Sulfur is another element required for animal
life. After carbon, hydrogen, oxygen and
nitrogen, the next most common element found
in organic molecules is sulfur. Sulfur is
a component of some essential amino acids
that are part of proteins and of humic substances.
You won't be surprised to find a "sulfur
cycle," even in the aquarium's limited
ecosystem, because all the elements essential
to life must be recycled, or else biological
processes would have exhausted the planet's
supply, after a couple of billion years.
In the sulfate ion (SO4), a sulfur atom is surrounded by four oxygen
atoms: oxidized, in other words. Sulfates
are harmless to fish, odorless and quite
stable. By contrast, in sulfides, such as
hydrogen sulfide (H2S), the sulfur atom is reduced, stripped
of its oxygen and joined to another atom.
Sulfides are reactive, and they can be quite
toxic.
Like other elemental cycles
represented in
the aquarium, the sulfur
cycle is driven
by bacterial processes.
Some bacterial metabolisms
can reduce sulfate, while
others oxidize
sulfide. The various sulfur-metabolizing
bacteria aren't all genetically
related.
They form a co-dependent
community, passing
sulfur back and forth between
sulfate and
sulfide states, even sometimes
excreting
a little elemental sulfur.
The aerobic sulfur
bacteria help use up the
last of diminishing
supplies of oxygen, so
they keep their anaerobic
neighbors safe from the
dangerous, reactive
O2.
Link. Few aquarium sites are very informative
about sulfur. Dr Gilles-Gonzalez offers a
good overview of microbial sulfur cycling in the context of an Ohio State course in
general microbiology.
Where is the aquarium's sulfur? At any given
moment, most of the sulfur in the aquarium
is in the form of sulfates (SO4). Though most of the organic sulfur is in
the form of sulfates, not all sulfates are
built into organic compounds: sulfates can
also be mineralized. Gypsum, for one example,
is calcium sulfate (when ground fine it makes
plaster-of-Paris, the major component of
sheetrock and those weekend "feeder
blocks" embedding food flakes) Sodium
sulfate is another widely distributed mineral
sulfate. Potassium sulfate is only moderately
soluble, but you might be adding it as a
plant fertilizer. Magnesium sulfate is Epsom
salts. Aluminum sulfate is the alum you might
occasionally use to clarify water.
Sulfate is the form of sulfur plants can
use, and like phosphate it can only re-enter
the food web through algae and plants. They
take it up and convert it into their characteristic
proteins, which may be consumed by animals.
Animals can't use sulfates directly; instead
they convert these plant-produced amino acids
into their own animal proteins. The death
of plants or animals begins the process of
decomposition, communal processes that involve
cooperating fungi and bacteria. The decomposers
break down animal and plant proteins, releasing
amino acids that animals can use or "mineralizing"
the amino acids, that is, breaking them down
all the way to release the sulfates, which
plants take up once more.
Sulfide (SO), stripped of oxygen, is the other form
of sulfur normally involved in biological
processes. (Thiosulfates--SO3-- are a less common form of sulfur. You
know them most likely from
sodium thiosulfate,
the active ingredient in
de-chlorinator.
Thiosulfates don't last
in the aquarium,
because bacterial communities
metabolize
them.)
Sulfur bacteria in the substrate. In the undisturbed substrate there are communities
of sulfur bacteria, playing various metabolic
roles. Their combined effect is to oxidize
sulfides to sulfate. Typical habitats for
sulfur bacteria are freshwater lake sediments
and intertidal mudflats. Sulfur is much less
common in freshwater sediments than it is
in marine environments. In sediments sulfur
bacteria communities form densely-populated
mats (denser than cyanobacterial mats) that
are confined to the narrow layers where the
oxygen and sulfide gradients overlap.
Sediment banding. Distinct layers form in sandy sediments--
which are more like most aquarium substrates
than mud sediments are.
Bands in an undisturbed substrate form in
reaction to four chemical gradients: light,
oxygen, sulfate and sulfide.
On the interface between water and substrate,
diatoms coat most surfaces, mixing with cyanobacteria.
The diatom layer protects cyanobacteria from
the effects of oxygen. On a mud substrate
this community might form a thin dense, somewhat
slimy mat: a biofilm.
