Iodine
in Reef Tanks 2: Effects on Macroalgae Growth
In
a previous
article1 I detailed many aspects of iodine in the ocean
and in marine aquaria, including what forms it takes, what organisms are
known to use it, and the sources of iodine in reef aquaria.One thing that is lacking from the available literature, however,
is any clear information on whether specific iodine supplementation is
beneficial to any organisms in coral reef aquaria.This information is lacking for two reasons:
1.There are few reports available in the literature on whether most
of the organisms that we typically keep in aquaria use iodine, and if
they do, how they obtain it (that is, from the water column or from
food).Most likely, these
tests have never been done on many of our favorite tank inhabitants,
such as corals, despite the fact that many do contain considerable
amounts of iodine (e.g.,
iodine in Xenia sp. and Sarcophyton sp.).2
2.Even if an organism is known to use iodine from the water column,
or even require it for survival, it is unclear whether the natural
sources of iodine in reef aquaria (such as foods) will fulfill the
requirement.Only testing
under actual aquarium conditions can give an answer, though the answer
given may well vary depending on the nature of the aquarium chosen (that
is, on the amounts of iodine in the imports and exports).
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Of
all of the organisms described in my previous article, macroalgae
are clearly the most studied with respect to iodine.Perhaps this emphasis on macroalgae is because they have so
many different ways of using iodine and are among the very few
organisms known to have a need for dissolved iodine from the water
column.[Alternatively,
one might argue that this is known because they have been
extensively studied, and not the other way around, but we’ll
leave the philosophy for another day.]
As
a starting point for investigating whether iodine supplements are
of value in reef aquaria, I have elected to study the effect of
supplemental iodine on the growth rate of two species of
macroalgae (an unidentified species of Chaetomorpha that I
obtained years ago from Inland Aquatics, and Caulerpa racemosa).
I selected macroalgae for this initial study because:
1.They are known to take up soluble iodine from the water column.
2.They are easy to grow and study in relatively short periods of
time.
3.More and more aquarists are using macroalgae for nutrient export,
so maximizing the growth of macroalgae is important to many aquarists
In
fact, that last reason is why I choose the macroalgae that I did.They are not among the species of macroalgae with the highest
iodine levels (Table 1), but they are two primary species for which I
believe aquarists would most like to optimize growth rates.It is interesting to note in Table 1 how much the iodine levels
in macroalgae actually vary between species, and they also seem to vary
with season, water temperature, and depth, even within the same species.6,7
Consequently, people growing other species of macroalgae may get quite
different results with iodine supplementation that I obtained with the
two species described in this article.
Table 1.Literature iodine concentrations for wild macroalgae (except
three Caulerpa values from aquaria).Those with the highest values are marked in red, and the yellow
sections relate to the types tested later in this article.
The
first question to address in testing whether macroalgae will be
stimulated to grow with the addition of iodine supplements is whether
they can absorb the forms dosed.There
have been many studies on what forms of iodine are taken up by
macroalgae, though none specifically on the two species of macroalgae to
be tested in this article.It
turns out that macroalgae in general can take up both iodide and iodate,
with a preference for iodide.8-12 The degree of preference seems to vary from species to
species, with most able to use both.For this reason, I chose iodide for my growth studies.
What
is Already Known About Iodine Requirements of Macroalgae?
For the two species of interest here
I could find nothing in the literature about iodine requirements.Some species of macroalgae do have a requirement for iodine
in order to grow.The
brown algae Ectocarpus
siliculosus, for example, requires at least 0.0015 ppm I-
for vegetative growth and 0.0025 ppm I- for normal
formation and maturation of pluriolocular sporangia, though it can
survive without growth in iodine deficient solutions.13At increased iodine levels, growth of this species
increases up to about 0.2 ppm I-, where growth levels
off, and stays constant up to at least 20 ppm I-.13
Laminaria
saccharina,
as another example, shows meiospore
germination increasing with iodine concentration up to about 0.1
ppm.In this case,
either iodide or iodate is suitable.That same species is insensitive to iodine concentration
after germination.14
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The red macroalgae Polysiphonia
urceolata has growth rates that increase linearly with iodine
concentration between 0.1 and 1 ppm iodine.15The same researchers showed that Nemalion
sp. showed no change in growth with iodine concentration, and Goniotrichum
elegans was inhibited at iodine concentration above natural levels.The macroalgae Asparagopsis
armatahas
been shown to grow optimally at iodide or iodate concentrations of 0.6
ppm iodine, and at levels above 1.8 ppm, the growth becomes inhibited.16
In
short, it appears that some species of macroalgae need more iodine than
others, and some may not need it at all.Consequently, it is not really possible to make any useful
conclusions about whether Chaetomorpha sp. or Caulerpa
racemosa will likely have any boost to growth from iodine
supplementation without actually testing them under real aquarium
conditions.
