The importance of
boron in marine aquaria is a subject that is not often discussed by
hobbyists, despite the fact that many people dose it every day as part
of their alkalinity supplements.In fact, most commentary on boron derives from manufacturers
that are selling it in one fashion or another as a “buffering”
agent.Unfortunately,
these discussions nearly always lack any quantitative discussion of
boron or the effects that it has (both positive and negative).This article describes in detail the forms and concentrations
that boron takes in seawater, and shows how much pH buffering it
actually provides, and why it does so.You may, in fact, be surprised to learn that boron actually
contributes only a minor fraction of the buffering of normal seawater.At the end of the article, I also make suggestions on how to
reduce the diurnal pH swing in reef tanks.
This article also
discusses the sources and sinks for boron in reef tanks, and shows that
while typical tanks may be slightly depleted in boron with respect to
natural levels, most are not so significantly depleted that any
correction is required.Nevertheless,
how to test for and supplement boron is addressed.
Finally, this article also touches briefly on the limited knowledge of
biological effects of boron in marine systems.Boron appears to be a necessary or desirable nutrient for certain
organisms, but is also toxic to others at levels not far above natural
levels (and below that present in one artificial salt mix).
Boron in
Seawater
In
natural seawater, boron is present at about 0.41 mM (4.4 ppm total
boron) and takes two different chemical forms.The predominate form is boric acid, comprising about 70% of the
total boron present.Boric acid, B(OH)3,
consists of a central boron atom and three hydroxyl groups arranged in a
triangle (Figure 1).The
second form is borate, B(OH)4-.It consists of a central boron atom and four hydroxyl groups
arranged in a tetrahedron (Figure 1). It carries a net negative charge,
while boric acid is neutral.These
two forms can interconvert in less than a second, so the two forms are
in chemical equilibrium with each other.
Figure 1.Space-filling molecular models of borate (left) and boric
acid (right). The boron is yellow, oxygen is red, and hydrogen
is blue.
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Boric acid
and borate have a variety of chemical and biological effects in
normal seawater, though biological effects are rarely discussed in
the marine aquarium hobby.The
biological effects are, however, important in certain situations,
and will be discussed later in this article.The most commonly known property of this system is its
ability to buffer seawater against pH changes, and this aspect is
discussed in detail in the next section.
Buffering
of Normal Seawater
The exact
percentage of boric acid and borate in any aqueous system is
dependent on pH.The
pKa of boric acid in seawater is about 8.55, depending on the
temperature.1That
is, the pH where both forms are equally represented is about 8.55
in a normal tropical reef tank.At lower pH values, such as those in most reef tanks, there
is more boric acid than borate.
The fact that the
two forms are related by equation 1 explains why boric acid and borate
together form a buffer system:
1.B(OH)3+H2OßàB(OH)4-+ H+pKa ~8.55
If the pH in a
system containing both forms were to begin to rise for any reason, some
of the B(OH)3 would be converted into B(OH)4-,
releasing a proton, H+.The pH would then not rise as much as it otherwise would.Likewise, if the pH in that system were to begin to drop for any
reason, some of the B(OH)4- would be converted
into B(OH)3, taking up a proton.The pH would then not drop as much as it otherwise would.
This effect is
exactly how a standard buffer works.
The other system
that significantly buffers the pH in normal seawater is the
bicarbonate/carbonate system:
2.HCO3-ßà
CO3-- +H+pKa ~ 8.92
The relative
buffering of these two systems (equations 1 and 2) depends on the
amounts of each system present, and also on the pH. For any given buffer
system, it turns out that the pH at which it gives optimal buffering
corresponds to the pKa, where there are equal amounts of each of the two
forms of the buffer present.Why
exactly this is true is beyond this article, but it relates to the fact
that for a given incremental change in pH, more of the buffer will
change form at that pH than would change form at any higher or lower pH,
and moreover, the farther that the pH is from the pKa, the smaller the
effective capacity of that buffer system to resist pH changes.
