TOXICITY
IN THE REEF TANK: ARE YOU AWARE? by D. WADE LEHMANN
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"All substances
are poisons; there is none which is not a poison. The right [concentration] differentiates
a poison from a remedy." - Paracelsus
Toxicology, or the study of poisons and their
modes of action, can be a very relevant addendum to any aquarist’s background.
Toxicity can be elicited from sources within a tank or from sources outside of the tank.
This discussion leads us through many of the more salient points regarding reef aquaria
and the relationships of toxicology.
We are all familiar with the presence of
toxic organisms in our tanks or at least the possibility of them. Cucumbers can produce
the ever fearful ‘cuke nuke’ or we might reach in and find ourselves nursing a
sting from a foxface rabbitfish. But, toxicity in our tanks can have a diverse array of
sources. These can range from the use of aerosols or solvents (such as ammonia, hairspray,
or spray lubricants) in close proximity to our tanks to the presence of certain corals,
fish, or alga within the system to that frying pan in the kitchen that occasionally smokes
up the house.
In discussing toxicity in our
tanks there are a few terms with which we should all be familiar:
Toxins are compounds that are natural in
origin, such as those from invertebrates, fish, or alga. These can be proteins, small
peptides, or other organic compounds.
Toxicants are those compounds that are
anthropogenic in nature (human made) such as pesticides, solvents, or residues from
burning fossil fuels or waste.
Bioaccumulation is the process of a chemical
or toxin passing from the environment into living tissue, such as a solvent used in paint
gradually making its way into the fish in your tank. Bioaccumulation consists of uptake
from both the environment and food minus that portion of a compound lost to metabolism or
excretion (termed depuration). (figure 1)
Biomagnification is what most people think of
when they hear the term bioaccumulation. This is the passing of compounds from prey items
into predator (or plant material to herbivore) with a resulting concentration higher in
the consumer than in the individual food sources.
Bioconcentration is the increase in a
concentration of toxicant in the body of an organism directly from the environment. This
is generally a passive process although certain compounds have been shown to be taken up
actively. A good example is mercury compounds through the gills of fish during
respiration.
Partitioning is a term used to describe the
movement of a compound from one media (i.e., air) to another (i.e., an organism or water
in an aquarium).
Detoxication describes the process of
removing an agent from a site by an exogenous means. Activated carbon export is a good
example relative to our tanks.
Detoxification is a natural process that
occurs when animals or bacteria/fungi convert compounds into less toxic forms. These
processes can also cause activation in rare instances.
LC50 or lethal concentration is a
measure of the concentration it takes to kill 50% of the population of interest. (LD is
lethal dose, EC is effective concentration; subscripted by the % mortality or effected).
Now that you have a basic set of definitions,
I would like to remind you that very few compounds of real interest in our aquaria have
known toxic values (LC50 or otherwise). The only data that exist tend to be
derived from real world ecosystems or closed laboratory systems. Each type of animal we
keep has different susceptibilities, different mechanisms for dealing with the insult, and
wholly different responses to the presence of toxic chemicals. Murky estuarine systems
will have a very different outcome regarding many chemicals than will a pristine reef.
Organisms from different areas also have a wide range of susceptibilities. Most compounds
discussed in this current context will only be represented in basic terms and generally
oversimplified. Much of what I will present to you is based on organic compounds (an
organic molecule is a molecule containing carbon in its structure) that have been
thoroughly studied and modeled in different environments, such as pesticides and various
hydrocarbons, so whether these assumptions can be extrapolated to a home aquarium or not
is still unknown. Proceeding conservatively though, we should consider the worst possible
scenario to preserve the animals we house.
There are a number of physical-chemical
properties of individual compounds that allow for more movement into our aquaria from the
surrounding environment. The water solubility of compounds is of primary concern. Some
compounds have a very strong aversion to water, which precludes them from readily entering
our systems (hydrophobic or lipophilic). If a compound has extremely high water
solubility, this will allow it to enter the water in a tank, but frequently prevents entry
of the toxin into organisms due to the presence of lipid based cell membranes. (figure 2)
However, as with most things, there are complicating factors. The presence of various
organic compounds and surfactants in the waters of our tanks actually assist the transfer
of some organic compounds into our tanks. This has been well studied in organic
contaminants such as PCBs, PAHs, and pesticides. Many inorganic contaminants and metals
are taken up readily (due to small size or the presence of specific carrier molecules in
the cell surface). In general, those toxicants or toxins with moderate to moderately high
lipophilicity values and little to no ionic charge are the most likely to cause issues
within a tank and to our animals.