Just below, purple bacteria (with bacterial
chlorophyll a), lie in a layer just above green sulfur
bacteria (with bacterial chlorophyll b). Purple and green (especially in alkaline
environments) bacteria are the phototrophic
"non-sulfur" bacteria of freshwater
sediments. In the layer where silica sand
transmits some light, especially in the infrared
range, "blind" but motile sulfur
bacteria exist in permanent symbiotic relations
with phototrophic green sulfur bacteria that
cover them. Phototrophic sulfur bacteria
require the simultaneous presence of reduced
inorganic sulfur compounds-- diffusing upwards
from anoxic layers-- and light coming from
above. They get their carbon from CO2. Green sulfur bacteria have two distinguishing
colors, bright grass green and chocolate-brown,
arising from phototrophic pigments.
In a nutrient-rich sediment,
heterotrophic
bacteria may overwhelm
the slower-growing
phototrophs.
Photosynthesizing sulfur bacteria. Anaerobic photosynthesizing sulfur bacteria
sometimes form distinct dark layers in the
substrate next to the tank glass, where daylight
hits it. The bacteria involved are Purple
Sulfur Bacteria (with red, brown, purple
and orange photosynthetic pigments) and,
usually beneath them, the olive-green to
brown Green Sulfur Bacteria. In anoxic environments,
these bacteria are able to metabolize sulfide
or elemental sulfur, using a primitive and
ancient kind of photosynthesis that doesn't
produce any oxygen. Very low levels of light
will suffice for them.
On one of my tanks (and yet not on others),
beginning a half-inch below the substrate
surface, there's a clear-edged area that
is black-green, perhaps dense with these
sulfur bacteria probably mixed with cyanobacteria.
My hunch is that these particular photosynthesizers
are discouraged by the oxygen in the topmost
gravel layer. What has been mysterious to
me was, why does this photosynthesizing zone
appear equally strongly in gravel that's
not exposed to daylight from a window? Then
RTR explained this phenomenon well in an
AC post, 3 Dec 2002:
"A significant
part of this may well be only on those sand
grains or the glass itself exposed to internally
relected light. Light from the tank's lighting
entering the glass at certain angles passes
through the interior glass surface, but is
reflected back from the exterior and comes
back out within the tank just below the substrate
surface-- the glass itself acts a light pipe
for a short distance. In one small area,
pull back or siphon up the sand. If the algae
is still there in the glass with a bit on
the sand removed, it is just internal reflection
promoting growth, and it may be removed at
will. If the patch extends into the tank
away from the glass a completely different
process is occuring."
Hydrogen sulfide. For
generations hydrogen sulfide has been
a bugaboo to aquarists, who have sniffed
their tanks for the tell-tale whiff of rotten
eggs that would confirm their dark fears.
Hydrogen sulfide can be produced by two kinds
of bacteria. Surprisingly, one kind are aerobic
bacteria: H2S can be formed in the normal process of
aerobic bacterial decomposing of plant and
animal remains. Atoms of sulfur form part
of the molecules in living tissues, notably
in proteins. When tissues are broken down,
the sulfur is first released as sulfides,
contributing to the stink of putrefaction.
In the decay process, where organic substances
from cells are being decomposed, a group
of "sulfur bacteria" scavenge oxygen
from the organic sulfate and use it to oxidize
carbon. Minute quantities of sulfur are released
throughout the aquarium, some of it as infinitesimal
amounts of H2S. Other bacteria, however, are right at
hand to oxidize the sulfides to sulfates;
they are a wide-ranging group of aerobic
bacteria, including thiobacilli. This sulfide/sulphate
regeneration is a normal component of the
mature community of the biofilm. But at points
in the cycle where oxygen is locally scarce,
such as microzones deep in a well-developed
biofilm, sulfur-reducing bacteria can short-circuit
the cycle by reducing free elemental sulfur
or SO4 directly back to sulfides, by-passing plants
and animals.