Buffering capacity
can be quantified using the buffer intensity, b,
defined mathematically in a way that is easy to calculate, but that
isn’t worth detailing here.2The units of the buffering intensity can be expressed as meq/L or
meq/L/pH unit (these are equivalent since pH is really a dimensionless
parameter).Thinking
about it as meq/L/pH unit makes it easier to understand that it is a
measure of the amount of alkalinity (or acidity; either one measured as
meq/L) that needs to be added to impact the pH up or down by one unit
(though that is a substantial simplification).
In the case of
normal seawater at pH 8.2, b
= 0.19 meq/L/pH unit for the boric acid/borate system, and 0.63 meq/L/pH
unit for the bicarbonate/carbonate system.These values are additive, and result in a total buffering of b
= 0.82 meq/L/pH unit.Under
these conditions, the boric acid/borate system provides about 23% of the
total buffering, while the bicarbonate/carbonate system provides about
77%.
If the pH of
normal seawater is raised to 8.5, the total buffering is b
= 1.2 meq/L/pH unit, or about 40% greater than at pH 8.2 (because both
systems are closer to the pKa).At
this pH, the relative contribution of the two systems to the total
capacity is only slightly different than at pH 8.2, with 20% from borate
and 80% from carbonate.
If the pH of
normal seawater is lowered to 7.8, the total buffering is b
= 0.42 meq/L/pH unit, or about half that at pH 8.2 (because both systems
are farther from the pKa).At
this pH, the relative contribution of the two systems to the total
capacity is also only slightly different than at pH 8.2, with 29% from
borate and 71% from carbonate.
Buffering in
Artificial Seawater
In artificial
seawater the buffering can be very different than in natural seawater.In artificial seawater at pH 8.2 made using Seachem salt, where
the boric acid/borate system is artificially very high (12X
normal)3, the boric acid/borate system would provide a
huge b
= 2.3 meq/L/pH unit, which is a whopping 78% of the total buffering of b
= 2.9 meq/L/pH unit .That
amount of total buffering (3.5X the amount in normal seawater) will go a
long way toward making the pH much more stable than in natural seawater,
and is the reason that Seachem incorporates it into the salt mix.Whether this benefit is worth the concerns presented later in
this article about unusually high boron concentrations is up to
aquarists to decide for themselves.
Likewise, in
artificial seawater with a normal amount of boron, but artificially high
total alkalinity (say, 4 meq/L or 11 dKH, as is present in many reef
tanks), the total buffering is about b
= 1.2 meq/L/pH unit, or about 50% more than normal seawater.In this case, the relative contribution of the borate system to
total buffer is less, or only about 16% of the total.
Would Depleted
Boron Cause Big pH Swings?
Boron may be
slowly depleted in some reef aquaria, and that possibility is discussed
in detail below.If it is,
would that impact the buffering in such a way that aquarists will
experience substantially larger pH swings?Asked another way: Should aquarists worry about borate from a
buffering perspective?Generally,
no, in my opinion.In some
situations, where pH stability is paramount, normal or excess borate is
potentially preferred.In a
normal reef tank, it is unlikely to be important at all.
Let’s look at
the most extreme case: Would a tank with no boron experience excessive
pH swings?First, let me
qualify this entire discussion with the idea that there is little
information available on the effect of pH swings on tank inhabitants.For example, there is no evidence that a tank with a diurnal pH
swing of 0.10 or 0.01 pH units is any better (or worse) than one with a
swing of 0.2 or 0.3 units.Larger
swings may well swing you out of the overall optimal range (which I
typically describe as pH 7.8 to 8.5), but the swing itself is not the
concern there. If it turned out that pH swings of some particular
magnitude were found to be problematic, that might change the discussion
that follows.Note also
that the ocean is not pegged to a particular pH, and lagoons, in
particular, can shift by several tenths of a pH unit daily.1
So, a tank with a
total alkalinity of 3 meq/L (typical of reef tanks) and normal boron
levels, has 20% of it buffering, b,
provided by boric acid/borate. Will removing that 20% of the buffering
make the diurnal pH swing larger?Yes,
certainly.How much larger?
While those
effects can be calculated precisely (though it is tedious), or measured
exactly in an experiment, here is a reasonable simplification that holds
for small pH swings:
3.pH change=(acid or based added)/ b
So if one has a pH
swing of 0.10 pH units, and b
were decreased by 20%, then the swing would become 0.125 pH units.Likewise a swing of 0.20 pH units becomes a change of 0.25 pH
units.