Another chemical property is vapor pressure
of the compound. This is directly related to water solubility via a factor called
Henry’s Law (Eq. 1). Higher H values (Henry’s Law values) indicate less aqueous
risk. Vapor pressure is a measure of the ability of compounds to escape from one media
(i.e., water) into air. The specifics of the interactions of these physical-chemical
properties will be left out of this discussion to keep it easier to follow. If you want
more information, any good environmental chemistry text is a great source, please see the
references.
The pH of our tanks is also a point that
needs to be made in reference to toxicity. Many compounds have a state that shifts from
ionic to nonionic around a given pH. This fact makes some compounds more toxic in a reef
tank versus a low pH freshwater tank. Cyanide is a great example as it is nonionized (HCN)
at low pH and ionized (CN-) at high pH (pKa of 9.1, which indicates 50% of each
at a pH of 9.1). While cyanide as CN- is more deadly to fish, there will be a
greater proportion of HCN in a tank that is at pH 8.0. Generally ionized compounds tend to
be less of a threat to living systems than nonionized due to passive permeability through
membranes, ammonia as NH3 or NH4+ is a prime example.
Certain compounds that are charged can mimic endogenous or necessary substrates (lead
mimicking calcium for example) and are readily taken up by cells.
Figure 1 - Hypothetical
uptake and depuration curve based on an animal placed into a tank that is polluted then
moved to a clean environment or exposure such as a solvent used near a tank that peaks
then declines after use.
Passage of toxins or toxicants into an organism is also
highly dependant on the specific physical-chemical characteristics of a given toxicant.
The ability of a compound to pass through a cellular membrane is dependant on a property
much like water solubility, only in this instance it’s the reverse. Typically
referred to as lipophilicity, it is a measure of how much a compound likes to dissolve in
lipid membranes or hydrophobic solvents. Lipophilicity is usually measured as the
octanol:water coefficient (Kow). (Eq. 2) This partitioning coefficient is a
good indicator of the ability of compounds to pass into cellular membranes. As with water
solubility, too much potential to dissolve into lipids causes the toxins to get stuck in
cellular membranes and fat deposits and be of little importance to the inner workings of a
cell. This partitioning effect can account for some forms of toxicity to cells, although
the vast majority of compounds are bound away from their site of action. Those toxicants
or toxins with the moderate to moderately high lipophilicity values are the most likely to
cause issues within a tank. Lipophilicity is not a true concern for inorganic compounds as
most are regulated by different mechanisms and have no associated Kow.
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Figure 2 - Diagramatic
representation of a cell membrane and typical organic toxicants ranging from fully
hydrophilic to fully hydrophobic. The ideal toxicant has some degree of water and lipid
solubility and no ionic charge. Note that many inorganic compounds do not follow this
example.
The case of lipophilic compounds
entering our tanks gets a bit more complex when you also consider what is in the water. A
truly lipophilic or hydrophobic compound theoretically would not enter the system;
however, our tanks have a significant amount of organics in the water column. This organic
matter can serve as a transport vehicle for those compounds. Binding of organic molecules
to one another is common, and the organics in our tanks can include surfactants as well as
residual slime, fatty acids, and other organic byproducts from corals, fish, or plants
that readily serve as transport media.
Equations:
Equation 1: H = VP/S
VP = vapor pressure
S = solubility constant
Equation 2: KOW = concentration in octanol/concentration in water
Sources of Toxicants
Anthropogenic sources come in all forms:
vapors/aerosols, gases, liquids, and solids. Those of most concern to the average aquarium
hobbyist (assuming no one tosses pennies in the tank for good luck!) are vapors/aerosols.
These can be in the form of a pesticide sprayed in proximity to the tank (sprays applied
outside the home are found inside in measurable quantity and within a short amount of
time, Leidy et al.) or an abundance of solvent vapor from painting or cleaning.
