important to calculate the toxic un-ionised ammonia when looking at ammonia toxicity as TAN can remain constant throughout the experiment, but with changes in pH and temperature, the un-ionised ammonia changes.
In general, at low pH and temperature, the lethal level of un-ionised ammonia is low.
Tests conducted by UCT scientists on abalone growth rates showed a significant
reduction in growth rate and length of abalone exposed to sub-lethal levels of
ammonia. Higher levels of ammonia can be lethal to the abalone. An abalone
farmer needs to know at what ammonia concentrations the abalone will die. Toxic
levels vary with the size of the abalone. Research identified a lethal
concentration for the cocktail size abalone (5 to 8cm) to have an LC50 of 8.06
ug/l. In toxicity tests LC50 values (which is the mean lethal concentration killing 50% of your test animals) are used as standards.
These scientists something interesting: allowing the abalone to acclimatise to
sub-lethal doses of ammonia increased the LC50 value to 12.65ug/l.
A possible mechanism for this increased tolerance to ammonia is the so-called stress or heat shock proteins (hsps).
It has been said that all organisms when exposed to elevated temperatures respond by
synthesizing a group of highly conserved proteins called the heat-shock proteins
or hsps. Hsps are activated not only by heat but by other physiological stresses that
shut down the synthesis of most other proteins. Ammonium chloride
is one such agent that activates hsp genes by inhibiting proteolysis. Hsps allow for the refolding of partially stress-denatured proteins. There are different groups of hsps designated according to their molecular weights. The 70
kilodalton hsp is the most conserved of proteins. Hsp70 has been reported in a number of invertebrates and a few molluscs (limpets and mussels) but
not in abalone.
To determine the presence of hsps in abalone, heat shock experiments were
performed at the Allergollogy Unit UCT Medical School. These initial experiments
have identified a 70 kilodalton protein from the foot of an abalone which may
very well be the 70 kilodalton heat shock protein.
Further investigation involving amino acid analysis of this protein will confirm the presence of
hsps. These heat shock proteins may prove to be of immense importance to
abalone farmers. They may be indicators of chemical stresses (such as
sub-lethal ammonia levels) which could be utilised to control water quality in abalone farms.
One final word to abalone farmers: When analysing water samples from farms, one should calculate the toxic un-ionised ammonia present and not just Total ammonia nitrogen.
High sub-lethal levels of ammonia do retard growth by over 50% and therefore one should look at these levels as
a warning of ammonia toxicity and not when 5% of your animals start dying.
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