While different
aquarists may react differently to the magnitude of this effect,
remember that it is the effect caused by a TOTAL lack of boron in the
tank.In a recent
study4 of reef tank water, it was found that typical reef
tanks were not far from natural boron levels, and even the very lowest
tank examined had half of the value, so would have about half of the
swing increase shown above.
For
comparison, a reef tank (pH 8.2) with no boric acid/borate at all, and a
total alkalinity of 4 meq/L (11 dKH) has b
= 1.0 meq/L/pH unit, or about 25% more total buffering than normal
seawater.
These
various comparisons should help aquarists understand the relative
contribution of borate to buffering, and to understand that slight
depletion (or excess) of boron will go unnoticed from a pH variation
perspective.Even the total
absence of borate might not be detected in the magnitude of the daily pH
swing present in most reef tanks.
Depletion of
Boron in Reef Tanks
Most artificial
salt mixes3 have boron in approximately natural levels
(0.41 mM; 4.4 ppm boron), with the exceptions of the brands Seachem (4.9
mM) and Coralife (1.26 mM), though a recent
test4 showed boron in Instant Ocean a bit lower (0.31 mM)
than the previous test (0.44 mM). 3Starting with natural levels, if the amounts of boron entering
the tank do not balance that being taken out, the boron levels may
either rise or fall over time.In
the 1 ½-year old reef tank of tank of Steve Nichols, for example, he
used Instant Ocean salt mix, used no other specific boron additives and
few water changes, and his tank was recently measured to have a boron
concentration of 0.28 mM (3.0 ppm).That result is on the order of 40% lower than natural seawater,
but only slightly lower (12%) than Instant Ocean salt mix in the same
test4.In
that study4,
the average tank was depleted by only about 10% compared to natural
seawater (although there were outliers at 2X natural levels and also at
half the natural level).
Boron Sinks:
Calcification
One
primary sink for boron in reef tanks is expected to be co-precipitation
with calcium carbonate.Typically,
boron is found to be incorporated at about 50-100 ppm into corals, but
the actual values vary considerably.5,6Boron in coral skeletons has recently been a hot topic of study
by chemical oceanographers.Specifically,
the incorporation of different boron isotopes into coral skeletons has
been hypothesized to be an indicator of the pH of the seawater, and
hence a way to measure ancient seawater pH.
The
reason for the differential isotope incorporation is that boric
acid is slightly (2%) enriched in 11B relative to
borate, which is enriched in 10B.The reason for this enrichment is that boric acid
containing 10B is very slightly more acidic than boric
acid containing 11B, and 10B is consequently
more in the borate form at any given pH.The actual enrichment will depend somewhat on pH, since the
two acids have different pKa values.Consequently, since it turns out boron is preferably
incorporated into CaCO3 from the borate form, then
looking at the isotope ratio in coral and foraminifera skeletons
can lead one back to the pH that was present when the CaCO3
was deposited.While the isotope issue is fascinating, and open to debate,
further discussion is beyond this article.Nevertheless, it has resulted in a great deal of study of
boron in corals and other ancient sources of CaCO3, so
the concentrations are well studied.
The
amount of boron that is expected to be co-precipitated with
calcium carbonate in a reef tank is, of course, a function of how
much calcium carbonate is deposited.At the high end of calcification
rates in reef aquaria,7 about 10 kg CaCO3/year
per 100 gallons (a daily consumption of 1.4 meq/L of alkalinity),
we can calculate that 50 ppm incorporation of boron results in a
depletion of boron by 500 mg in a year, or 0.12 mM, about 30% of
the natural boron level.
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Similarly,
at the middle to low end of calcification rates in reef aquaria, about 2
kg CaCO3/year per 100 gallons (a daily consumption of 0.28
meq/L of alkalinity), we can calculate that 50 ppm incorporation of
boron results in a depletion of boron by 100 mg in a year, or 0.02 mM,
about 6% of the natural boron level.
