Abstract
Molybdenum (Mo) is an essential micronutrient for biological assimilation of nitrogen gas and nitrate because it is present in the cofactors of nitrogenase and nitrate reductase enzymes. Although Mo is the most abundant transition metal in seawater (107 nM), it is present in low concentrations in most freshwaters, typically <20 nM. In 1960, it was discovered that primary productivity was limited by Mo scarcity (2–4 nM) in Castle Lake, a small, meso-oligotrophic lake in northern California. Follow up studies demonstrated that Mo also limited primary productivity in lakes in New Zealand, Alaska, and the Sierra Nevada. Research in the 1970s and 1980s showed that Mo limited primary productivity and nitrate uptake in Castle Lake only during periods of the growing season when nitrate concentrations were relatively high because ammonium assimilation does not require Mo. In the years since, research has shifted to investigate whether Mo limitation also occurs in marine and soil environments. Here we review studies of Mo limitation of nitrogen assimilation in natural microbial communities and pure cultures. We also summarize new data showing that the simultaneous addition of Mo and nitrate causes increased activity of proteins involved in nitrogen assimilation in the hypolimnion of Castle Lake when ammonium is scarce. Furthermore, we suggest that meter-scale Mo and oxygen depth profiles from Castle Lake are consistent with the hypothesis that nitrogen-fixing cyanobacteria in freshwater periphyton communities have higher Mo requirements than other microbial communities. Finally, we present topics for future research related to Mo bioavailability through time and with changing oxidation state.
Highlights
Molybdenum (Mo) is an essential micronutrient in all three domains of life
Mo is important for microbial nitrogen (N) assimilation due to its presence in nitrogenase, the enzyme that performs N2 fixation and in nitrate reductase, the etdroisienreodpczniurrynseecmhsaoteisiefobneMtnithtbsooaeovt,cNfeasprNeOueeasrO−3efSco−3ieNrugrmeptHtlaotsiaa+4ntnknhedatiethsrS(sfiriDiietrmgessoehtilr(losNta(tcl2tedhOi0po,0−2init12n)9i)d.sn.9oFeWi0etoi)rtsrh,haneteaoenornt(thpdaNerrmreeMOqfmbe−3ourioio)rrrnelaeeodisqugMsoumiimcviorae.(eilrlNmWafNutHeihOnon+4icnlt−3e)-s, Mo is the most abundant transition metal in seawater (107 nM; Collier, 1985), Mo concentrations are low in most freshwaters (
We found that the green alga Scenedesmus acutus grown in chemostats contained 60% higher chlorophyll a, 3000% higher nitrate reductase activity and 80% higher cellular Mo when grown on high concentrations of Mo (90 μM) than under Mo limitation (1 nM; Glass, 2011)
Summary
We review both laboratory and field experiments in aquatic (and briefly, soil) environments that have tested the effect of varying Mo concentration on N assimilation via N2 fixation and NO−3 reduction. We summarize new results on the effect of Mo additions on total protein abundance and activity of enzymes involved in NO−3 assimilation in Castle Lake, as well as high-resolution Mo depth profiles that suggest Mo may be limiting N2 fixation by lake periphyton communities.
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