Nitrogen fixation in freshwater, estuarine, and marine ecosystems. 2. Biogeochemical controls1

TLDR: The tendency toward less nitrogen fixation by plankton in estuaries and coastal marine ecosystems than in lakes subject to similar loadings of nitrogen and phosphorus may be due to a lower availability in oxic seawater of one or more trace elements required for nitrogen fixation, such as molybdenum and iron.

Abstract: Planktonic nitrogen fixation in lakes is strongly related to lake trophic status, with moderate and high rates usually occurring only in eutrophic lakes. Among eutrophic lakes, nitrogen fixation is related to the ratio of nitrogen loading to phosphorus loading to the lake; significant nitrogen fixation by planktonic organisms generally occurs only when the N: P ratio of the nutrient loading is near or below the Redfield ratio of 16: 1. In contrast, nitrogen fixation by planktonic organisms is generally low in estuaries even when the N: P ratio of nutrients inputs is low. The tendency toward less nitrogen fixation by plankton in estuaries and coastal marine ecosystems than in lakes subject to similar loadings of nitrogen and phosphorus may be due to a lower availability in oxic seawater of one or more trace elements required for nitrogen fixation, such as molybdenum and iron. Iron concentrations are generally lower in estuarine waters and seawater than in most lakes. And although molybdenum concentrations in seawater are actually higher than in lakes, molybdenum availability is probably lower, since sulfate inhibits molybdate assimilation by microbes. Molybdate is the primary form of molybdenum in oxic seawater, and the ratio of sulfate: molybdate is greater than in lakes. However, even in lakes sulfate is several orders of magnitude more abundant than molybdenum, and the ratio of dissolved sulfate to dissolved molybdenum typically is much greater than the ratio of sulfur to molybdenum apparently required by nitrogen-fixing cyanobacteria. Consequently, assimilation of molybdate by cyanobacteria is probably an energetically expensive process in all natural waters, but more so in seawater than in freshwaters. High concentrations of dissolved organic matter are known to favor blooms of cyanobacteria, perhaps by increasing iron and/or molybdenum availability through chelation. The primary controls on nitrogen fixation in sediments, wetlands, macrophyte beds, and cyanobacterial mats may be different from those for fixation by planktonic organisms. Both molybdenum and iron are probably more available in these systems than in oxic waters, since reducing conditions and high DOC concentrations will increase iron solubility and favor the stability of reduced forms of molybdenum; sulfate should not inhibit the assimilation of these reduced molybdenum compounds. Consequently, nitrogenase synthesis (and, therefore, nitrogen fixation) in wetlands and in sediments may be less energetically expensive than in oxic water columns. A major control on nitrogen fixation in sediments may be repression of nitrogenase synthesis by high concentrations of ammonium, a factor less important to planktonic fixation because of the much lower concentrations of ammonium generally found in water columns than in sediments.