Boron Sinks:
Algae
Another sink for
boron is biological uptake by algae.Algae are not nearly so extensively studied with respect to boron
as are corals and other organisms that deposit CaCO3, but the
amounts involved can be significant.One study of 26 marine algae showed boron incorporated at 50-300
ppm of the dry weight.8
Calculations
similar to those for calcium carbonate deposition (above) apply to the
growth and harvesting of macroalgae.In many tanks, the deposition of calcium carbonate is likely to
be a larger contributor to boron export than is the harvesting of
macroalgae.However, algal
removal may be the biggest boron export mechanism for tanks with lower
calcification, but heavy macroalgae harvesting (like my own, where
macroalgae harvesting may outpace calcium deposition on a weight basis).
Sources of
Boron
Sources of boron,
other than the salt mix used in marine aquaria, can be any of the
following:
Foods
The
dissolution of CaCO3/CO2 reactor media that
contains substantial boron
Other calcium
and alkalinity supplements with incidental boron
Alkalinity
supplements that contain borate as an intentional additive
1.Foods.In a recent study
of foods9,10 supplied to reef aquaria, it was shown that
most contain very little in the way of boron.None had quantifiable levels, meaning they were all less than 2.5
ppm boron.If you added 5
grams of a food with 2.5 ppm boron to a 100 gallon tank every day, you
would be adding a total of 4.5 mg of boron in a year, or 0.001 mM per
year, which is less than 1% of the natural level of boron.
Since algae can
contain substantial boron,8 feeding macroalgae might be a
significant source.If you
fed 5 grams of dry macroalgae per day to a 100 gallon tank (a fairly
hefty amount), then at 200 ppm boron in the macroalgae, that 5 grams
contributes 1 mg of boron, or 0.24 mM
to the tank per day.In a
year, that would add 0.09 mM, which is about 22% of the natural boron
level.
2. Dissolution of
CaCO3/CO2 media.This method seems like it should be a good source of boron,
assuming that people are using crushed coral.For some reason, however, the commercial materials around are
very low in boron, much lower than reported values for boron in coral
skeletons.In two studies,11,12
it was found that boron ranged from less than 0.25 ppm (Koralith
and also quarried
limestone) to 0.7 ppm (Nature's
Ocean brand, Atlantic crushed coral)to 1.7 ppm (Super
Calc Gold).
All of these media
end up being small contributions to tank boron, even in cases with
fairly high calcification.For example, a tank with a yearly calcification rate of 10 kg
CaCO3 per 100 gallons of tank water (a daily addition of 1.4
meq/L of alkalinity) results in delivery of only 17 mg total in that 100
gallons, or only 0.004 mM(0.045 ppm)boron
using the Super Calc Gold.That
amount comprises only 1% of the natural level of boron.Of course, a media with higher boron concentrations, like coral
skeletons matching those reported in the literature, could result in a
net addition of fifty times that level, which then becomes quite
significant.
Boron in
Alkalinity Supplements
Some brands of
alkalinity supplements or “buffers” are claimed to contain added
boron (such as Marine Aquarium Buffer, Pro●Buffer dKH, and
Superbuffer dKH made by Kent), while others do not (such as Reef
Builder, Reef Buffer, and Reef Carbonate made by Seachem).Unfortunately, those that contain boron do not indicate how
much, either on the bottle, or when the manufacturer is directly asked.The only statement made is that it is an “important
contribution.”Consequently,
it is not apparent, without lab testing, whether the amounts of boron
present are large enough to be useful.It is also not apparent whether the amounts present might be high
enough to invoke some of the undesirable effects of boron that are
described below.
Two-Part
Calcium and Alkalinity Additives: The Effect on Boron
Another series of
products that should contain boron are the two-part calcium and
alkalinity supplements that claim to leave only a natural seawater ionic
residue after removal of calcium carbonate. That is, they contain all
ions, including boron, in natural seawater ratios with the exception of
calcium, carbonate, and bicarbonate (as is the case in B-ionic according
to Bob Stark of ESV).Assuming
that such supplements are properly formulated, then they should not
cause an increase in boron above natural levels, though they may help
“maintain” it if it is otherwise being depleted (just as a water
change would).