Obligate anaerobes and the dreaded hydrogen
sulfide. But, pretty rarely in most aquaria, pockets
of hydrogen sulfide can also form in deep
substrate layers that are never touched by
oxygen. In entirely oxygen-free zones of
the substrate, de-nitrating
bacteria can thrive, stripping the oxygen from nitrate
and nitrite. Their activities produce a nitrate
gradient. In sufficiently deep substrates,
nitrates may become entirely used up. Below
the de-nitrating zone, where there is neither
nitrate to work on, nor oxygen to interfere,
sulfate (SO4) can become the next-best electron receptor
for obligate anaerobes, those bacteria who
can't handle oxygen at all.
This metabolic process is much less efficient
as a source of energy, but as long as they
are utterly protected from the deadly oxygen,
a range of anaerobic sulfate-reducing bacteria strip the oxygen from sulfate and use it
to get carbon from carbon dioxide. Their
waste products include H2S. Hydrogen sulfide (H2S) is deadly to all aerobic organisms, so
the noxious gassy byproduct can help stabilize
a safely anoxic environment for the sulfate-reducing
bacteria that produce it, surrounding themselves
with a "killing zone" called a
sulfuretum.
Hydrogen sulfide is highly
reactive, part
of what makes it toxic
at the nanomolar (µM)
level, acc. to a 1997 California
Academy
of Sciences BioForum lecture
"Living with toxic sulfide" given by Dr Alissa Arp, who has been exploring
marine H2S metabolisms in deep oceanic thermal vents
and methane seeps and the
black mud of tidal
estuaries. They are a long
way from the freshwater
aquarium, but these are
good places to study
sulfur metabolisms.
For instance, Dr. Arp relates,
H2S reacts with iron to make black iron sulfide.
In the substrate, iron
in the ferric state,
Fe(III), will oxidize H2S, turn it to thiosulfate. This useful reaction
of Fe(III) and H2S has been harnessed by a curious California
mudflat worm, Urechis, which lives in an excavated burrow where
water is stagnant at low
tide. There is a
lot of bacterial activity
in the mud, which
is highly enriched organically,
so the oxygen
gradient is very steep.
The bacteria in anoxic
mud produce sulfide, and
if you walk on the
mudflat at low tide, you
smell the hydrogen
sulfide. So the worm Urechis
is faced with
environmental challenges,
which it meets
by detoxifying the H2S in its coelomic fluid, which is rich in
heme compounds, though
not associated with
a protein as in our hemoglobin.
The iron
in the heme group is in
the ferric state.
Its extra positive charge
oxidizes the sulfide
to non-toxic forms, principally
thiosulfate.
These sulfate-reducing bacteria giving off H2S are "obligate anaerobes," the
kind of primitive bacteria
that are poisoned
by a breath of oxygen.
There is another group
of specialized anaerobic
sulfur bacteria
that can also metabolize
sulfate for energy,
converting it to sulfide.
These bacteria
also require a strictly
anoxic environment
to work in. Oxygen doesn't
kill them outright,
but in the presence of
oxygen these sulfate
reducers can't grow and
multiply. They need
an organic substrate, such
as acids generated
by the fermentative activities
of other anaerobic
bacteria. Nitrate also
retards their action.
Only in deep, richly organic
substrates that
are disturbed at long intervals
could they
become a problem.
Sulfate-reducing bacteria tend to create
a blackened layer in the substrate, because
iron reacts with some of the the sulphide
they produce to form dark-colored iron sulfide
(FeS).
Sulfate reduction in biofilms. Though the sulfate reducers are obligate
anaerobes, suitable oxygen-free microzone
environments aren't necessarily buried in
the deeper layers of the substrate. Sulfate-reducing
bacteria tend to multiply in undisturbed
matured biofilms as well. A sulfuretum doesn't
get established there because neighboring
bacteria are waiting to oxidize the hydrogen
sulfide to harmless sulfate.
Sulfate-oxidizing bacteria. So, if H2S has formed in a deeper anoxic layer in
substrate or a thick biofilm, various aerobic
bacteria are waiting to scavenge any available
hydrogen sulfide and oxidize it to harmless
sulfate. In an undisturbed substrate, bacteria
like these would tend to congregate in a
thin layer at the limits of diffused oxygen,
subsisting on any H2S that might diffuse up from a deeper anoxic
layer.
