Journal of Zoo and Wildlife Medicine 35(3): 347±355, 2004 Copyright 2004 by American Association of Zoo Veterinarians EFFECTS OF OZONATION ON THE SPECIATION OF DISSOLVED IODINE IN ARTIFICIAL SEAWATER Johanna Sherrill, D.V.M., M.S., Brent R. Whitaker, M.S., D.V.M., and George T. F. Wong, Ph.D. Abstract: Iodine in the form of iodide is required for synthesis of tri-iodothyronine and thyroxine in ®sh. Iodine chemical speciation in aliquots of raw arti®cial seawater mix was measured before, during, and after exposure for ®xed time periods to air only and to concentrations of ozone required to achieve oxidation±reduction potentials typical of a protein skimmer (400 mV) and an ozone contact chamber (800 mV). Chemical species of iodine were also measured in tank water from a large, recirculating, ozonated aquarium system that has a low-grade incidence of thyroid lesions (e.g., thyroiditis, hyperplasia, adenoma, and adenocarcinoma) in its ®sh. With increasing exposure to ozone, concen- trations of iodide and dissolved organic iodine (DOI) decreased, whereas iodate levels increased. As a result of exposure to 400 mV, iodide concentration dropped to less than half the amount found in raw arti®cial seawater mix. After exposure to 800 mV, initial iodide levels decreased by 67%, and DOI became undetectable, whereas iodate concentration increased by 155%, with no remarkable change in total iodine concentration. These results indicate ozone-induced conversions from iodide to iodate, and DOI to iodide or iodate (or both). Iodide and DOI were not detectable in the aquarium system's water samples. Ozonation of arti®cial seawater may alter the relative concentrations of iodine species in a closed tank system, so that iodide supplementation of the diet or tank water of captive teleosts and elasmobranchs living in ozonated seawater is advisable. Key words: Fish, arti®cial seawater, iodine, ozone, thyroid disease, goiter. INTRODUCTION have been documented in a variety of ®sh within Thyroid function and hormone regulation in ®sh natural and arti®cial systems, and a lack of iodine have been reviewed in relation to hormone concen- has been implicated as a causative agent in these 9,34 trations and conversions, growth stages, migration conditions. A gold®sh (Carassius auratus) goiter patterns, osmoregulation, and natural epizootics of was successfully treated with an organic iodine thyroid hyperplasia (goiter).4,10,12±14,17,23,24,31,34 In gen- compound at a concentration of 10 ppm.8 eral, ®sh thyroid hormone metabolism mimics that Fish in closed seawater aquaria may experience of most other vertebrates.32,35 In bony and cartilag- gradual or sudden decreases in the bioavailability inous ®sh, thyroid metabolism and thyroid disease30 of iodine, leading to the development of I2 de®- are affected by dietary and environmental9,22 iodine ciency, thyroid disease, and even death.29,35 During levels.23,32 Studies of the chemical form of iodine the past several years, thyroid hyperplasia and neo- that is used in teleost or cartilaginous ®sh thyroid plasia have been detected in necropsied ®sh from metabolism are scarce.7,17,24 Iodide (I2) is required several different tanks at the National Aquarium in for the synthesis of thyroid hormones tri-iodothy- Baltimore (NAIB), including the Atlantic Coral 4,17,24 ronine (T3) and thyroxine (T4) in ®sh and other Reef (ACR) exhibit. The ACR is a three-story, 1.3 vertebrates.7 Iodine in an aqueous environment is 3 106±L habitat designed to look like a natural cor- more ef®ciently absorbed by diffusion across ®sh al reef that contains a variety of tropical bony and gill membranes than through the alimentary tract.33 cartilaginous ®sh species. Iodine in water provided rainbow trout (Oncorhyn- Speci®c lesions found in NAIB ®sh include thy- chus mykiss) with up to 84% of their plasma I2.22 roid hyperplasia in a neon goby (Elacatinus ocean- Thyroid hyperplasia9,21,30,34 and neoplasia2,29,30,41 ops) and in several bluestriped grunts (Haemulon sciurus), a thyroid adenoma penetrating major ar- From the Department of Animal Health, Smithsonian terial blood supply in a lookdown ®sh (Selene vo- National Zoological Park, 3001 Connecticut Avenue, mer), an expansive thyroid adenoma with marked, Washington, D.C. 20008, USA (Sherrill); the Department multifocal, granulomatous thyroiditis in a yellow- of Biological Programs, National Aquarium in Baltimore, headed jaw®sh (Opistognathus aurifrons), a thyroid Pier 3, 501 East Pratt Street, Baltimore, Maryland 21202, adenocarcinoma in®ltrating overlying epidermis in USA (Whitaker); and the Department of Ocean, Earth and an angel®sh (Pomancanthus sp.), and an invasive Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Avenue, Norfolk, Virginia 23529-0276, USA thyroid adenoma causing pericardial compression (Wong). Present address (Sherrill): P.O. Box 5051, Rolling (tamponade) in a trumpet ®sh (Aulostomus macu- Hills