Biological
Aspects of Boron in Marine Aquaria
For such a
ubiquitous chemical, there is actually very little known about the
biological effects of boron.In one recent review,13 it is stated:
Vascular
plants, diatoms, and some species of marine algal flagellates have abs.
requirements for B, although the primary role remains unknown.
Further,
some types of macroalgae (e.g., Gracilaria tikvahiae) apparently grow
more rapidly in the presence of natural levels of boron than it its
complete absence.14In
one case, it is known that blue-green alga Nostoc linckia uses boron to
form borophycin, a potent cytotoxic compound that it may use for
defense. 15
It
has also been hypothesized by some in the hobby, but not demonstrated,
that borate may be important for calcification.Just as with the issues around strontium, I’m not aware of any
study that has tested the growth of a calcifying coral in the absence of
borate, so the lack of any data may simply reflect the lack of anyone
actually running a test.Nevertheless,
there is no evidence that calcification is altered if boron is allowed
to become depleted (or raised, for that matter).
While
the data are very sparse, these studies suggest that it is desirable to
maintain natural levels of boron in reef tanks.
Elevated
Boron: Toxicity
At boron levels
above that present in natural seawater, as is supplied in some
artificial salt mixes and as may develop from overuse of boron
supplements, boron begins to exert undesirable toxicity on a number of
organisms.The studies on
marine organisms are not wide ranging, so one must be careful in how to
interpret levels above natural seawater since tests have not been run on
most of the organisms that we keep.
In
general, marine organisms (invertebrates and fish) are seemingly more
prone to experience toxicity from boron than are freshwater species.The marine isopod Limnoria lignorum has a 24-hour LC50
(that is, the concentration at which 50% die in 24 hours) of only 2.6 mM(28 ppm boron).16That is only about 6 times the concentration in natural seawater
(and is BELOW the concentration in Seachem salt mix!).Similarly, the dab, Limanda limanda (a North Sea Fish), has a
96-hour LC50 of 6.8 mM (74 ppm boron).16
Elevated
Boron: Confounding Interpretation of Alkalinity Tests
One additional
complication that comes from substantially elevated borate is the
confounding of the interpretation of alkalinity
tests. 19When
reef aquarists are concerned about alkalinity, they are almost
invariably concerned with the alkalinity that comes from bicarbonate and
carbonate, and it is largely used as a surrogate measure of bicarbonate,
which is necessary
for calcification.20Nearly all hobby test kits measure alkalinity with a single
titration that provides total
alkalinity,19 which is the sum of bicarbonate, carbonate,
and borate alkalinity.When the levels of boron are similar to natural levels, then
the contribution of borate to that test is minimal, and is generally
safely ignored in guidelines for alkalinity (for example, keeping a reef
tank at 2.5-4 meq/L total alkalinity).
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However, if
the boron level is substantially above natural levels, as it is in
the Seachem salt mix with 12x normal levels, borate can actually
begin to dominate
such tests, 19 and makes knowing the real
bicarbonate and carbonate alkalinity much more difficult.Seachem sells a special borate alkalinity test kit to try
to disentangle these effects, but that is only really necessary
with tank water that contains greatly elevated boron levels.
Testing
and Supplementing Boron
If you are
interested in measuring boron in your tank, Salifert makes a boron
test kit, as well as a supplement for adding boron. I’d advise
maintaining a level close natural levels (0.41 mM; 4.4 ppm boron),
rather than any elevated level with the goal of trying to attain
better pH stability, but that is just my personal preference and
you may feel differently about pH stability.
If you have a test kit, and want
to supplement boron, you can also just use borax (from the grocery
store; borax is 21.5% boron by weight) dissolved in water.One
teaspoon will weigh about 4 grams,21 so 1 teaspoon in a
100 gallon tank will raise the boron level by 0.21 mM (2.3 ppm).
In general, I don’t think
that most reef tanks are likely to suffer much if you don’t measure
boron and just let the level be determined by the ebb and flow of boron
from your salt mix and the various sinks in the tank.For whatever it is worth, I’ve not measured the boron level in
my tank.
