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…geo…chemistry. Let's 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 nature.

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 21^st 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 20^th 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 energy.

Nutrients

Living organisms 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.

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) contain nitrogen.

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.

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 organisms.

Biogeochemical Cycles

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 the biosphere.

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, NH₃, PO₄^-3 and many other inorganic compounds to the environment. The organic material is mineralized.

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, 2001).

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 must act.

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 biomolecules.

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.

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.

Conclusion

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 for ecosystems.

References:

Adey, W.H. and K. Loveland. Dynamic Aquaria: Building Living Ecosystems. Academic Press: San Diego, 1991.

Atkinson, M.J., J.L. Falter, and C.J. Hearn. 2001. Nutrient dynamics in the Biosphere 2 coral reef mesocosm: water velocity controls NH₄ and PO₄ uptake. Coral Reefs 20(4): 341.

Bashkin, V.N. and R.W. Howarth. Modern Biogeochemistry. Dordrecht: Kluwer Academic Press, 2003.

Besaw, L. 2005. Personal communications.

Davidson, E.A, M. Keller, H.E. Ericson, L.V. Verchot and E. Veldkamp. 2000. Testing a conceptual model of soil emissions of nitrous and nitric oxides. BioScience 50(8): 667-680.

Houghton, J. Global Warming: The complete briefing. Second ed. Cambridge, 1997.

Hovenac, T. and E.F. Delong. 1996. Comparative analysis of nitrifying bacteria associated with freshwater and marine aquaria. Applies and Environmental Microbiology 64: 258-264.

IPCC (Intergovernmental Panel on Climate Change). 1996. Climate change 1995: The science of climate change. Cambridge.

IPCC. 2001. Summary for policymakers: A report of Working Group I Of the Intergovernmental Panel on Climate Change. See http://www.usgcrp.gov/ipcc/.

Post, W.M. et al. 1991. The global carbon cycle. Am. Sci. 78: 310-326.

Schlesinger, W.H. Biogeochemistry: An analysis of global change. San Diego: Academic Press, 1997.

Winogradsky, S. 1890 Sur les organisms de la nitrification. C.R. Acad. Sci. 110: 1013-101.