Most solvents used in household application have little overt toxicity and are unlikely to
be soluble enough in water due to very high vapor pressure to partition into our tanks,
but the effects on invertebrates and fishes are rarely, if ever, known. As a general rule,
if a product has warnings for ventilation on it or toxicity on contact information, follow
even more stringent rules than you would for human safety. There are sources more remote
as well. Compounds emitted from the combustion of fossil fuels and wastes are known to be
toxic in quantity and are found ubiquitously. Pesticides can also be widely distributed,
although most of concern (on a large scale) is traditional and no longer in use. One such
pesticide is DDT, of which we all carry detectable amounts. There is, of course, a concern
with gases as well, but in general terms the conditions under which these can occur are
rare as it would also be unhealthy for people. High concentrations of carbon monoxide (CO)
and carbon dioxide (CO2) are possible in a well insulated house with natural
gas appliances. These would have the same effect on your tank’s residents as it would
have on you; namely suffocation. Cleaning solvents such as bleach (sodium hypochlorite) or
ammonia are of concern in this regard. Both are volatile to a sufficient degree and water
soluble. This gives them the correct combination of characteristics to be toxic to our
animals. Use caution and ventilate the house when cleaning with these products.
Metals are also a concern in a reef
tank. We all know we shouldn’t use metal accessories as they corrode or rust with
saltwater contact. But what you may not realize is that many metal alloys contain various
metals which can become ‘entrapped’ in our tanks due to a lack of export
mechanisms. Galvanization commonly employs zinc for example. While some metals are not
directly toxic, there can be other indirect effects such as algal blooms with increased
iron content. Many cases of nutrient limitation are not actually N or P, but in reality
Fe. We all know that copper can destroy invertebrate populations. Mercury is another of
concern. Although rare to find, old thermometers used mercury and a single thermometer
breaking in a tank could have dire consequences (note that red liquid in thermometers is
an alcohol and not nearly as dangerous as the silver mercury type). The mercury danger
would also extend to the reefkeeper upon contact with the water. I will cover mercury more
thoroughly in a future article. Metals and other toxic inorganic molecules can accrue due
to poor input water quality (arsenic is a good example in the west and southwest),
dropping metal items into the tank and allowing them to corrode, or from the atmosphere.
Another source of toxicants, although
some might possibly call them toxins, is the tank itself. The nitrogen cycle produces
ammonia, nitrite, and nitrate all of which are fairly toxic although via different
mechanisms, especially in the marine environment. Nitrite is the most toxic on a
concentration basis. A completely cycled and balanced tank with sufficient media (i.e.,
that with bacterially colonized surface area such as live rock and sand) is prevention
enough for these inherent threats.
Movement and Clearance of
Toxins
Persistence is a measure of the time a
toxin or toxicant stays in a system. Scientifically noted as half-life (t1/2), it is an
actual measure of the time it takes for 50% of the compound to be removed or altered to
another form. Some compounds are relatively unstable and are broken down quickly, whereas
others are highly resistant to breakdown. A good example of this process is seen in DDT.
DDT is the parent compound, an insecticide, used to treat fields. DDT is broken down to
DDE (and DDD) by various chemical and biological processes. DDE is very much more stable
than DDT (although both are relatively stable hence they are now illegal to use) so that
measurements taken today can still detect quantities of DDE and only traces of DDT from
historical use. Persistence is affected by the above listed mechanisms of removal and
metabolism/breakdown.
Chemical processes
Toxins in natural systems, whether in
estuaries, the open ocean, or our aquaria, undergo movement and transformation. There
exist both chemically and biologically based mechanisms that destroy or remove these
compounds. Chemically, the main removal processes are photolysis - the energy from light
radiation causing the breakage of molecules; hydrolysis - the destruction of chemical
bonds by insertion of a water molecule; and oxidation - the process of removing electrons
usually by adding groups such as oxygen molecules into a chemical structure. These
processes likely occur in our tanks (assuming the UV/light intensity is enough in the case
of photolysis) helping to remove some less stable organic molecules. For those aquarists
using ozone or UV sterilizers, oxidation may be a leading mechanism of elimination.
However, breaking organic molecules is not always beneficial. The broken molecules can be
detrimental in their own right due to increased reactivity, but that is beyond the scope
of this article.
Complexation can be another mechanism
that keeps toxicants from having any ill effects in our tanks. Copper is a great example.
In regions where there is a high degree of near anoxic space, such as internal to live
rock and deep in sand beds, copper is combined readily with sulfur causing a precipitation
into a solid form. This mechanism is taken advantage of in wetlands during bioremediation.
This is probably not a major source of removal for our tanks, however, as ideally we keep
them highly oxygenated and the surface area to water volume ratio is too low.
