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The carbon cycle. CO2 from the atmosphere moves into plants, soils, surface ocean, and rivers. From plants, the carbon moves back to the air. From the water, the carbon moves to marine biota, the deep ocean, and sediments. Carbon also moves back to the air from fossil fuel and cement production.
This figure summarizes the carbon cycle. Eukaryotes participate in aerobic respiration, fermentation, and oxygenic photosynthesis. Prokaryotes participate in all the steps shown. (credit: modification of work by NOAA)
  • Describe the interaction between heterotrophs and autotrophs in the carbon cycle.

Nitrogen cycle

Many biological macromolecules, including proteins and nucleic acids, contain nitrogen; however, getting nitrogen into living organisms is difficult. Prokaryotes play essential roles in the nitrogen cycle ( [link] ), transforming nitrogen between various forms for their own needs, benefiting other organisms indirectly. Plants and phytoplankton cannot incorporate nitrogen from the atmosphere (where it exists as tightly bonded, triple covalent N 2 ), even though this molecule composes approximately 78% of the atmosphere. Nitrogen enters the living world through free-living and symbiotic bacteria, which incorporate nitrogen into their macromolecules through specialized biochemical pathways called nitrogen fixation . Cyanobacteria in aquatic ecosystems fix inorganic nitrogen (from nitrogen gas) into ammonia (NH 3 ) that can be easily incorporated into biological macromolecules. Rhizobium bacteria ( [link] ) also fix nitrogen and live symbiotically in the root nodules of legumes (such as beans, peanuts, and peas), providing them with needed organic nitrogen while receiving fixed carbon as sugar in exchange. Free-living bacteria, such as members of the genus Azotobacter , are also able to fix nitrogen.

The nitrogen that enters living systems by nitrogen fixation is eventually converted from organic nitrogen back into nitrogen gas by microbes through three steps: ammonification, nitrification , and denitrification . In terrestrial systems, the first step is the ammonification process, in which certain bacteria and fungi convert nitrogenous waste from living animals or from the remains of dead organisms into ammonia (NH 3 ). This ammonia is then oxidized to nitrite ( NO 2 ) , then to nitrate ( NO 3 ) , by nitrifying soil bacteria such as members of the genus Nitrosomonas , through the process of nitrification. Last, the process of denitrification occurs, whereby soil bacteria, such as members of the genera Pseudomonas and Clostridium , use nitrate as a terminal electron acceptor in anaerobic respiration , converting it into nitrogen gas that reenters the atmosphere. A similar process occurs in the marine nitrogen cycle, where these three processes are performed by marine bacteria and archaea.

Human activity releases nitrogen into the environment by the use of artificial fertilizers that contain nitrogen and phosphorus compounds, which are then washed into lakes, rivers, and streams by surface runoff. A major effect from fertilizer runoff is saltwater and freshwater eutrophication , in which nutrient runoff causes the overgrowth and subsequent death of aquatic algae, making water sources anaerobic and inhospitable for the survival of aquatic organisms.

The nitrogen cycle. Gaseous atmospheric nitrogen shore; this moves into organic matter (R-H2) through bacterial and lightning fixation. Fertilizers and mineralization produce ammonium (NH4+). This can enter waterways via run off and leaching. Ammonium is converted to nitrates (NO2-) via nitrification. These are then converted to nitrates (NO3-) via nitrification. Both of these can end up in waterways causing eutrophication. Nitrates can be taken in by plants or converted to gaseous nitrate (N2) by denitrification.
This figure summarizes the nitrogen cycle. Note that specific groups of prokaryotes each participate in every step in the cycle. (credit: modification of work by NOAA)

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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