Ozonation Effects on the Speciation of Dissolved Iodine in Artificial Seawater at the National Aquarium in Baltimore
Numerous studies and reviews have been conducted on the control, regulation, and functions of thyroid
tissue in fish.2,4,5,7 In both bony and cartilaginous fishes, thyroid metabolism is affected by dietary and
environmental iodine levels.3,7,10 In fish, absorption of iodine from the aqueous environment, via diffusion
across the gill membranes, is considered to be more efficient than absorption from the alimentary tract.10 In
fish and other vertebrates, low levels of environmental iodine and goitrogenic substances (e.g., ammonia, nitrates,
nitrites, urea, chemical toxins) have been associated with the development of thyroid lesions, specifically hyperplasia
(goiter)3,6,8,9 and neoplasia (e.g., adenomas, adenocarcinomas).8-10
As a trace element in the marine environment, iodine occurs in several forms due to ongoing oxidative
processes. In seawater, total dissolved iodine consists of two inorganic chemical species, iodide (I -) and
iodate (IO3-), plus dissolved organic iodine (DOI).11,12 Iodide is required for synthesis
of the thyroid hormones tri-iodothyronine (T3) and thyroxine (T4) in vertebrates, including
fish.2,5,7 Large aquarium systems, however, often use ozone, a strong oxidant, in order to remove organic debris,
including bacteria and viruses, from tank water.1 It is likely that ozone treatment in aquaria also alters the
speciation and thus the bio-availability of dissolved iodine in artificial seawater, with subsequent effects on iodine
metabolism and thyroid health in fish.
The primary goal of our study was to examine the effects of ozonation on the speciation of dissolved
iodine in artificial seawater made at the National Aquarium in Baltimore (NAIB). The effect of adding ozone to seawater is
measurable in millivolts (mV) as oxidation-reduction potential (ORP). We hypothesized that ozone concentrations required to
achieve ORP levels approximating those in an ozone contact chamber (800 mV) and protein skimmer (400 mV) would significantly
change iodine species concentration in artificial seawater, resulting in a relative iodine deficiency for resident fish.
Because the NAIB Atlantic Coral Reef exhibit tank (ACR), a closed, ozonated 1.3 x 106 L (335,000-gallon) system,
has a low-grade incidence of thyroid lesions in its fishes (e.g., thyroiditis, hyperplasia, adenoma, adenocarcinoma), the
secondary goal of our study was to examine iodine levels present in the ACR and any possible correlations with thyroid
A laboratory experiment was performed which compared iodine speciation of aliquots of 25.5°C NAIB
raw seawater mix before, during, and after exposure for fixed time periods to air only, or to concentrations of ozone needed
to achieve an ORP of 400 mV or 800 mV. Aliquots were immediately frozen at -20°C until analyzed. Differential pulse
polarography and cathodic stripping square wave voltammetry,12 both electrochemical methods, were used to detect
levels of I -and IO3-, respectively, in each sample. Total dissolved iodine was measured
separately according to other recently developed methods,12 allowing calculation of DOI as the difference between
total dissolved iodine and total inorganic iodine (I-+ IO3-).11,12 Results
revealed a systematic decrease in concentrations of I- and DOI and concomitant increase of
IO3-with increasing exposure to ozone. At an ORP of 800 mV, approximately two-thirds of the initial I
-disappeared, DOI became undetectable, and there was no significant change in the concentration of total iodine.
This suggests an ozone-induced conversion of I -to IO3-and a conversion of -DOI to I
-and/or IO3. Additionally, ACR water samples were found to be virtually free of I-and DOI.
Based on these findings, we conclude that ozonation of artificial seawater may alter the relative
concentrations of iodine species present in a closed tank system. Furthermore, it is possible that depletion of
I- and DOI via oxidation by ozone and/or biologic processes is linked to thyroid disease documented in ACR fish
over recent years. Thus, it may be advisable to supplement the diet and/or tank water of captive teleosts and elasmobranchs
living in ozonated seawater with iodide. Iodide concentrations ranging from 0.1-0.15 µM the tank water may prevent
goiter in several species of elasmobranchs housed in marine aquaria.3 Ideally, frequent testing of tank water for
I -, IO3-, and total dissolved iodine levels is recommended to detect fluctuations in
available iodine and aid in calculation of appropriate supplement dosages for resident fish. Concentrations of phytoplankton
and organic debris, temperature, pH, and other factors are also reported to affect iodine speciation in artificial and
natural seawater systems.3,11,12 Some of these factors, as well as the presence of possible goitrogens or
thyrotoxins, may be examined in future investigations.
The authors wish to thank the generous staff at the National Aquarium in Baltimore, especially Ryan
Bromwell, April Smith, Jill Arnold, Liz Neely, and Andy Aiken (technical support); Susie Ridenour (library services);
Valerie Lounsbury (editorial input); Michele Martin, and Ian Walker (animal health). Financial support for iodine analyses
was granted through the NAIB Biological Programs Department.
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