Limitation of N2 fixation in coastal marine waters: Relative importance of molybdenum, iron, phosphorus, and organic matter availability1

Abstract

Phytoplankton growth in many coastal and pelagic marine waters is chronically limited by nitrogen availability. Such conditions potentially favor the establishment of N2‐fixing microorganisms (eubacteria and cyanobacteria). However, planktonic and benthic N2 fixation is often either absent or present at ecologically insignificant rates. It has been proposed that deficiencies in inorganic nutrient (specifically molybdenum) availability could help explain this paradox. We examined both inorganic and organic nutrient limitations of marine N2 fixation in nitrogen‐deficient coastal North Carolina waters. Inorganic nutrient (phosphorus, iron, and molybdenum) availability consistently exceeded demands by N, fixers. In contrast, enrichment with the sugars fructose, glucose, sucrose, and maltose and the sugar alcohol mannitol either elicited N2 fixation or enhanced existing rates of N2 fixation. Supplementation with particles (organic detritus) also enhanced N2 fixation potentials; the combined addition of particles and organic compounds yielded maximum rates of N2 fixation. This combination promotes the development of O2‐reduced microenvironments (microzones) in which N2 fixers can reside. A functional explanation for the observed stimulation of N2 fixation is that it is an anaerobic process which, in aerobic marine waters, can only proceed in O2‐poor microzones. Hence, deficiencies in organic matter rather than inorganic nutrient availability may play key roles in limiting and regulating marine N2 fixation.