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Growth Test
Description
This section gives a
brief description of the growth tests such that most hobbyists can
understand what was done.In
a more detailed section at the end of the article I give all of
the details for anyone wanting to understand it more fully.
The
water that I used for the growth tests was made from Instant Ocean
salt that had been in a normal aquarium for 4-8 weeks prior to
use.This aquarium
contained live rock and sand, various invertebrates, and a fairly
large amount of macroalgae growing in it, both Chaetomorpha sp.
and Caulerpa racemosa.No
iodine had ever been specifically dosed to this tank, but it was
fed twice a day with OSI flake food.It was also supplemented with iron as described in a previous
article to ensure that the macroalgae was not growth limited
by lack of iron.17
The samples of
macroalgae and the water used in the studies were both taken from this
aquarium.The water is
expected to be at the low end of iodine levels found in aquaria since
iodine had not been specifically dosed and since there was a large
amount of macroalgae growing in the aquarium.Nevertheless, I do not report any measurements of iodine in this
water because I am not presently convinced of the reliability of
available hobby kits.I did
not detect any iodine by ICP-OES (which in these samples cannot detect
values below 0.3 ppm iodine) as I described in my first
iodine article.1 The water in this aquarium had no
detectable nitrate using Salifert and LaMotte kits (<0.5 ppm), and no
detectable phosphate using the Hach kit (<0.02 ppm).
The growth studies were
carried out in small clear plastic saucers lit by fluorescent lights.
There were 14 samples of each species tested.Half of these got supplemental iodide (0.05 ppm) in their water,
and half got no iodine supplementation.All samples also got an additional 4 ppm of nitrate and 0.06 ppm
of phosphate added to them to mimic typical aquaria and to try to ensure
that growth was not nutrient limited.There was no stirring or aeration of the samples. All
samples had a 100% water change every day.
The
macroalgae samples were weighed at the start of the study, and then
again at then end.The
saucers were placed in a double row along the middle 2.5 feet of the 4
foot fluorescent lamps, about 5” below the lights as shown in Figure
1. The Chaetomorpha sp. was grown for 10 days, and the Caulerpa
racemosa for 21 days.
Figure
1.Schematic showing
the arrangement of the double row of sample saucers under the
fluorescent lamps (shown in blue).The samples with supplemental iodine (red) alternated
positions with those that did not (green).
Iodine Effects on Chaetomorpha
sp.
It
turns out that the Chaetomorpha sp. grew tremendously during the
course of this study.One
specimen grew to more than six times its starting weight during this
10-day period, and even the slowest specimens grew to more than three
times their starting weight.
The average sample
getting supplemental iodine grew to 5.1 times its initial weight during
the 10-day period.The
average sample getting no supplemental iodine grew to 4.7 times its
initial weight during the 10-day period.Statistical analysis of this 9% difference in growth (5.1x vs.
4.7x) shows it to not be statistically significant (p= 0.47 using a
standard t-test).
Figure
2.The growth of Chaetomorpha
sp. samples as a function of position across the test
apparatus.
Interestingly, the
growth seems to be a function of placement along the fluorescent tubes. Samples
at the ends of the rows apparently grew more slowly.This data is shown in Figure 2, where the “Position under
Lamps” refers to the side to side positioning, and each position has
one that has added iodine and one that does not.The samples in the middle of the experiment had a higher growth
rate than those near the sides of the water bath.This result is likely to have come from differences in lighting
intensity as a function of position, since those at the ends were
partially shielded from those portions of the lamps off to one side by
the sides of the water bath (Figure 1).
Taking the assumption
that lighting intensity did cause differences in growth, then a
different sort of statistical analysis can be carried out, where samples
under equivalent lighting can be compared.In this case, a paired t-test can be used to see if the
samples adjacent to each other (that is, in the same lighting but with
and without added iodide) have different growth rates.Using this type of paired t-test, the results are still not up to
the level to claim true statistical significance (in this case, p =
0.16, meaning that there is a 16% chance that the iodine treatment has
no effect and the differences were due to random variations, and an 84%
chance that there was a real difference between the supplemented and
unsupplemented samples; scientists usually like to see that 16% chance
drop to 5%, or preferably much less, before claiming
“significance”).Perhaps
a larger study would have had the power to demonstrate that the observed
9% difference was “significant.”
