ADENOSINE TRIPHOSPHATE (ATP)

An organic, phosphate-rich compound important in the transfer of energy in organisms. Its central role in living cells makes it an excellent indicator of the presence of living material in water. A measure of ATP therefore provides a sensitive and rapid estimate of Biomass. ATP is reported in micrograms per liter of the original water sample.

Main source of energy for biochemical reactions within the cell.
  

Known as the "molecular currency" of intracellular energy transfer, this nucleotide can store and transport chemical energy. It also plays a part in nucleic acid synthesis.