Biological processes
Inherent in the term biological is the
fact that living things go through certain cycles of growth, birth, lactation, diet and
death. Each of these can have a role in removal or dilution of toxicants. If a body burden
of a specific hydrocarbon is unchanged and a fish grows, the concentration per unit of
body weight is reduced. Birth or deposition of eggs is a process by which a great deal of
lipophilic toxicant can be shed. Fish laden with heavy stores of glycogen and fat (fish
use muscle as a storage mechanism where mammals use fat cells) that are forced to fast due
to breeding or migratory behaviors can see a release of compounds from those stores as
they are consumed. This can be very detrimental in cases of highly lipophilic compounds in
that they are suddenly available again and can cause cellular injury. The activity
associated with life can be both beneficial, as is the case with removal of toxins, or
detrimental as stated above.
Biological processes leading to the
breakdown of toxicants or toxins use many of the same chemical means as listed above. The
major difference is that organisms have a suite of enzyme driven systems for altering and
eliminating these compounds from their cells. Let’s consider a toxicant such as an
organophosphate insecticide (parathion is a good example). The water solubility is
sufficient to allow it to enter your tank water via the air passing over its surface and
concomitant contact with organics in the water. The lipophilicity is sufficient to allow
the compound to enter into the body, either via ingestion or through simple diffusion. Now
that the compound is within the cell, how is it removed? It obviously could simply diffuse
out, but if it were carried in actively, what then? It had all the characteristics that
allowed it to accumulate within the cell and those same characteristics will allow it to
stay in the cell. In defense, the cell calls into action a series of enzymes that allow
for alteration or metabolism of the pesticide. Small side groups are added or removed, or
the molecule is split into smaller pieces. These altered molecules now have altered
properties. They are typically more water soluble and have sites that allow for
recognition and removal from the cell by specific transporters. This is the process of
metabolism and excretion. These processes are what allow organisms to survive in an
ever-changing world filled with unknown and potentially toxic compounds. There are also
situations where these compounds cannot be removed readily. In this case, the potential
for biomagnification exists.
Figure 3 - Input
and elimination routes for aquaria and fish. Green arrows represent uptake. Pink arrows
represent depuration (elimination).
Figure 4 - Biomagnification. Increase
in toxicant concentration in tissue at higher trophic levels due to dietary uptake versus
biomass.
Just as in the case with excretion from
individual cells, organisms are also capable of removing toxins from the body. In mammals,
the bile system is used to clear compounds from the bloodstream via the liver. The bile
then flows outward into the intestines where some compounds are reabsorbed and others are
excreted. This process is much the same in all vertebrates.
Plants may serve as a source of export
as well: Macroalgaes specifically as they are readily pruned back and physically removed
from the system. Macrophytes, however, are not a good primary source of elimination.
Organic uptake tends to be limited to smaller organic compounds such as sugars, amino
acids, nucleic acids, peptides, and various others. In order for plants to be effective as
an elimination route, bacteria and other micro-life must convert the larger molecules into
smaller molecules. This service as a secondary export is useful, but not in the context of
detoxication.
Another facet of biological metabolism
and elimination is the presence of bacteria and fungi. In most natural systems, and
likely, in most reef tanks, the biomass of the bacterial and fungal populations far
exceeds that of any other biological agent. These bacteria and fungi are responsible for
the low residual free nitrogen oxide levels as well as many other highly specific and
important functions. Among those functions is the ability to convert compounds to their
mineralized state. Mineralization is the process, whether driven by a single species or by
many species, of taking a chemical and reducing it to its smallest components, such as H20,
CO2, O2, etc.
Remember as well that our reef tanks are
not open systems with backing by heavily populated mangrove or estuarine systems. Our
closed systems, while we attempt to mimic natural processes, prohibit complete mimicry.
Microbial action and recycling of nutrients help us in our attempt to provide oligotrophic
waters, although export remains critical. A striking example of the need for export
mechanisms (not just transformation processes) is in the bacterial recycling of conjugated
toxins. Vertebrates (and likely other higher organisms as well) have systems, as
previously mentioned, for conjugating toxins to help eliminate and reduce toxicity of some
compounds. The system works very efficiently to prevent toxicity in the organism. However,
many conjugates are a substrate for bacterial growth and are cleaved from the parent
molecule, thereby freeing the original toxin from its altered state and allowing it to
once again be available for uptake.
