The energy harnessed from the sun is used by these organisms to form the covalent bonds that link carbon atoms together. These chemical bonds store this energy for later use in the process of respiration. However carbon dioxide is acquired, a by-product of the process is oxygen.
The photosynthetic organisms are responsible for depositing approximately 21 percent of the oxygen content in the atmosphere that we observe today. Heterotrophs acquire the high-energy carbon compounds from the autotrophs by consuming them and breaking them down by respiration to obtain cellular energy, such as ATP. The most efficient type of respiration, aerobic respiration, requires oxygen obtained from the atmosphere or dissolved in water.
Thus, there is a constant exchange of oxygen and carbon dioxide between the autotrophs which need the carbon and the heterotrophs which need the oxygen. Gas exchange through the atmosphere and water is one way that the carbon cycle connects all living organisms on Earth. The movement of carbon through the land, water, and air is complex and, in many cases, it occurs much more slowly than the biological carbon cycle.
As stated, the atmosphere, a major reservoir of carbon in the form of carbon dioxide, is essential to the process of photosynthesis. The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir of carbon in the oceans.
The exchange of carbon between the atmosphere and water reservoirs influences how much carbon is found in each location; each affects the other reciprocally. Carbon dioxide CO 2 from the atmosphere dissolves in water, combining with water molecules to form carbonic acid.
It then ionizes to carbonate and bicarbonate ions. Formation of bicarbonate : Carbon dioxide reacts with water to form bicarbonate and carbonate ions. More than 90 percent of the carbon in the ocean is found as bicarbonate ions. Some of these ions combine with seawater calcium to form calcium carbonate CaCO 3 , a major component of marine organism shells.
These organisms eventually form sediments on the ocean floor. Over geologic time, the calcium carbonate forms limestone, which comprises the largest carbon reservoir on earth. On land, carbon is stored in soil as a result of the decomposition of living organisms or the weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface runoff. Deeper underground, on land and at sea, are fossil fuels: the anaerobically-decomposed remains of plants that take millions of years to form.
Fossil fuels are considered a non-renewable resource because their use far exceeds their rate of formation. A non-renewable resource is either regenerated very slowly or not at all. Another way for carbon to enter the atmosphere is from land by the eruption of volcanoes and other geothermal systems. Carbon sediments from the ocean floor are taken deep within the earth by the process of subduction: the movement of one tectonic plate beneath another.
Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal vents. Carbon dioxide is also added to the atmosphere by the breeding and raising of livestock.
This is another example of how human activity indirectly affects biogeochemical cycles in a significant way. Although much of the debate about the future effects of increasing atmospheric carbon on climate change focuses on fossils fuels, scientists take natural processes, such as volcanoes and respiration, into account as they model and predict the future impact of this increase. Nitrogen, the most abundant gas in the atmosphore, is cycled through the biosphere via the multi-step process of nitrogen fixation, which is carried out by bacteria.
Getting nitrogen into the living world is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere which exists as tightly-bonded, triple-covalent N 2 , even though this molecule comprises approximately 78 percent of the atmosphere.
Nitrogen enters the living world via free-living and symbiotic bacteria, which incorporate nitrogen into their macromolecules through nitrogen fixation conversion of N 2. Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play a key role in nitrogen fixation. Rhizobium bacteria live symbiotically in the root nodules of legumes such as peas, beans, and peanuts , providing them with the organic nitrogen they need.
Free-living bacteria, such as Azotobacter , are also important nitrogen fixers. Organic nitrogen is especially important to the study of ecosystem dynamics as many ecosystem processes, such as primary production and decomposition, are limited by the available supply of nitrogen.
The nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by bacteria. This process occurs in three steps in terrestrial systems: ammonification, nitrification, and denitrification. Third, the process of denitrification occurs, whereby bacteria, such as Pseudomonas and Clostridium , convert the nitrates into nitrogen gas, allowing it to re-enter the atmosphere.
