Abstract:
The marine nitrogen cycle is important ecologically as nitrogen is a nutrient required by phytoplankton, which as primary producers, form the basis of the marine food chain. Nitrate and nitrite ions are utilised as terminal electron acceptors for respiration by microorganisms during the unavailability of oxygen. The anaerobic processes of nitrogen transformation, such as denitrification and anammox are prolific in the oxygen minimum zones of the marine environment. Eukaryotic microorganisms or protists form a major part of picoplankton and nanoplankton in the pelagic water column of the oceans and seas. These also thrive and flourish in the marine sediments and are involved in various biogeochemical activities along with bacteria. These are known to be involved in the nitrogen cycle by various enzymes involved in denitrification, nitrification, and ammonification. The fungi, diatoms, dinoflagellates, and foraminifera are the most widely known eukaryotes to be involved in nitrogen cycling. These eukaryotic microorganisms belong to various supergroups in the tree of life and contribute to the marine nitrogen cycle in some way or the other. This chapter summarises the various eukaryotic microorganisms involved in the different processes of the nitrogen cycle, as well as those whose involvement is still ambiguous.