If you found that the roots of plants are
blackened (they should be white) you might
suspect hydrogen sulfide poisoning. But healthy
active plant roots have a natural defense
against H2S; they release some oxygen, which creates
a protective microzone surrounding each rootlet,
where these H2S-oxydizing bacteria can thrive. In an undisturbed
substrate, as H2S rises up to the rootzone, it is increasingly
unlikely to avoid getting oxidized to sulfate.
Diana Walstad notes that H2S was found to be negligable in oxygenated
swamp water, even when
it was present in
the underlying sediment.
My H2S conclusions. In sum, hydrogen sulfide could only be an
issue in a substrate that
was too deep (over
4 inches, say), that was
entirely anoxic,
was also depleted in nitrate
and was enriched
with decaying organics
and sulfate, perhaps
from fertilizer. Then,
to get the H2S up into the water column, though, you'd
have to get in there at
long intervals and
vigorously stir up the
deepest layers of
substrate with a gravel
cleaner. My point
is that several poor aquaristic
practices
would have to be combined.
I did once have an unpleasant brush with
H2S. An Aponogeton bulb had died back but never
renewed itself. After some months I went
to root it out. It was reduced to a shell-
the heart was rotten and softer than a French
cheese. When I managed to get it to the surface
in one piece, the notorious stink of H2S greeted my nose. I figure that the resistant
rind of the bulb had protected the interior
from the destructive powers of oxygen. Inside
it, an isolated community of stinky anaerobic
decomposers had uninhibited free play, and
H2S could accumulate, safe from the substrate
bacteria that would otherwise have oxidized
it to harmless sulfate.
...Other fishkeepers have had similar experiences.
Apparently the fleshy tubers of a Banana
Lily (Nymphoides aquatica) can provide a similar oxygen-free haven
for sulphur-reducing anaerobes. G.S. Mollin
posted at Tom'sPlace 10Jan 2002: "Actually
it took a dead banana plant to get the anaerobic
pocket started. It smelled pretty bad and
was jet black wastewater when vacuumed."
But on the whole, I think aquarists tend
to mistake the funky odors of thiols
for the legendary rotten-egg
fumes of hydrogen
sulfide.
Thiobacilli. Among the varied
community known as colorless
(i.e. non-photosynthesizing) sulfur bacteria
are the small tribe of thiobacilli. Microbiologists
have identified five or so species. Some
thiobacilli are definitely exotic: two live
in sulfurous hot springs, and a couple more
live in waters so acid that nothing else
can survive. In fact, one thiobacillus is
the most acid-tolerant organism known. Most
need oxygen, but at least one, Thiobacillus
denitrificans, is a facultative anaerobe: provided some
nitrate is available, it can reduce nitrate
to dinitrogen gas, then oxidize sulfide to
sulfate, using the freed oxygen. Because
of these few highly-specialized thiobacilli,
it is misleadingly easy to associate thiobacilli
only with extreme environments. In oxygen-rich
water, thiobacilli utilize various forms
of sulfur as a source of energy, just as
we use various forms of carbon. They burn
sulfur with oxygen--— oxidize it--— to obtain
energy, forming stable sulfate. As the metabolizing
of carbon produces carbon dioxide, metabolizing
sulfur produces sulfur dioxide, which thiobacilli
can further use to produce sulfuric acid.
Many thiobacilli can also use hydrogen sulfide,
which is much more common in healthy aquaria
than we imagine, as I've suggested.
Thiobacilli multiply in narrow zones and
gradients where some sulfide is diffusing
upwards from anoxic zones below and where
some oxygen is fitfully available, diffusing
down from the interface between water and
substrate or biofilm surface. Rivers and
estuaries support vast natural populations
of thiobacilli, a very desirable inhabitant
of the deeper strata of the aquarium substrate.
Thiobacilli and a few other bacteria oxidize
H2S (and elemental sulfur if they can get it)
to sulfate (SO4). In the presence of some oxygen, on the
outer fringes of a sulfate-reduction zone,
thiobacilli and various other aerobic bacteria
congregate to feast on hydrogen sulfide,
oxidizing it to harmless sulfate. Some specialized
photosynthetic anaerobes can also metabolize
H2S. The only place in your aquarium where
they could operateis where a deep, anoxic
substrate is exposed to sunlight against
a glass pane.
Thiobacilli links you might be curious to work through, because
these bacteria are distinctly underrated
in the aquarium:
