Talk with anyone
who keeps a reef aquarium long enough and he'll use the word
"nutrients." "Nutrients," we are often
told, are "bad" for our reef tanks. At the same
time most realize that proper diets and adequate nutrition
are paramount to keeping the organisms in our tanks alive
and healthy. Manufacturers come out with more and fancier
concoctions every year to feed the creatures we keep, a few
of which actually work, yet "nutrients" are still
lambasted. If "nutrients" are "bad" for
our reef tanks, why are we spending so much time, effort and
money figuring out how to better put nutrients into them?
As I will explain through this series, the idea that "nutrients"
are "bad" is truly a misinterpretation of the natural
world. Living things must have nutrients in order to live,
but there is a bit more to the story than that. On a broad
scale, this series will address the ways in which Mother Nature
provides food for the organisms that live on coral reefs,
as well as how she processes their wastes.
While reef aquarists must always give
consideration to the individual organisms their tanks contain,
it is also important to consider tanks holistically. Coral
reefs are not merely populations of bacteria, or algae, or
corals or fish. While focus on groups like these is useful
much of the time, it does not show the complete picture. Reefs
include all of these groups and more, working in concert and
with profound influence over one another. Therefore, this
series will focus on how coral reef ecosystems operate with
regard to nutrients and energy, and their connections to adjacent
habitats. Complex terminology will be avoided whenever possible,
but some amount of jargon is inevitable. Because most people
likely are not very familiar with many of the terms and concepts
that are necessary for a discussion of ecosystems' function,
I will begin by reviewing several pertinent items before moving
on to specifics about coral reefs.
In order to understand how organisms and,
indeed, how whole ecosystems survive and interact with the
world around them, a scientific discipline called biogeochemistry
has developed. About that terminology-I realize I may have
just lost you. Seriously, how many prefixes can a person fit
onto one root? That word is bio
break it down a little bit with some definitions that will
help to clarify its meaning.
Chemistry: The scientific study of matter, its properties
and its interactions with other matter and with energy.
Geology: The science and study of the Earth, its
composition, structure, physical properties, history and
the processes that shape it.
Geochemistry: The study of the distribution and
amounts of the chemical elements in minerals, ores, rocks,
soils, water and the atmosphere, and their circulation in
Biology: The branch of science dealing with the
study of life.
Ecology: The scientific study of the interactions
of organisms with their environments and with one another.
Biogeochemistry: The study of the geochemistry of
the Earth's surface, which is subject to major influences
by living systems (derived from Schlesinger, 1997).
Bashkin (2003) has called biogeochemistry
"the interdisciplinary science of the 21st
century." Historically, chemistry and geology developed
independently but found an area of overlap in a field that
came to be called geochemistry. Biology was also independent
and eventually subdivided into a variety of fields, including
ecology. Ecology was found to overlap with geochemistry at
about the turn of the 20th century, and the field
we now call biogeochemistry developed. This component of ecosystem
ecology is usually invoked to explain regional or even global
phenomena, though it also addresses microscopic transformations.
It describes how materials and energy are used by organisms
and by ecosystems, how those materials and that energy move
through an ecosystem and which forms they take during these
processes. The answers to questions about how and why an organism
might do a certain thing or be adapted a certain way are often
found in the availability of the things that organisms need
to live and to reproduce, namely, nutritive compounds and
require a variety of compounds in order to stay alive and
produce offspring. See Figure 1 for a brief summary of essential
plant nutrients. This is the least exhaustive list possible.
Most organisms require additional nutrients that they are
unable to synthesize (e.g., vitamins, some fatty acids, some
amino acids). It should also be noted that only particular
forms are acceptable nutrient sources to organisms. For example,
a tree readily takes up CO2 as an available form
of carbon, but a tree can't digest a piece of bread and get
carbon from it. Alternatively, a person cannot use CO2
as a source of carbon, but readily digests and uses carbon
when consuming a piece of bread. While there are a number
of essential nutrients and all of them have received attention
in understanding ecosystem function, carbon, nitrogen and
phosphorus tend to be especially well studied.
1. Essential elements required by plants.
Carbon is the seat of energy storage
and tissue building in living organisms. All energy-rich compounds
used in metabolism are also very rich in carbon (including
carbohydrates, lipids, amino acids, etc.). Many autotrophs
also use carbohydrates in structural compounds such as cellulose.
Nitrogen is required to build a
vast assortment of critical compounds within living things.
It forms the amine group in amino acids, which are the subunits
of proteins. Many proteins are used as structural compounds
(e.g., muscles, organs, bones, etc.), especially in heterotrophic
organisms. Many enzymes and other biomolecules (e.g., chlorophyll)
Phosphorus is critical to cellular
respiration in the form of ATP.
It is also required in the structure of DNA, RNA, cellular
membranes and other substances. Phosphorylation and dephosphorylation
are also ubiquitous processes that control many activities
essential for cellular survival.
Why, might we ask, do these three elements
get so much attention, considering the number of other nutrients
required by living things? In order to understand why this
is so, it's necessary to consider Liebig's Law of the Minimum
and the concept of nutrient limitation.
Imagine a female fruit fly (Drosophila
melanogaster) that lays an average sized clutch of eggs
(about 50). On average, half of the clutch would be females
and half males. If the original female lays several more clutches
during an average lifespan (about six weeks) she will have
laid several thousand eggs in her lifetime. Let us also imagine
that all of her offspring survive, mature in about a week
(average) and reproduce in the same way that their mother
did. Let us imagine that these flies continue reproducing
without any constraints except the rate at which their life
cycle normally progresses, and that this happens for a period
of one year. How many fruit flies would there eventually be?
If we were to pack them 1000 per cubic inch they would make
a ball of tightly packed fruit flies about the size of the
planet Earth (Besaw, pers. comm.). Obviously this does not
happen, nor is it possible in the real world. Organisms in
the real world are constrained somehow in their reproductive
effort. Malthus was the first to formally describe this phenomenon.
Let us consider another scenario. Imagine that you are a
mason and you are building a wall. The wall is patterned with
three shades of bricks, white, gray and slate, and three of
each are used to form a pattern in the wall. Let us also imagine
that large piles of each of these bricks are all within easy
reach. The only constraint on how fast the wall can be built
is the rate at which you can build it. Plenty of materials
are available. Let us now imagine that you've been building
for awhile and that each pile of bricks is getting small.
You send someone out to buy some more of each style of brick
so that you can finish your work. Soon afterward he returns,
but with only gray and slate bricks. The supplier has run
out of white bricks and more won't be available until tomorrow
at the earliest. You are faced with a dilemma. You have plenty
of gray and slate bricks to continue building, but the white
bricks are nearly gone. If you wish to maintain the same pattern
you started with you will have to wait until tomorrow, when
white bricks (hopefully) will be available again. On the other
hand, you could change the pattern you've made by using fewer
white bricks and more gray and slate bricks.
Such is the dilemma organisms face when experiencing nutrient
limitation. While it might be possible to continue building
tissue (growing and reproducing) when there is not enough
of some constituent (nutrient), this is an option only when
the composition of the product (that is, the new tissues being
built) can be allowed to vary. If a precise structure must
be made, building stops until the limiting constituent becomes
more available. While many autotrophs can alter their
tissue composition a bit by using more or less of a particular
nutrient under certain circumstances, many heterotrophs
are much more limited in this ability. For living organisms,
nutrient limitation can slow growth and reproduction, lead
to deformities or even cause outright mortality. As an example
of such a limitation, an oak tree growing in an oak-hickory
forest near my home is most likely limited in its growth during
the summer by the amount of available nitrogen. As less and
less nitrogen is available in the soil, the oak's growth rate
slows down. It can continue to grow for a little while if
it can build tissues containing progressively less and less
nitrogen, and because it is a tree it is able to do just that.
It produces more carbon-rich substances such as cellulose
(strengthening its trunk) and fewer nitrogen-rich compounds
such as chlorophyll. There comes a point when there is not
enough nitrogen to build even the most N-poor tissues, and
the oak tree stops growing. Eventually its leaves may even
begin to yellow due to a lack of chlorophyll. This would be
described as a nutrient deficiency. If this deficiency is
not alleviated, eventually the tree will die (though this
rarely happens because the tree has adapted over evolutionary
time to a condition of N-limitation). If a small, mid-summer
shower comes, the sprinkle of water on the leaf-litter may
allow nitrogen to leach from the top layer of humus to a deeper
soil layer where it might be more readily available to the
oak. Now, once again, sufficient nitrogen is available and
the tree resumes normal growth. It is important to understand
that while all other conditions for growth may have been optimal,
without enough nitrogen (in this case) the tree cannot use
these other resources. Its growth was limited by the deficiency
of the single, least-abundant (relative to need) nutrient.
This, in essence, is Liebig's Law of the Minimum. The
growth rate of a plant (or truly any organism or population)
will be limited by the least available resource.
The reason carbon, nitrogen and phosphorus (C, N, and P)
receive so much attention in ecology is that these three nutrients
are the elements most required, relative to their supplies,
by most organisms. In other words, they are the nutrients
most likely to limit the growth and reproduction of living
things, though they are not the only nutrients that can limit
wild populations, as I will detail later. For the sake of
completeness it must also be noted that the availability of
nutrients such as these is not the only factor that
can and does limit the growth and reproduction of living things.
Plants often are limited by the availability of light (as
in a forest understory) or water (as in a desert). In the
coral reef environment, space is often the limiting factor.
Additionally, interspecific and intraspecific competition,
predation, disease and many other factors can and do limit
Think back to a
natural or environmental science course you took. In that
class you probably recall learning about certain cycles in
the natural world, such as the water cycle or the nitrogen
cycle. These are, in fact, biogeochemical cycles. They explain
the movement of huge amounts of material and energy through
Before diving into a discussion of the
global carbon, nitrogen and phosphorus cycles, let's quickly
define a bit of important, yet simple, terminology. In any
discussion of nutrient dynamics in an ecosystem the terms
source, sink and flux are used. A source is exactly
what it sounds like. This is a portion of an ecosystem from
which some material or energy originates. It is from this
origin that a material or energy is added to the ecosystem
in a biologically accessible manner. A sink is just
the opposite. This is the portion of an ecosystem where a
material or energy becomes trapped in such a way that it is
no longer readily available for the ecosystem to use. Often
this action is called sequestration. For example, when
an organism grows it removes nutrients from the environment
and holds onto them in its tissues. These nutrients therefore
are no longer readily available to the ecosystem. The organism
has sequestered those nutrients. Flux is just a more
rigorous way to describe the movement of something from one
area to another through some imaginary plane. For example,
if I throw a baseball to my friend through a plane halfway
between us, there is a flux of one baseball to him per unit
of time. If I throw the ball five seconds after receiving
it and he does the same there is a flux of twelve baseballs
per minute through this plane-six from me to him and six from
him to me. Neither of us represents a source or a sink because
the same number of baseballs is constantly kept in play. This
is different from saying that nothing is happening. Indeed,
baseballs are moving constantly, but they are not being added
or subtracted from the system. Rather, they are being kept
in motion. If instead he throws a ball only every 60 seconds
but I continue to throw one ball every 10 seconds as before,
I will become a net source of baseballs and he will become
a net sink.
A few other terms are common to these biogeochemical
cycles. One is assimilation, the act of taking up some
nutrient from the environment and converting it into tissues
and biomolecules. When a tree sucks up CO2 in order
to produce sugars, and then converts those sugars into cellulose,
it has assimilated this carbon. When a lion catches a wildebeest
and digests the meat, it absorbs amino acids, phospholipids
and other compounds. When it converts these into proteins
(RNA, DNA, phospholipids, etc.) in its own body it assimilates
these nutrients. Because the atmosphere has significant pools
of both carbon and nitrogen, several organisms have evolved
to be able to utilize these gaseous nutrient sources, though
most cannot. The process of converting inorganic carbon (as
CO2 on land or as primarily HCO3-
in the water) or inorganic nitrogen (as N2 gas)
to usable, organic forms is called fixation. Usually,
fixed carbon and nitrogen are assimilated immediately by the
organism that perform the fixation, or by a symbiont.
Finally, mineralization is the process of converting
organic compounds to inorganic compounds during decomposition.
As bacteria decompose detritus they release CO2,
and many other inorganic compounds to the environment. The
organic material is mineralized.
global carbon cycle, courtesy of NASA.
The Carbon Cycle
The diagram above
depicts the global carbon cycle. It illustrates not only the
qualitative transformations that occur, but also supplies
estimates of sources, sinks and fluxes in gigatons of carbon
(Gt = 1×1012 t) per year. Broadly, the carbon
cycle's pattern is circular. Most of the carbon available
in the biosphere has been reused over and over again over
geologic time. In order to appreciate what is happening in
the diagram above, let's track the movement of a few atoms
of carbon through this cycle. At this point we'll focus on
the major global patterns and will build on this next month
by examining the ocean more carefully.
In order to enter an ecosystem, carbon
in the form of CO2 from the atmosphere or dissolved
in the water (usually as HCO3- in seawater)
is assimilated by a plant or algal cell. Plants and algae
take up these inorganic species of carbon and use their photosynthetic
machinery to convert them (along with water) into carbohydrates
using the captured energy from sunlight. Cyanobacteria are
also photosynthetic, as are certain other types of bacteria.
So as not to forget them, chemoautotrophic bacteria and archaeans
also assimilate inorganic carbon, but use energy sources besides
sunlight. On a global scale, however, they likely do not contribute
significantly to the total uptake of inorganic carbon.
Once a carbon atom has been captured by an autotroph several
things can happen to it. It might just be used to fuel that
organism's metabolism and get respired right back out as CO2.
It might also be used to build tissues in that organism. In
the ocean, a very large percentage of this carbon actually
leaks out of algal cells into the water. We'll examine what
happens to all this carbon next month. If the autotroph dies
without being eaten, then its tissues will eventually be decomposed.
Then the carbon atom is respired by a decomposer as CO2
(or put into tissue until the decomposer gets eaten or dies
and decomposes). If the autotroph gets eaten, the carbon atom
might be respired by the predator, or might be put into tissue.
Eventually, this organism is either eaten (and the carbon
atom is again either respired or put into new tissues) or
dies and is decomposed. One way or another, even if the carbon
atoms pass through many organisms in a food chain, eventually
almost all of them end up going back into the atmosphere or
being dissolved in the ocean, and eventually get fixed again
by some other autotroph. This starts the whole process anew.
Thus, there is a significant flux of C between living organisms
and the atmospheric-oceanic system every year. In fact, this
amounts to more than 210 Gt C yr-1 (Post et al.,
1991). There also are known sources and sinks of C in
the biosphere. A very small portion of the carbon taken up
by marine organisms or washed into sea from land actually
sinks to the bottom of the ocean and is buried before it fully
decomposes. This eventually produces oil and natural gas.
Rapid carbon uptake by vegetation without decomposition (especially
in wetlands) can lead to coal deposits such as those produced
during the Carboniferous period (notice the name; wink, wink).
This is an extremely slow process, however, and only a small
amount of carbon relative to the total amount on Earth has
been stored in this way. However, because there is so little
carbon in the atmosphere relative to other reserves (the ocean),
a little bit of CO2, say, from burning oil, coal
and natural gas, can drastically alter the atmosphere's composition.
Again, fossil fuel formation is a very slow process. Right
now the world is using about one million years' worth of fossil
fuel production every year (IPCC, 2001). Many estimate that
we'll actually use up all the world's significant oil reserves
within the next 50 years. In addition to fossil fuels as a
sink for carbon, the living things on the planet have sucked
up significant amounts of carbon to build their bodies. Some
of this is present in living organisms, but much (most) of
it is actually in the form of undecomposed detritus in the
soil on land and in the mud, sand and water in aquatic habitats.
But all other reserves of carbon pale in comparison to that
dissolved in the ocean. A full 85% of all the actively available
carbon on the planet is in the ocean, mostly as bicarbonate
(HCO3-) (Post et al., 1991).
No other reservoir comes anywhere close to this amount.
As for the sources of carbon in the biosphere,
until historic times the major sources were volcanic eruptions
and wildfires. Volcanoes, even fairly large ones, don't add
that much C to the atmosphere relative to the amount that
is already there. In fact, during a typical year there are
many eruptions, large and small, yet they usually make no
easily perceptible impact on atmospheric CO2. Over
geologic time, what volcanoes have done is offset the slow
accumulation of carbon in fossil fuels (in geologic reserve).
Naturally occurring fires have tended to have an even smaller
effect by quickly releasing carbon that had been trapped by
living things, but again without a readily perceptible change
in atmospheric chemistry. What was released by the fire was
eventually taken up and sequestered by the biosphere.
Beginning a few thousand years ago, however, humans began
to change the face of the Earth. Fire was use to clear land
at unprecedented rates. The grasslands and forests that once
held large amounts of carbon in their living tissues and detritus
now released this carbon to the atmosphere as they burned
at rates far higher than was natural. Large tracts of forest
were eventually cleared with the same effect, especially in
Europe. More recently, North America has been logged along
with most of Asia. Much of Africa, South America and other
regions are following suit. While these changes in land use
certainly have added a fair amount of carbon to the atmosphere,
they are not nearly as dramatic as the use of fossil fuels.
Fossil fuels were previously locked in a non-cycling pool
of carbon; a sink as discussed above. By burning these fuels
we have released this carbon into the atmosphere. Before the
industrial revolution the content of CO2 in the
atmosphere was approximately 280 ppmv, and it has fluctuated
from about 180-280 ppmv over the last 750,000 years-low during
ice ages, high during interglacial periods (IPCC, 1996). Due
to human activities, especially the use of fossil fuels, the
concentration of CO2 in the atmosphere has risen
to 380 ppmv. In other words, we've raised the concentration
of carbon dioxide in the atmosphere by more than 35% in the
last 150 years. Interestingly, it seems this is totally unprecedented
in the past 750,000 years. An increase of this magnitude usually
takes 2000 years or more to accomplish through natural climatic
oscillations, yet we have made this change in atmospheric
chemistry in just 150 years, with the majority being realized
in only the last 50 years. Also very interesting is the fact
that only half of the carbon we have put into the atmosphere
actually remains there. The ocean has actually absorbed about
50% of the carbon from fossil fuels (and a very small portion
has gone into terrestrial sinks) (Post et al., 1991).
Understanding this phenomenon is critical to understanding
the oceanic system, though it is actually explained by very
straightforward concepts. I'll explain this mechanism further
next month. If none of this carbon had been sequestered in
the ocean, the atmospheric carbon dioxide concentration would
likely be near 480 ppmv, an increase of more than 70% in just
150 years. An increase of that magnitude, to say nothing of
the rate, is completely unprecedented in the last 750,000
years. Most optimistic estimates expect the atmospheric CO2
concentration to peak at double its current level sometime
this century. Less optimistic estimates call for levels three
times the current concentration and higher (IPCC, 1996; IPCC,
As explored in many publications, there is no more meaningful
debate among scientists in peer-reviewed literature (excepting
those reports with affiliations to oil and gas industries)
than about whether global warming is happening, whether humans
are causing it, or if it will have severe, negative repercussions
in the near future (there has been no debate about this in
the scientific community for a long time, but the message
has gotten spun on its way to the public). Actually, that
last point on repercussions is perhaps only partly true: global
warming has already had severe, negative repercussions
for humanity. For instance, parts of Africa have suffered
unprecedented droughts in the last few decades that are expected
to intensify in the near future, killing millions. China is
expected to suffer severe droughts as well, and much of the
Midwest is expected to dry out, too (Houghton, 1997). The
poverty that will come to pass if we do nothing to prevent
the worst effects will not be visited strictly upon some future
society. Rather, within the next few decades we will see devastating
natural catastrophes that we have no control over (e.g., whole
ecosystems dying) if we falter on this issue. If we do not
decide as a global society to address this problem (thankfully,
it seems as though the tide is turning favorably where it
hasn't already) the future looks very grim indeed. Even conservative
estimates of what will happen if we turn a blind eye predict
trillions of dollars in property damage, countless lives lost,
the possible destruction of 13 of the world's 20 largest cities
(all at sea level) and the very frightening likelihood that
significant portions of the globe will become uninhabitable
(Houghton, 1997). Thankfully, popular opinion supports rectifying
this issue, and I expect that the United States will lead
the charge to fuel-efficiency and remove the monkey of reliance
on fossil fuels from the world's back over the next few years.