Nitrogen fixation : Nitrogen enters the living world from the atmosphere via nitrogen-fixing bacteria. This nitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply terrestrial food webs with the organic nitrogen they need. Human activity can release nitrogen into the environment by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and the use of artificial fertilizers in agriculture, which are then washed into lakes, streams, and rivers by surface runoff.
A major effect from fertilizer runoff is saltwater and freshwater eutrophication: a process whereby nutrient runoff causes the excess growth of microorganisms, depleting dissolved oxygen levels and killing ecosystem fauna. A similar process occurs in the marine nitrogen cycle, where the ammonification, nitrification, and denitrification processes are performed by marine bacteria.
Although the movement of nitrogen from rock directly into living systems has been traditionally seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process may indeed be significant and should be included in any study of the global nitrogen cycle.
Phosphorus is an essential element of living things, but, in excess, it can cause damage to ecosystems.
Phosphorus is an essential nutrient for living processes. It is a major component of nucleic acid, both DNA and RNA; of phospholipids, the major component of cell membranes; and, as calcium phosphate, makes up the supportive components of our bones. Phosphorus is often the limiting nutrient necessary for growth in aquatic ecosystems. In addition to phosphate runoff as a result of human activity, natural surface runoff occurs when it is leached from phosphate-containing rock by weathering, thus sending phosphates into rivers, lakes, and the ocean.
This rock has its origins in the ocean. Phosphate-containing ocean sediments form primarily from the bodies of ocean organisms and from their excretions. However, in remote regions, volcanic ash, aerosols, and mineral dust may also be significant phosphate sources.
Weathering of rocks and volcanic activity releases phosphate into the soil, water, and air, where it becomes available to terrestrial food webs. Phosphate enters the oceans via surface runoff, groundwater flow, and river flow. Phosphate dissolved in ocean water cycles into marine food webs.
Some phosphate from the marine food webs falls to the ocean floor, where it forms sediment. Phosphorus is also reciprocally exchanged between phosphate dissolved in the ocean and marine ecosystems. The movement of phosphate from the ocean to the land and through the soil is extremely slow, with the average phosphate ion having an oceanic residence time between 20, and , years. Excess phosphorus and nitrogen that enters these ecosystems from fertilizer runoff and from sewage causes excessive growth of microorganisms and depletes the dissolved oxygen, which leads to the death of many ecosystem fauna, such as shellfish and finfish.
This process is responsible for dead zones in lakes and at the mouths of many major rivers. Dead zones : Dead zones occur when phosphorus and nitrogen from fertilizers cause excessive growth of microorganisms, which depletes oxygen, killing flora and fauna.
Worldwide, large dead zones are found in coastal areas of high population density. A dead zone is an area within a freshwater or marine ecosystem where large areas are depleted of their normal flora and fauna. These zones can be caused by eutrophication, oil spills, dumping of toxic chemicals, and other human activities. The number of dead zones has been increasing for several years; more than of these zones were present as of One of the worst dead zones is off the coast of the United States in the Gulf of Mexico, where fertilizer runoff from the Mississippi River basin has created a dead zone of over 8, square miles.
Phosphate and nitrate runoff from fertilizers also negatively affect several lake and bay ecosystems, including the Chesapeake Bay in the eastern United States, which was one of the first ecosystems to have identified dead zones. Sulfur is deposited on land as precipitation, fallout, and rock weathering, and reintroduced when organisms decompose. Sulfur is an essential element for the macromolecules of living things.
As a part of the amino acid cysteine, it is involved in the formation of disulfide bonds within proteins, which help to determine their 3-D folding patterns and, hence, their functions. Sulfur cycles exist between the oceans, land, and atmosphere. Sulfur cycle : Sulfur dioxide from the atmosphere becomes available to terrestrial and marine ecosystems when it is dissolved in precipitation as weak sulfuric acid or when it falls directly to the earth as fallout.