Mechanical processes
While the chemical processes above will
occur in our tanks, there are other means by which we can eliminate toxins and toxicants
so as to make their presence less deleterious. Activated carbon functions well in this
regard. Activated carbon is known to bind organic molecules (note that it will not help
with nitrogen cycle components or many other inorganic compounds). It has commonly been
employed in clarifying water such as removing the yellow coloration from older tanks. The
molecules which cause yellowing in the water are organic wastes, typically large organic
acids, which readily bind to activated carbon. Carbon is consumed once all of the
available sites are covered with these organics or a biofilm (bacterial mat) develops on
the granule surfaces preventing diffusion through the pores. Carbon is a highly useful,
readily available, and easy to use method for removal of smaller organic contaminants. I
highly recommend at least occasional use of carbon for the removal of wastes and other
released compounds, especially those released by corals competing for space in our closed
systems.
Another common method of organic removal
is via a protein skimmer. This method has many of the same properties as carbon usage,
only by a different chemical interaction. This takes advantage of the fact that organics
will cling to one another, in function, much the same process as activated carbon. In the
end, it is another great mechanism for removal of toxins.
Conclusions
In conclusion, I would like everyone to
come away with a better understanding of what could be affecting our tanks. We have all
either been involved in or known someone who had ‘something’ happen in the tank
and never been able to identify a source of the problem. Be aware of the consequences of
using aerosols and other household cleaners and solutions in proximity to your aquaria. Be
smart. The processes, from input to export are all dynamic. This manuscript is to make you
aware of possibilities, but don’t forget that the dynamics of the system may
eliminate sources of stress before they ever have an obvious effect. Here are common
mistakes that people make and their cures:
-Spraying glass cleaner
directly on the aquarium.
-Instead, spray it onto a rag or paper
towel well away from the tank. Glass cleaners frequently contain small amounts of ammonia,
but more importantly have various alcohols of unknown toxicity.
-Use of pesticides in proximity to a
tank. Many pesticides travel very well, especially when put into aerosol form. Our bodies
deal with the toxicant without ill effect in most instances, but our fish cannot do so as
readily.
-Instead, use less toxic forms of insect
control or very careful application (the 3 foot spray instead of the 22 foot!).
-Cooking in hot oil with a tank nearby.
This vaporizes the oils which then condense on cool surfaces. This type of toxicant can
actually kill birds in acute exposures.
-Always ventilate well and place tanks
away from the kitchen.
-Allowing pesticide application to the
entire house. Even spot applications spread throughout the air in a house quickly. Many
pesticides are extremely toxic to aquatic life, especially invertebrates.
-Stick to low toxicity pesticides or
remove the tank prior to application (I suggest for months afterwards of heavy application
that carbon be used and changed weekly).
In summary, a toxin must have three
properties to be of concern. First, it must be toxic. If a compound is present, but has no
toxic consequences, then it is of little concern. Secondly, it must be available.
Availability includes lipophilicity and vapor pressure. If a compound immediately leaves
the aqueous environment, it is of little concern. Thirdly, the toxin must be persistent.
If a chemical has a half-life of only minutes, it will not likely be of concern.
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Remediation of a tank that
has been polluted by a toxin or toxicant is highly dependant on the specific compound
involved. Basically, any organic compounds would best be dealt with using activated carbon
and water changes. Water changes serve to dilute most toxic compounds, but remember that
stress can occur from environmental changes which amplify toxic effects. Be certain to
match all parameters to the tank’s current levels. The carbon acts as a sink for the
compounds, removing them from being active against your animals. As an example, I have
seen a number of cases where sand eating holothurians (cucumbers) have been assaulted by
powerheads and have released a fairly potent toxin that hits vertebrates. Immediate use of
carbon seems to be critical in saving fish affected by the toxin, although if ingestion of
cucumber flesh occurs, there is almost no option to save the fish.
While this article deals with the various
processes behind poisonous compounds entering our tanks and their movement and removal
from tanks, it listed only mild examples of various toxins and toxicants. The next
articles in the series will deal with specific examples that are more relevant to our home
aquaria, including fish, invertebrates, alga, and anthropogenic compounds.
References:
Wright CG, Leidy RB, Dupree, Jr HE. "Chlorpyrifos in the Air and Soil
of Houses Eight Years after Its Application for Termite Control." Bull. Environ.
Contam. Toxicol. 1994. 52:131-134
