Biogeochemical Cycles

The enzyme that participates in the Calvin-Benson cycle is called RuBisCO. It is the most abundant enzyme on the earth and is crucial for the fixation of carbon dioxide to make sugar. There are alternative cycles used to fix CO2 in certain green sulfur bacteria that are also photoautotrophic in nature.

BIOGEOCHEMICAL CYCLES

Biogeochemical cycles are important in all ecosystems. The most common atoms used in organic compounds are carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus. These are found in many forms in nature and will move from one aspect of nature to another with the different organisms contributing to this. Many of these atoms are recycled in what’s called a biogeochemical cycle. There are several of these cycles. The carbon cycle is very important. It connects all the different organisms in the world’s ecosystems. It is completely oxidized as CO2 and is reduced in organic molecules. Heterotrophs will produce CO2 and autotrophs will fix carbon. Autotrophs also participate in respiration and fermentation for their own specific metabolic needs. Certain archaea and bacteria will use methane as a carbon source. These are called methanotrophs. Methanogens are organisms that make methane as part of their fermentation process. This builds up in the ecosystem. Cattle are a big source of methane because the bacteria in their gut produce it. Methane is a greenhouse gas.

The nitrogen cycle involves the different forms of nitrogen in the ecosystems. Proteins and nucleic acids have nitrogen in them. Most plants and phytoplankton are unable to fix nitrogen so it takes symbiotic bacteria and some free-living bacteria to help fix them through nitrogen fixation. Cyanobacteria can fix nitrogen from nitrogen gas to make ammonia, which can be made into biological molecules.

Nitrogen goes back into nitrogen gas by certain microbes that undergo ammonification, followed by nitrification and denitrification. The ammonia and organic waste get oxidized to make nitrite and nitrate. Then some bacteria, such as Clostridium and Pseudomonas, make nitrogen gas during anaerobic respiration, to make gas that can get into the atmosphere again. Artificial fertilizers contribute to nitrogen gas in the

environment by adding nitrogenous compounds to the water supply through runoff into the water supply.

Sulfur also gets cycled in the environment. The amino acids methionine and cysteine have sulfur in them and there are other biological molecules that have sulfur in them. Anoxygenic photosynthetic bacteria and some archaea use hydrogen sulfide and oxidize it to elemental sulfur and to sulfate. Sulfate can be used as a sulfur source by certain bacteria and plants.