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.

Models of biogenic carbon (BC) flux assume that short herbivorous food chains lead to high export, whereas complex microbial or omnivorous food webs lead to recycling and low export, and that export of BC from the euphotic zone equals new production (NP). In the Gulf of St. Lawrence, particulate organic carbon fluxes were similar during the spring phytoplankton bloom, when herbivory dominated, and during nonbloom conditions, when microbial and omnivorous food webs dominated. In contrast, NP was 1.2 to 161 times greater during the bloom than after it. Thus, neither food web structure nor NP can predict the magnitude or patterns of BC export, particularly on time scales over which the ocean is in nonequilibrium conditions.

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Diel variations in N 2 fixation (acetylene reduction), CO 2 fixation, and oxygen concentrations were measured, on three separate occasions, in a marine microbial mat located on Shackleford Banks, North Carolina. Nitrogenase activity (NA) was found to be inversely correlated with CO 2 fixation and, in two of the three diel periods studied, was higher at night than during the day. Oxygen concentrations within the top 3 mm of the mat ranged from 0 to 400 μM on a diel cycle; anaerobic conditions generally persisted below 4 mm. NA in the mat was profoundly affected by naturally occurring oxygen concentrations. Experimentally elevated oxygen concentrations resulted in a significant depression of NA, whereas the addition of the Photosystem II inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea decreased oxygen concentrations within the mat and resulted in a significant short-term enhancement of NA. Mat N 2 -fixing microorganisms include cyanobacteria and heterotrophic, photoautotrophic, and chemolithotrophic eubacteria. Measured (whole-mat) NA is probably due to a combination of the NA of each of these groups of organisms. The relative contributions of each group to whole-mat NA probably varied during diel and seasonal (successional) cycles. Reduced compounds derived from photosynthetic CO 2 fixation appeared to be an important source of energy for NA during the day, whereas heterotrophic or chemolithotrophic utilization of reduced compounds appeared to be an important source of energy for NA at night, under reduced ambient oxygen concentrations. Previous estimates of N 2 fixation calculated on the basis of daytime measurements may have seriously underestimated diel and seasonal nitrogen inputs in mat systems.

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