Weathering of rocks also makes sulfates available to terrestrial ecosystems. Decomposition of living organisms returns sulfates to the ocean, soil, and atmosphere. On land, sulfur is deposited in four major ways: precipitation, direct fallout from the atmosphere, rock weathering, and decomposition of organic materials. Atmospheric sulfur is found in the form of sulfur dioxide SO 2. As rain falls through the atmosphere, sulfur is dissolved in the form of weak sulfuric acid H 2 SO 4 , creating acid rain.
Sulfur can also fall directly from the atmosphere in a process called fallout. The weathering of sulfur-containing rocks also releases sulfur into the soil.
These rocks originate from ocean sediments that are moved to land by the geologic uplift. Upon the death and decomposition of these organisms, sulfur is released back into the atmosphere as hydrogen sulfide H 2 S gas. Sulfur may also enter the atmosphere through geothermal vents. Sulfur vents : At this sulfur vent in Lassen Volcanic National Park in northeastern California, the yellowish sulfur deposits are visible near the mouth of the vent.
Sulfur enters the ocean via runoff from land, fallout, and underwater geothermal vents. Some marine ecosystems rely on chemoautotrophs, using sulfur as a biological energy source. This sulfur then supports marine ecosystems in the form of sulfates. Human activities have played a major role in altering the balance of the global sulfur cycle. The burning of large quantities of fossil fuels, especially from coal, releases large amounts of hydrogen sulfide gas into the atmosphere, creating acid rain.
Acid rain is corrosive rain that causes damage to aquatic ecosystems and the natural environment by lowering the pH of lakes, which kills many of the resident fauna; it also affects the human-made environment through the chemical degradation of buildings.
For example, many marble monuments, such as the Lincoln Memorial in Washington, DC, have suffered significant damage from acid rain over the years. These examples show the wide-ranging effects of human activities on our environment and the challenges that remain for our future.
Privacy Policy. Skip to main content. Search for:. Biogeochemical Cycles. Biogeochemical Cycles The elemental components of organic matter are cycled through the biosphere in an interconnected process called the biogeochemical cycle. Learning Objectives Summarize the concept of biogeochemical cycles.
Materials are recycled via erosion, weathering, water drainage, and the movement of tectonic plates. Water is essential to all living processes, while carbon is found in all organic macromolecules. Nitrogen and phosphorus are major components of nucleic acids and play major roles in agriculture.
Well-studied forms of eukaryotic phytoplankton include the opal-secreting diatoms, prymnesiophytes including the CaCO 3 -secreting coccolithophorids , and the organic wall-forming dinoflagellates. The centrality of these organisms in early oceanographic thought was due to their accessibility by standard light microscopy.
Only with recent technological advances have smaller organisms become readily observable, revolutionizing our view of the plankton. In particular, the cyanobacteria, which are prokaryotes lacking a nucleus and most other organelles found in eukaryotes , are now known to be important among the phytoplankton.
Initially, the cyanobacteria were identified largely with colonial forms such as Trichodesmium that play the critical role of "fixing" nitrogen see below. It is now recognized that two cyanobacterial genera — Synechoccocus and Prochlorococcus — dominate phytoplankton numbers and biomass in the nutrient-poor tropical and subtropical ocean Waterbury et al.
In addition, new methods, both microscopic and genetic, are revealing a previously unappreciated diversity of smaller eukaryotes in the open ocean. Mapping ecological and biogeochemical functions onto the genetic diversity of the phytoplankton is an active area in biological and chemical oceanography.
Based on observations as well as theory, the smaller phytoplankton such as the unicellular cyanobacteria are thought to dominate regenerated production in many systems, whereas the larger eukaryotes appear to play a more important role in new production i.
The food source of a given form of zooplankton is typically driven by its own size, with microzooplankton grazing on the prokaryotes and smaller eukaryotes and multicellular zooplankton grazing on larger eukaryotes, both phytoplankton and microzooplankton. Because of their relative physiological simplicity, microzooplankton are thought to be highly efficient grazers that strongly limit the biomass accumulation of their prey.
