Sulfate inhibition of molybdenum-dependent nitrogen fixation by planktonic cyanobacteria under sea water conditions: a non-reversible effect

Abstract

The trace element molybdenum is a central componentof several enzymes essential to bacterial nitrogenmetabolism, including nitrogen fixation. Despitereasonably high dissolved concentrations (for atrace metal) of molybdenum in seawater, evidencesuggests that its biological reactivity andavailability are lower in seawater than infreshwater. We have previously argued that thisdifference is related to an inhibition in the uptakeof molybdate (the thermodynamically stable form ofmolybdenum in oxic natural waters) by sulfate, astereochemically similar ion. Low molybdenumavailability may slow the growth rate ofnitrogen-fixing cyanobacteria, and in combinationwith an ecological control such as grazing byZooplankton, keep fixation rates very low in evenstrongly nitrogenlimited coastal marineecosystems. Here we present results from a seawatermesocosm experiment where the molybdenumconcentration was increased 10-fold under highlynitrogen-limited conditions. The observed effects onnitrogen-fixing cyanobacterial abundance andnitrogen-fixation inputs were much smaller thanexpected. A follow-up experiment with sulfate andmolybdenum additions to freshwater microcosms showedthat sulfate (at seawater concentrations) greatlyreduced nitrogen fixation by cyanobacteria and thatadditions of molybdenum to the levels present in theseawater mesocosm experiment only slightly reversedthis effect. In light of these results, were-evaluated our previous work on the uptake ofradio-labeled molybdenum by lake plankton and bycultures of heterocystic cyanobacteria. Our newinterpretation indicates that sulfate at salineestuarine levels (>8 10 mM) up to seawater (28mM) concentrations does inhibit molybdenumassimilation. However, the maximum molybdenum uptakerate ( V max ) was a function of the sulfateconcentration, with lower V max values at highersulfate levels. This indicates that this inhibitionis not fully reversed at some saturating level ofmolybdenum, as assumed in a simple competitiveinhibition model. A multi-enzyme, mixed kineticsmodel with two or more uptake enzyme systemsactivated in response to the environmental sulfateand molybdate conditions may better explain therepressive effect of sulfate on Mo-mediatedprocesses such as nitrogen fixation.