Regardless
of whether it is statistically significant or not, or whether a larger
study would have been clearer, the actual observed difference in growth
rate (9%) is small.Most
aquarists will not be so concerned with optimizing the growth rate that
9% would be critical.
Iodine Effects on Caulerpa
racemosa
It
turns out that the Caulerpa racemosa grew more slowly during the
course of its 21-day study.Two specimens even died (one in each group). Each Caulerpa
racemosa sample started as just a stolon (the growing tips) of the
plant, with no thalli (the round balls), though a few had rhizoids
(roots).By the end of the
test, many samples had rhizoids and several had thalli, and most lost a
portion of the stolon near the end that was cut.
The
average sample getting supplemental iodine grew by 17% wet weight during
the test.The average
sample getting no supplemental iodine grew by 12% wet weight during the
test.These values are,
however, misleading.Most
of the samples did have considerable new growth, but they also died from
the end where they were cut off of the mother macroalgae.The variation between samples was quite large, with some samples
actually losing weight overall, and several gaining more than 40% of
their original weight.Statistical
analysis of this 41% difference in growth (17% vs. 12%) shows it to not
be statistically significant (p= 0.66 using a standard t-test and 0.70
using the paired t-test).
Figure
3.The growth of Caulerpa
racemosa samples as a function of position across the test
apparatus.
Interestingly,
the growth of the Caulerpa racemosa does not seem to be as dependent on
the position under the lamps (Figure 3).Perhaps Caulerpa racemosa does not require as much
lighting for optimal growth as does Chaetomorpha
sp.
Summary
Neither species of
macroalgae tested were shown to grow statistically faster with
supplemental iodine.Perhaps
larger studies would show such an effect.Nevertheless, it does not seem that a strong case can be made for
iodine supplementation if the goal is growth of these two species, most
especially in the case of Chaetomorpha sp., where the growth is
very good with and without supplemental iodine.
Whether any other
organisms benefit from iodine supplementation is even less clear.The literature does not support the need, but testing has been
very limited.I’ve initiated some additional experiments on mushroom
corals (Actinodiscus sp.) by attaching them to the same types of
saucers as in this study and I’ve let them equilibrate in the refugia
of my main tank for several weeks.These mushroom corals may, in the future, be the basis of
additional growth studies to be carried out in a similar fashion to
those described here for macroalgae.
In future articles, I
hope to report on depletion rates of iodine in aquaria, as well as on
testing methods, but so far I’m not comfortable with the results that
I’ve obtained with the iodine kits that I’ve tried.Before saying anything definitive about them, I plan to do more
extensive testing of them.
Growth Test Details
This section gives a
more detailed description of how the growth tests were carried out.
I started with Instant
Ocean salt mix adjusted to natural seawater salinity (35 ppt) measured
by conductivity (53 mS/cm).The
water was then aged for 4-8 weeks in an aquarium containing live rock
and sand, various invertebrates such as bristle worms, amphipods, and
mushroom corals that had all moved from my main tank system. This system
had no addition filtration and only an air stone for aeration and
circulation.It was
maintained at approximately 80 ºF. It also had a fairly large amount of
macroalgae growing in it, both Chaetomorpha sp. and Caulerpa
racemosa.No iodine
hade been specifically dosed to this tank, but it was feed twice a day
with OSI flake food (0.15 grams per day).It was also supplemented with iron as described in a previous
article to ensure that the macroalgae was not growth limited by lack
of iron.17
The samples of
macroalgae and the water used in the studies were both taken from this
aquarium.I am not
reporting any measurements of iodine in this water because I am not
presently convinced of the reliability of available hobby kits.I did not detect any iodine by ICP-OES (which in these
samples cannot detect values below 0.3 ppm iodine) as I described in my first
iodine article.1 The water in this aquarium had no
detectable nitrate using Salifert and LaMotte kits (<0.5 ppm), and no
detectable phosphate using the Hach kit (<0.02 ppm).