In contrast, the multicellular zooplankton , because they typically have more complex life histories, can lag behind the proliferation of their prey, allowing them to bloom and sometimes avoid predation altogether and sink directly. The multicellular zooplankton also often facilitate the production of sinking organic matter, for example, through the production of fecal pellets by copepods.
In the nutrient-poor tropical and subtropical ocean, the small cyanobacteria tend to be numerically dominant, perhaps because they specialize in taking up nutrients at low concentrations. Small phytoplankton have a greater surface area-to-volume ratio than do large phytoplankton. A greater proportional surface area promotes the uptake of nutrients across the cell boundary, a critical process when nutrients are scarce, likely explaining why small phytoplankton dominate the biomass in the nutrient-poor ocean.
The microzooplankton effectively graze these small cells, preventing their biomass from accumulating and sinking directly. Moreover, these single-celled microzooplankton lack a digestive tract, so they do not produce the fecal pellets that represent a major mechanism of export. Instead, any residual organic matter remains in the upper ocean, to be degraded by bacteria.
All told, microzooplankton grazing of phytoplankton biomass leads to the remineralization of most of its contained nutrients and carbon in the surface ocean, and thus increases recycling relative to organic matter export.
In contrast, larger phytoplankton , such as diatoms, often dominate the nutrient-rich polar ocean, and these can be grazed directly by multicellular zooplankton. By growing adequately rapidly to outstrip the grazing rates of these zooplankton , the diatoms can sometimes accumulate to high concentrations and produce abundant sinking material.
In addition, the zooplankton export organic matter as fecal pellets. Figure 3 The most broadly accepted paradigm for the controls on surface nutrient recycling efficiency. NPP is supported by both new nutrient supply from the deep ocean and nutrients regenerated within the surface ocean. In the nutrient-poor tropical and subtropical ocean a , the small cyanobacteria tend to be numerically dominant.
The microzooplankton that graze these small cells do so effectively, preventing phytoplankton from sinking directly. Moreover, these single-celled microzooplankton do not produce sinking fecal pellets. Instead, any residual organic matter remains to be degraded by bacteria. In nutrient-rich regions b , large phytoplankton are more important, and these can be grazed directly by multicellular zooplankton.
By growing adequately rapidly to outstrip the grazing rates of zooplankton, the large phytoplankton can sometimes accumulate to high concentrations and produce abundant sinking material. The relationships between nutrient supply, phytoplankton size, and sinking thus dominate this view of upper ocean nutrient cycling. Satellites can measure the color of the surface ocean in order to track the concentration of the green pigment chlorophyll that is used to harvest light in photosynthesis Figure 4.
Higher chlorophyll concentrations and in general higher productivity are observed on the equator, along the coasts especially eastern margins , and in the high latitude ocean Figure 4a and b. Figure 4 Composite global ocean maps of concentrations of satellite-derived chlorophyll and ship-sampled nitrate NO 3 - ; the dominant N-containing nutrient. Northern hemisphere summer is shown in the left panels and southern hemisphere summer on the right.
In the vast unproductive low- and mid-latitude ocean, warm and sunlit surface water is separated from cold, nutrient-rich interior water by a strong density difference that restricts mixing of water and thereby reduces nutrient supply, which becomes the limiting factor for productivity. These "ocean deserts" are dissected by areas, mainly at the equator and the eastern margins of ocean basins, where the wind pushes aside the buoyant, warm surface lid and allows nutrient-rich deeper water to be upwelled.
In the high latitude ocean, surface water is cold and therefore the vertical density gradient is weak, which allows for vertical mixing of water to depths much greater than the sunlit "euphotic zone" as a result, the nutrient supply is greater than the phytoplankton can consume, given the available light and iron, see text. Sea ice cover impedes measurement of ocean color from space, reducing the apparent areas of the polar oceans in the winter hemisphere upper panels. There are caveats regarding the use of satellite-derived chlorophyll maps to deduce productivity, phytoplankton abundance, and their variation.
