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

Phytoplankton growth in many coastal and pelagic marine waters is chronically limited by nitrogen availability. Such conditions potentially favor the establishment of N,-fixing microorganisms (eubacteria and cyanobacteria). However, planktonic and benthic N, 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 N, 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 N, fixation or enhanced existing rates of N, fixation. Supplementation with particles (organic detritus) also enhanced N2 fixation potentials; the combined addition of particles and organic compounds yielded maximum rates of N, fixation. This combination promotes the development of O,-reduced microenvironments (microzones) in which N, fixers can reside. A functional explanation for the observed stimulation of N, fixation is that it is an anaerobic process which, in aerobic marine waters, can only proceed in O,-poor microzones. Hence, deficiencies in organic matter rather than inorganic nutrient availability may play key roles in limiting and regulating marine N, fixation. Nitrogen availability is a key factor regulating primary productivity in pelagic, coastal marine, and estuarine waters (Dugdale 1967; Mann 1982). Chronic nitrogen deficiencies in such systems would appear to offer a selective advantage to N,-fixing microorganisms, which by way of the enzyme complex nitrogenase, are able to convert Nz to ammonia, a biologically utilizable form of nitrogen (Stewart 1974; Postgate 1978). However, previous studies of pelagic, coastal, and estuarine N, fixation have ’ This study was supported by the National Science Foundation (OCE 8500740 and BSR 83-14702) and North Carolina Sea Grant (R/MER-5). 2 Present address: Dept. of Biological Sciences, Univ. California, Santa Barbara 93 106. come to two common conclusions; N2 fixation is often absent in such nitrogen-depleted waters (Carpenter and Capone 1983) and when detectable, N2 fixation rates generally satisfy only a fraction of plant and animal community nitrogen demands (Carpenter 1973; Mague et al. 1977; McCarthy and Carpenter 1979). It must therefore be concluded that factors other than the mere presence of nitrogen depletion regulate marine N, fixation. Recently, Howarth and Cole ( 1985) proposed that insufficient MO availability, with respect to nitrogenase molybdenum requirements, may explain the lack of appreciable N2 fixation rates in N-depleted waters. In the same study they postulated that constraints on molybdenum assimilation by Nz-fixing microorganisms