Let us all ensure that our lawmakers, whether liberal or conservative,
Republican or Democrat, support the work necessary to save
us. Global warming is no more a partisan issue than is providing
national security or enforcing the laws. All policymakers,
regardless of their affiliation, should support this issue
because it will quite literally determine whether the world
is prosperous or impoverished during our lifetime. Signs are
all around us of a dying world. In 1989 the world coughed
when the coral reefs bleached at unprecedented levels. In
the 1990s the world hacked as hundreds of species of neotropical
frogs were lost due to a fungus made virulent by high temperatures.
In 1997 the reefs bleached again. In 2002 they bleached again.
In 2005 and now in 2006 they are bleaching again. Montane
species race up mountains to escape the heat and drought that
chase them. Eventually they reach the top and fade away. All
the wonder in the natural world of our parents' generation
is fading away. Our very life-support system is dying. We
nitrogen cycle. Image courtesy of Michael Pidwirny.
The Nitrogen Cycle
Some of the transformations
occurring in the global nitrogen cycle parallel those in the
carbon cycle, but several others are unique. For clarity's
sake I will briefly define these transformations with regard
to nitrogen. Inorganic nitrogen sources are primarily nitrate
(NO3-) and ammonia/ammonium (NH3/NH4+).
Organic sources include proteins, amino acids and various
Assimilation: the process of converting nitrogen
into tissues and biomolecules within an organism.
Ammonification: mineralization that produces ammonia.
Denitrification: the conversion of inorganic nitrogen (primarily
nitrate) into gaseous forms (N2, N2O).
Fixation: the conversion of N2 gas into
usable, inorganic forms ("combined forms").
Mineralization: the conversion of organic nitrogen
into inorganic nitrogen.
Nitrification: the conversion of ammonia to nitrite
and then to nitrate through microbial action.
As reef aquarists, many readers probably
are already somewhat familiar with many of these processes.
The vast majority of available nitrogen on this planet is
N2 gas in the atmosphere. Only a very few organisms
can use this as a source of nitrogen, however, and they are
called nitrogen-fixers. These include mainly certain microbes
and fungi. The nitrogen gas in the air is, to most organisms,
totally useless. Think of the old observation about being
stranded at sea, "Water, water, every where, nor any
drop to drink." Indeed, if organisms could use dinitrogen
gas as a nitrogen source they would get all they need just
from breathing. Not too many people would give up their favorite
protein-rich dish (meat, rice and beans, tofu, etc.) to take
a few full breaths of air. Before the ability to fix atmospheric
nitrogen evolved it is likely that organisms were totally
reliant on atmospheric deposition (Bashkin, 2003). Lightning
creates a small amount of combined nitrogen in the air, which
is deposited dry or in rainwater.
Usually fixed nitrogen is assimilated immediately
by the organisms that perform the fixation, or it might be
passed along to a symbiont (e.g., Rhizobia and legumes).