Second, chlorophyll concentration speaks more directly to the rate of photosynthesis i. Fourth, the depth range sensed by the satellite ocean color measurements extends only to the uppermost ten's of meters, much shallower than the base of the euphotic zone Figure 2. Compared to nutrient-bearing regions, nutrient-deplete regions e.
Thus, satellite chlorophyll observations tend to over-accentuate the productivity differences between nutrient-bearing and -depleted regions. Despite these caveats, satellite-derived ocean color observations have transformed our view of ocean productivity.
In some temperate and subpolar regions, productivity reaches a maximum during the spring as the phytoplankton transition from light to nutrient limitation. In the highest latitude settings, while the "major nutrients" N and P remain at substantial concentrations, the trace metal iron can become limiting into the summer Boyd et al. In at least some of these polar systems, it appears that light and iron can "co-limit" summertime photosynthesis Maldonado et al.
Our planet's climate has changed throughout its long history among various extremes and on different time scales, ranging from millions of years, to just a few millennia, to just a few centuries. Discover oceanic processes, productivity of life in the ocean, and how ocean organisms and circulation respond to climate change.
Our planet's surface is created by tectonic processes, but later molded into shape by water, wind, and ice. Discover the many terrestrial landscapes Earth contains and the processes that create them. Citation: Sigman, D. Nature Education Knowledge 3 10 Productivity fuels life in the ocean, drives its chemical cycles, and lowers atmospheric carbon dioxide.
Nutrient uptake and export interact with circulation to yield distinct ocean regimes. Aa Aa Aa. Figure 1. Productivity in the surface ocean, the definitions used to describe it, and its connections to nutrient cycling. Figure 2. Typical conditions in the subtropical ocean, as indicated by data collected at the Bermuda Atlantic Time-series Station in July, Effect of diversity on productivity.
Figure 3. The most broadly accepted paradigm for the controls on surface nutrient recycling efficiency. Geographic variation. Figure 4. Composite global ocean maps of concentrations of satellite-derived chlorophyll and ship-sampled nitrate NO 3 - ; the dominant N-containing nutrient. Depth variation. Due to the impoverishment of low latitude surface waters in N and P, the productivity of the low latitude ocean is typically described as nutrient limited.
However, limitation by light is also at work Figure 2. As one descends from sunlit but nutrient-deplete surface waters, the nutrient concentrations of the water rise, but light drops off. Phytoplankton at the DCM are compromising between limitation by light and by nutrients.
Phytoplankton growth at the DCM intercepts the nutrient supply from below, reducing its transport into the shallower euphotic zone. Thus, the DCM is not only a response to the depth structure of nutrients and light but indeed helps to set these conditions Figure 2. Conversely, in highly productive regions of the ocean, high phytoplankton density near the surface limits the depth to which light penetrates, reducing productivity in deeper waters.
Such self-limitation of primary productivity is a common dynamic in the ocean biosphere. Seasonality in productivity is greatest at high latitudes, driven by the availability of light Figure 4a and b. The areal intensity and daily duration of sunlight are much greater in summer, an obvious direct benefit for photosynthesis.
In addition, the wind-mixed layer or " mixed layer " of the upper ocean shoals such that it does not mix phytoplankton into darkness during their growth Siegel et al. The mixed layer shoals in the spring partly because increased sunlight causes warming and freshening the latter by the melting of ice , both of which increase the buoyancy of surface waters. Mixed layer shoaling is sometimes also encouraged by generally calmer spring and summer weather, which reduces wind-driven turbulence.
During the "spring bloom," NPP exceeds the loss of phytoplankton biomass to grazing and mortality, leading to transient net biomass accumulation and a peak in export production. The population of grazing organisms also rises in response to the increase of their feedstock, transferring the organic carbon from NPP to higher trophic levels. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable.
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