Estates, California 90274, USA. Correspondence lates). Cases of thyroid disease in ®sh inhabiting should be directed to Dr. Whitaker. the ACR may be caused by environmental or die- 347 348 JOURNAL OF ZOO AND WILDLIFE MEDICINE tary iodine de®ciency. Identi®cation of factors as- seawater mix from the NAIB mixing vat. All water sociated with functional iodine de®ciency in this samples to be used for subsequent experiments aquarium exhibit should aid in the prevention of were taken from either of these two containers. The thyroid disease in future resident ®sh and may ap- noncommercial, proprietary formula for NAIB ar- ply to similar closed systems in other aquaria. ti®cial seawater mix contains 0.5 mMofI2 or 0.08 As a trace element in the natural marine envi- mg/L (ppm) based on the addition of 38 g of po- ronment, dissolved iodine occurs either as inorgan- tassium iodide (KI) per 486,400 L of arti®cial sea- 2 2 ic I and iodate (IO3 ) or as dissolved organic io- water. 42 dine (DOI). The presence of molecular iodine (I2) Parameters of a separately obtained aliquot of the and of hypoiodous acid has been hypothesized in NAIB arti®cial seawater mix were analyzed by in- 42 seawater but neither has been observed directly. house techniques, adapted from standard methods The distribution of iodine chemical species, as well for the examination of wastewater.26 In brief, pH as the biologic or abiologic processes that may was measured with a combination electrode cause transformations among I2,IO2, and DOI, 3 (ThermoOrion Model 920Aplus pH/mV/ISE meter, have been a major focus of studies concerning the ThermoOrion, Beverly, Massachusetts 01915, biogeochemistry of iodine in natural marine envi- USA), calibrated at three points daily. Salinity was ronments.42±44 Evidence for the biologic use of IO 2 3 measured directly with a 0.475-cm, four-electrode or the simultaneous production of I2 by marine phytoplankton has been reported in laboratory cul- cell conductivity meter (ThermoOrion Model tures.28,44 Iodide may react with naturally occurring 150Aplus, ThermoOrion). Phosphate was measured reactive transient oxidants, including ozone and hy- in seawater by spectrophotometric ascorbic acid di- gestion (Hach DR-4000 UV/VIS, Hach Co., Love- drogen peroxide, to form I2 and hypoiodous acid, and these products may further react with organic land, Colorado 80539-0389, USA). Simple titration molecules to form DOI.39,40 using a weak acid as the titrant and bromocresol as Large aquaria often use ozone, a potent oxidant, an indicator was used to measure alkalinity. to remove organic debris such as bacteria and vi- For each step of the experiment, a 1.5-L aliquot ruses3 from tank water. Ozone treatment of seawa- of NAIB raw arti®cial seawater was equilibrated to ter is an important part of water quality and life 25.58C in a 2-L glass beaker set within an open support for aquatic animal exhibits1 and aquaculture water bath. Throughout the study, the temperature systems.37 Effects of ozonation on the chemical of sample water was monitored using a glass ther- speciation and therefore bioavailability of iodine in mometer immersed in the beaker of interest. The arti®cial seawater are important to measure because thermometer was rinsed copiously with deionized of the potential for altered iodine metabolism and water between uses. effects on thyroid health in ®sh housed in ozonated systems. Ozone measurement In an effort to eliminate several variables, the The ORPs were measured in millivolts using a effects of ozonation on speciation of dissolved io- portable ORP meter (Model 2000, VWR Scienti®c dine in arti®cial seawater made at NAIB were stud- Products, South Plain®eld, New Jersey 07080, ied experimentally. The result of adding ozone to USA) as an estimate of ozone concentration in the seawater is measurable in millivolts as the oxida- seawater samples. The probe stayed immersed in a tion±reduction potential (ORP). We postulated that pH-balanced probe solution when not in use and ozone concentrations required to achieve ORP lev- was rinsed thoroughly with deionized water be- els approximating those in an ozone contact cham- tween measurements of seawater samples. ber (800 mV) and a protein skimmer (400 mV) would measurably alter concentrations of iodine Sample description chemical species in arti®cial seawater, resulting in a de®ciency of bioavailable iodine for resident ®sh. Iodine chemical speciation in aliquots of NAIB Because the ACR exhibit has a low-grade, docu- raw arti®cial seawater mix at 258C was compared mented incidence of thyroid lesions in its ®sh, an before, during, and after exposure for ®xed time additional goal of our study was to measure the periods to air only or to concentrations of ozone existing levels of iodine chemical species in this needed to achieve ORPs of 400 and 800 mV. Three closed, ozonated aquarium system. separate laboratory trials were run to perfect meth- ods.