Is this not the whole meaning with using ozone? To oxidise organics (including yellowing substances) and reduce organisms without defence system for different free oxygen radicals? Most "higher" organisms have a defence system. IMO - photosynthesizing organisms must have it because free oxygen radicals is a intermediate step in their waste production of oxygen.
It is not generally ozone remaining in the water, but toxic species such as bromate.
Toxicity of Ozone Produced Oxidants (OPOs)
Two sorts of toxicity studies of ozone produced oxidants (OPOs, such as bromate, hypobromous acid, etc.) are relevant to reef aquarists. The first involves the testing of seawater that has been exposed to ozone, and the second involves the testing of specific compounds dissolved in seawater that are known to form when using ozone. Most of the OPOs are unstable, and so have few or no specific toxicity studies. Bromate (BrO3-) is the notable exception, and its toxicity is examined in the next section.
Much of the study of OPOs stems from applications slightly different from aquaria, and such studies must be viewed in that light. Often they relate to aquaculture facilities, where ozone is used at high doses to sterilize the water. Other studies are done on the disinfection of wastewater using ozone, another high dose application. Bear in mind that OPOs in reef aquarium applications will be at a maximum of about 0.3 ppm in typical reaction chambers, and will be lower (hopefully, much lower) once the water passes over activated carbon (assuming it does) and finally enters the aquarium. The concentration of OPO is always given in terms of the weight of ozone that produces that amount of oxidant.
In terms of the toxicity of ozonated seawater itself, one group concluded that fish were relatively insensitive to OPOs:
"Ozonation of estuarine or marine waters can produce significant amount of bromate…Toxicity studies showed that the concentrations of bromate which theoretically could be formed in an ozonated discharge were not toxic to the early life stages of striped bass (Morone saxatilis) and juvenile spot (Leiostomus xanthurus)."50
Larvae are, in general, more sensitive to OPOs than are eggs,51 adults or juveniles.52 Japanese flounder eggs were found to be impacted by OPOs to the extent that 50% did not hatch after one minute of exposure to 2.2 ppm OPO. Larvae aged 3-15 days were killed to the extent of 50% in 24 hours at 0.02-0.05 ppm OPO. Larvae aged 44 days were killed to the extent of 50% in 24 hours at 0.15 ppm OPO. In this case, the larvae were shown to have damage to their branchial tissues.53
The eggs and larvae of Japanese whiting (Silago japonica) also have been tested for toxicity by OPOs. In this case, half of the eggs and larvae died in about 24 hours when exposed to 0.18 and 0.23 ppm OPOs, respectively.54
Certain microalgae are also relatively insensitive to OPOs (perhaps to the disappointment of many aquarists). The growth of the microalgae Tetraselmis chuii was found to be unaffected at levels up to 0.7 ppm.55 At 1 ppm, growth was impacted negatively.
Toxicity tests of OPOs on shrimp show them to be less sensitive than fish. Penaeus chinensis and Paralichthys olivaceus were found to live up to 48 hours at OPO concentrations of more than 1 ppm, while ******* halibut (fish) in the same study lived only three hours at 1 ppm and 48 hours at 0.13 ppm.56
As for other organisms, the damage to the American oyster (Crassostrea virginica) by OPOs varied with their age. Even for adults, fecal matter accumulation was reduced at OPO levels as low as 0.05 ppm.57
The effect of OPOs on rotifers (Brachionus plicatilis) has also been determined.58 No effect on survival was seen at less than 0.22 ppm OPO, but effects became significant above that level. The authors point out that bacteria and other pathogens can be killed at that level, so rotifer cultures can be used with that amount of continuous ozone to reduce bacterial contamination.
Are these levels of OPO toxicity important to reef aquarists? That is difficult to answer without knowing the levels that are attained in reef aquaria. In a typical ozone application in reef aquaria that might produce 0.1-0.3 ppm OPO in a reaction chamber, the levels are quite significant compared to potential toxicity to fish larvae and other organisms at as little as 0.02-0.05 ppm. After passing the reactor's effluent over activated carbon, the OPO concentrations should be much lower, but exactly how low is unclear and will vary considerably in different setups.
Toxicity of Bromate
The toxicity of ozone and bromate at "natural" levels in the ocean has been assessed and usually found to be minimal.59 Few studies have examined the toxicity of excess bromate itself to marine organisms.60 One review article concluded:
"Bromate toxicity tests on marine animals indicate the levels of bromate produced by chlorination or ozonation of power plant cooling waters are not acutely toxic. The LC50 ranged from 30 ppm bromate for Pacific oyster, Crassostrea gigas, larva to several hundred ppm for fish, shrimp and clams."9
One individual study showed that Pacific oysters (Crassostrea gigas) had abnormal larval development at bromate levels of 30-300 ppm.61,62 Fertilized eggs of the oyster Crassostrea virginica were killed at 1 ppm.63 The clams Protothaca staminea (littleneck) and Macoma inquinata (bent-nosed) were killed by 880 ppm.64 The marine dinoflagellate Glenodinium halli showed changes in population growth at 16 ppm.65 The marine microalgae Isochrysis galbana showed changes in population growth at 8 ppm.65 The marine diatom (Skeletonema costatum) showed changes in population growth at 0.125 to 16 ppm.65 The marine diatom Thalassiosira pseudonana showed changes in population growth at 16 ppm.65 The salmon Oncorhynchus keta was killed at 500 ppm and the perch Cymatogaster aggregata at 880 ppm.64 Two shrimp (Pandalus danae and Neomysis awatschensis) were killed at 880 and 176 ppm, respectively.64
Are these levels of toxicity important to reef aquarists? That is difficult to answer without knowing the levels that are attained in typical reef aquaria. The one study in the literature of bromate in a seawater aquarium, described above, showed the accumulation of up to 0.6 ppm bromate, although that was an application in which ozone was used for disinfection, so it was used at high doses. That level is high enough, however, to cause toxicity to certain organisms, but not others. In a typical reef aquarium ozone application, the bromate in the aquarium water is likely to be much lower. How much lower will likely depend on the way it is used, especially the dose and whether it is passed over activated carbon before entering the aquarium. It may also depend on the other husbandry practices used in the aquarium, because some procedures (such as denitrification) may reduce bromate levels. In any case, the potential toxicity data for bromate support the practice of using activated carbon after ozone exposure.
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