This adds nitrogen to the ecosystem as a whole. When this
organism dies, it is decomposed. Usually bacteria absorb much
of the nitrogen and use it for their own growth and reproduction,
but any excess is typically left behind. This usually results
in the production of ammonia; hence this process is called
ammonification, a type of mineralization. A next possible
step, and probably the most familiar to aquarists, is called
nitrification. This is the classic portion of the nitrogen
cycle that is always taught to new aquarists. Nitrosomonas
spp. bacteria are primarily responsible for the oxidation
of ammonia to nitrite. Nitrobacter spp. are primarily
responsible for the oxidation of nitrite to nitrate (Winogradsky,
1980). Recent evidence from Hovenac and Delong (1996) suggests
that the bacteria responsible for the oxidation of nitrite
to nitrate in aquaria and some other aquatic environments
are actually Nitrospira-like spp. and not Nitrobacter
spp. which are ubiquitous on land. It should be noted that
heterotrophic bacteria can perform these transformations to
a small degree as well (Bashkin, 2003). Alternatively, most
of the ammonia produced by ammonification is actually absorbed
and used by plants and algae in nature (though nitrifying
bacteria do tend to be ubiquitous) (Adey and Loveland, 1991;
Atkinson et al., 2001). While new aquarists are always
taught the importance of nitrifying bacteria in their aquariums,
in reef tanks (which tend to have excellent lighting), algal
uptake of ammonia is likely happening much faster than nitrification.
In situations where gaseous oxygen is depleted, nitrate often
is used in lieu of oxygen as a final electron acceptor in
a process called denitrification. Nitrate (NO3-)
is converted to nitrite (NO2-), then
to nitric oxide (NO), then to nitrous oxide (N2O
-a small amount of this tends to escape), and eventually to
dinitrogen gas (N2) (Davidson et al., 2000).
Thus, nitrogen returns to the atmosphere and potentially can
be fixed again into combined forms.
phosphorous cycle. Image courtesy of wwtlearn.org.
The Phosphorus Cycle
While there are
certain obvious similarities between the carbon and nitrogen
cycles, the phosphorus cycle is very different. This is due
to P's chemically dissimilar nature compared to either C or
N. While the atmosphere has significant pools of both C and
N, it has no such pool of P. Also, while C and N atoms tend
to be highly recycled between the biosphere and these pools,
P has shown much less conservation over the geologic record.
If an atom of carbon or nitrogen is introduced from a volcano,
it might be used hundreds or thousands of times before it
is eventually deposited geologically and taken out of nature's
circulation. A phosphorus atom, on the other hand, tends to
originate from the weathering of phosphorus-rich rock. Most
of this is transported from the land to a river and eventually
is buried in oceanic sediments without ever being used by
the biosphere at all (Bashkin, 2003; Schlesinger, 1997). The
portion that is absorbed by living things tends to get recycled
a bit, but still does not show nearly the recycling that is
common for carbon and nitrogen before being sunk in oceanic
sediments. Phosphorus (primarily as orthophosphate, PO4-3)
also tends to show relatively low solubility in water, making
it less available to ecosystems. It also bonds to aluminum
and iron-rich sediments (terrestrial soils) and gets immobilized.
As is apparent, it often can be difficult for many living
things to get enough phosphorus. In fact, phosphorus availability
is often said to be the ultimate limitation on ecosystems.
If there is a deficiency of nitrogen or carbon in an ecosystem,
nitrogen- or carbon-fixers can continue growing and alleviate
this deficiency in the ecosystem as a whole. If there is a
phosphorus deficiency, there is no mechanism for organisms
to alleviate the limitation, except to survive until some
phosphorus is made available. Because of this tendency, many
organisms have gotten very, very good at drawing the soluble
reactive phosphorus concentrations that are very low in the
natural world. Despite all this, nitrogen remains the proximate
limiting nutrient in many terrestrial and oceanic ecosystems
due to the large demand for nitrogen and denitrification as
a method of nitrogen removal from the ecosystem.
For those interested
in biogeochemistry and its implications in ecosystem ecology,
I recommend Biogeochemistry: An Analysis of Global Change
by William H. Schlesinger as the most accessible modern treatise
on the subject. For everyone else, feel free to show off your
new knowledge by taunting family and friends. Seriously, how
many people have heard the word biogeochemistry, much less
know what it really means? Just think of how much better prepared
you'll be for Jeopardy now! Next month we'll begin discussing
the ocean more fully, as well as biogeochemical implications