The growth studies were
carried out in small clear plastic saucers, about 5”across and 1”
deep.They contained about
125 mL of water.It turns
out that the amount of iodine added to each saucer (0.05 ppm = 0.006
mg/saucer) was likely adequate to provide the growing macroalgae with
the amounts of iodine typically present in the new tissue that were
grown.For example, in Table 1 the various samples of Chaetomorpha
sp. are reported to contain 24, 68, 144, 362, and 1124 ppm dry
weight iodine.The amount
of new tissue grown in the saucer with the greatest growth had just
under a gram (wet) of total new growth, or about 0.13 grams dry weight
(most were significantly less).That
tissue would then take up 0.003, 0.009, 0.02, 0.05, and 0.15 mg of
iodine.Over the growth
period of 10 days, the amount of iodine added to each saucer was 10 days
x 0.006 mg/day = 0.06 mg.Consequently,
there was more iodine added than would be accounted for through growth
by all but the literature sample with the highest iodine content.Still, the macroalgae may use the iodide and then release it in
another form, and also the samples may have added more tissue at the end
of the test than at the start simply because they were larger then, and
so might have been iodide depleted on those later days.
The saucers were kept
at 80-82 ºF by being submerged about ½” in a circulating water bath.The bottoms of the saucers were almost completely in contact with
the water to maximize the temperature control on the samples by placing
the samples on thin metal rods.
The samples were lit by
placing them under 2 48” fluorescent lights (40 watts each).One was a “cool white” bulb by ABCO and the second was a
“Softtone” by Phillips.The
lights were on for 12 h/day.The
saucers containing the samples were placed in a double row along the
middle 2.5 feet of the 4 foot fluorescent lamps, about 5” below the
lights as shown in Figure 1.The entire apparatus was partially covered to reduce evaporation,
though some evaporation (and consequent increase in salinity in the
saucers) would presumably take place each day before the next 100% water
change.
There
were 14 samples of each species tested.Half of these got supplemental iodide (0.05 ppm) in their water,
and half got no iodine supplementation.The water for the test was made as follows:
Two liters of water from the
aquarium was collected.Nitrate
was added by spiking the entire 2 liters with 0.8 mL of a stock
solution of sodium nitrate in DI water (6.96 g of NaNO3
in 500 mL; 10,160 ppm nitrate ion).The final nitrate concentration in the test water was 4.1 ppm.
Phosphate was added by spiking the entire 2 liters with 2.0 mL of
a stock solution of sodium phosphate dodecahydrate (Na3PO4
· 12 H2O) in DI water (0.117 g in 500 mL; 58 ppm
phosphate ion).The
final phosphate concentration in the test water was 0.06 ppm. This
nitrate and phosphate was added to mimic typical aquaria and to
try to ensure that growth was not nutrient limited.
The
two liters of water was then split in half.Half was used as is (already spiked with nitrate and
phosphate) for the control samples.Iodide supplementation was accomplished in the other half
starting with a 1,090 ppm iodine stock solution comprising sodium
iodide in water (0.644 grams NaI in 500 mL of DI water).This standard was freshly diluted each day 1:10 in DI water
(making 109 ppm iodide), and then added to the other liter of the
nitrate- and phosphate-spiked aquarium water.
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There was no stirring
or aeration of the samples. All samples had a 100% water change every
day.
The macroalgae was
weighed at the start of the study, and then again at then end using a
high precision electronic balance.Each macroalgae sample was gently patted dry with a paper
towel prior to weighing.
The Chaetomorpha sp.
samples ranged in starting weight from 0.115 to 0.206 grams.They were obtained from a large growing ball of macroalgae by
gently pulling out several strands.These were weighed and then used immediately for the test. . The Chaetomorpha
sp. was grown for 10 days
The
Caulerpa racemosa samples consisted of just the growing tips (stolons)
of the plant, with no thalli (the round balls) and only a few had
rhizoids (roots).They were
cut to 3-7 inches and then allowed to recover in the aquarium for 4 days
prior to weighing and initiation of the test.By the end of the test, many samples had rhizoids and several had
thalli, and most lost a portion of the stolon near the end that was cut.The initial weights of the Caulerpa racemosa ranged from 0.125 g
to 0.715 g.Final weights
were only slightly higher, and in some cases lower.One sample in each cohort (+/- iodide) died and completely
disintegrated during the 21 days of the test.
References
1.
Iodine in Marine Aquaria: Part I by Randy Holmes-FarleyAdvanced Aquarist March 2003.
3.Concentration of iodine and bromine by plants in the seas of
Japan and Okhotsk.Saenko, G. N.; Kravtsova, I. I.; Ivanenko, V. V.;
Sheludko, S. I.Far
East Sci. Cent.,Inst.
Chem.,Vladivostok,USSR.Marine Biology (Berlin, Germany)(1978),47(3),243-50.
4.Instrumental neutron activation analysis of iodine levels in
fourteen seaweed species from the coastal belt of Ghana.Serfor-Armah,
Y.; Nyarko, B. J. B.; Carboo, D.; Osae, E. K.; Anim-Sampong, S.; Akaho,
E. H. K.Chemistry Department, National Nuclear Research
Institute,Ghana Atomic
Energy Commission,Legon-Accra,Ghana.Journal
of Radioanalytical and Nuclear Chemistry(2000),245(2),443-446.
5.Distribution of iodine in marine algae of Goa region.Solimabi;
Das, B.Natl. Inst. Oceanogr.,Goa,India.Indian Journal of Marine Sciences(1977),6(2),180-1.
6.Seasonal variation, bioaccumulation and prevention of loss of
iodine in seaweeds.Mairh,
O. P.; Ramavat, B. K.; Tewari, A.; Oza, R. M.; Joshi, H. V.Cent. Salt Mar. Chem. Res. Inst.,Bhavnagar,India.Phytochemistry(1989),28(12),3307-10.
7.Biological aspects of iodine behavior during interaction of
algae with sea water.Kravtsova,
Yu. Yu.; Saenko, G. N.USSR.Editor(s):
Krasnov, E. V.Vzaimodeistvie
Vodoi Zhivym Veshchestvom, Tr. Mezhdunar. Simp.(1979),Meeting Date
1975,1146-52.
8. The transformation
of iodate to iodide in marine phytoplankton cultures.Wong, George T. F.; Piumsomboon, Ajcharaporn U.; Dunstan,
William M.Department
of Ocean, Earth and Atmospheric Sciences,Old Dominion University,Norfolk,VA,USA.Marine Ecology: Progress Series(2002),23727-39.
9.
Absorption and elimination of chemical forms of iodine-131 by marine
macrophytes.Svetasheva,
S. K.Inst. Biol. Yuzhn. Morei,Sevastopol,USSR.Gidrobiologicheskii Zhurnal(1984),20(4),94-6.
10.
Chemical forms of radioactive iodine in seawater and its effects upon
marine organisms.Hirano,
Shigeki; Ishii, Toshiaki; Nakamura, Ryoichi; Matsuba, Mitsue; Koyanagi,
Taku.Natl.
Inst. Radiol. Sci.,Nakaminato,Japan.Radioisotopes(1983),32(7),319-22.
11.
Effect of stable iodine upon the concentration of radioactive iodine
by marine organisms.Hirano,
Shigeki; Matsuba, Mitsue; Koyanagi, Taku.Div. Mar. Radioecol.,Natl.
Inst. Radiol. Sci.,Nakaminato,
Japan.Radioisotopes(1983),32(8),353-8.
12.Absorption and elimination of chemical forms of iodine-131
by marine macrophytes.Svetasheva, S. K.Inst. Biol. Yuzhn. Morei,Sevastopol,USSR.Gidrobiologicheskii
Zhurnal(1984),20(4),94-6.
13.
Influence of iodine on growth and development of the brown alga
Ectocarpus siliculosus in axenic cultures.Woolery, Marti L.; Lewin, Ralph A.Scripps Inst. Oceanogr.,Univ. California,La
Jolla,CA,USA.Phycologia(1973),12(3-4),131-8.
14.
Environmental control of gametogenesis in Laminaria saccharina.III.Effects of
different iodine concentrations and chloride and iodide ratios.Hsiao,
Stephen I.; Druehl, Louis D.Dep. Biol. Sci.,Simon Fraser Univ.,Burnaby,BC,Can.Canadian Journal of Botany(1973),51(5),989-97.
15.
Influence of iodine and bromine on growth of some red algae in axenic
culture.Fries,
Lisbeth.Univ. Uppsala,Swed.Physiol.
Plantarum(1966),19(3),800-8.
16.Effects of iodine on the growth of the fronds of Asparagopsis
armata (Rhodophyceae, Bonnemaisoniales) in culture from spear bearing
branches.Codomier,
Louis; Segot, Madeleine; Combaut, Georges.Groupe Rech. Biol. Chim. Veg. Mar.,Cent. Univ. Perpignan,Perpignan,Fr.Giornale
Botanico Italiano(1979),113(5-6),387-93.
17.Iron in a Reef Tank by Randy Holmes-Farley Advanced
Aquarist August 2002
