Nitrogenase activity in tropical carbonate marine sediments

Abstract

Rates of nitrogenase acbvity were investigated in carbonate sediments of shallow tropical waters uslng the acetylene (C2H2) reduction method Nitrogenase achvlty was significantly h g h e r in fine grained sedments than m coarser grained sands The importance of oxygen, ammonium (NH:) and organic substrate availability in regulating nitrogenase activ~ty in these sedlments was evaluated The effect of O2 was vanable and appeared to be influenced by physical dsturbance of the sediment Ammonium did not appear to inhibit rates of N2 fixation over the range of in situ concentrahons observed, as there was no significant correlation of N2 fixation rates and pore water NH: Neither additions of NH: nor MSX (L-methonlne-DL-sulfoxlmine a nltrogenase derepressor) had any consistent effect on N, flxatlon N2 fixation rates were correlated w t h total organlc content of sediments Additions of mannltol and glucose led to significant, concentrahon-dependent stimulation of N2 fixahon We therefore conclude that organic substrate avallabllity is a malor factor controlling rates of nitrogen-ase activlty in these shallow carbonate sediments Based on our experiments benthic heterotrophic bactena in carbonate sediments could account for d d y N2 fixation lnputs of 0 07 to 5 2 mg N m-2 (0 03 to 1 9 g N m yr-l) These estimates indicate that the extensive areas of carbonate sedlments in these environments may contnbute a substant~al amount of nitrogen to shallow tropical and subtropical ecosystems