Increasing pH Stability
If you are concerned about pH
stability, here is a list of actions that can be taken to reduce the
diurnal pH swing in a reef tank:
Increase
aeration.Since the
diurnal pH swing largely derives from changes in CO2 in
the tank, “Perfect” aeration would nearly eliminate any pH
swing.Nevertheless,
perfect aeration is rather hard to accomplish in reef tank, but
better aeration can decrease a large pH swing.
Use
limewater and other high pH alkalinity supplements only between late
night and early morning.
Use
CaCO3/CO2 reactors only between late morning
and late evening.
Connect
a reverse daylight tank or lit refugium to the existing system.
Increase
the carbonate alkalinity.
Increase
the boron level.
Happy Reefing!
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References for
Further Reading
1.
Chemical Oceanography, Second Edition.Millero,
Frank J.; Editor.USA.(1996),496
pp.Publisher: (CRC,Boca Raton, Fla.)
2.
Aquatic Chemistry Concepts.Pankow,
J. F. (1991), 712 pp. Publisher: Lewis Publishers, Inc.The buffering intensity factor, b,
was calculated according to equation 8.30 on page 150:b
= 2.303Ca0a1
where C is the total concentration of the various forms of the buffer
system in question (from reference 1 for normal seawater values) and a0
anda1
are the fractions present in each the two forms.a0
anda1
were calculated using seawater pKa values given in reference 1, using
the equation 8.27 (p. 150): a0
= [H+]/([H+]
+ K) and a1
= 1-a0.
3.
The Composition Of Several Synthetic Seawater Mixes by Marlin
Atkinson and Craig Bingman
5.
Boron isotopic compositions of corals: Seawater or diagenesis record?Gaillardet,
Jerome; Allegre, Claude Jean.Paris,Fr.Earth and Planetary Science Letters(1995),136(3-4),665-76.
6.Coprecipitation and isotopic fractionation of boron in
modern biogenic carbonates.Vengosh, Avner; Kolodny, Yehoshua; Starinsky, Abraham;
Chivas, Allan R.; McCulloch, Malcolm T.Res. Sch. Earth Sci.,Aust.
Natl. Univ.,Canberra,Australia.Geochimica
et Cosmochimica Acta(1991),55(10),2901-10.
7.Calcification Rates in Several Tropical Coral Reef Aquaria
by Craig Bingman
8.
Boron content of seawater and marine algae of the Finistere coast
[France].Maurice,
J.Stn. Agron.,INRA,Quimper,Fr.C.
R. Seances Acad. Agric. Fr.(1983),69(17),1455-61.
9.
Necessary Nutrition, Foods and Supplements, A Preliminary
Investigation by Ronald Shimek
13.
Regulation of enzymatic activity: one possible role of dietary boron
in higher animals and humans.Hunt,
Curtiss D.Grand Forks Human Nutrition Research Center,USDA-ARS,Grand
Forks,ND,USA.Biological
Trace Element Research(1998),66(1-3),205-225.
14.
Inorganic nutrition of marine macroalgae in culture.McLachlan, J.Atl. Res. Lab.,Natl.
Res. Counc. Canada,Halifax,NS,Can.Editor(s): Srivastava, Lalit Mohan.Synth. Degrad. Processes Mar. Macrophytes, Proc. Conf.(1982),Meeting Date
1980,71-98.
15.
Structure and biosynthesis of borophycin, a new boeseken complex of
boric acid from a marine strain of the blue-green alga Nostoc linckia.Hemscheidt, Thomas; Puglisi, Melany P.; Larsen, Linda
K.; Patterson, Gregory M. L.; Moore, Richard E.; Rios, Jorge L.; Clardy,
Jon.Department
of Chemistry,University of
Hawaii,Honolulu,HI,USA.Journal of Organic Chemistry(1994),59(12),3467-71.
16.
A comparative analysis of the toxicity of boron compounds to
freshwater and saltwater species.Hovatter,
Patricia S.; Ross, Robert H.Health and Safety Research Division,Oak Ridge National Laboratory,Oak Ridge,TN,USA.ASTM
Special Technical Publication(1995),STP 1218(Environmental Toxicology and Risk Assessment: 3rd Vol.),288-302.
