Vertical distribution of nitrogen-fixing phylotypes in a meromictic, hypersaline lake.
We investigated the diversity of nitrogenase genes in the alkaline, moderately hypersaline Mono Lake, California to determine (1) whether nitrogen-fixing (diazotrophic) populations were similar to those in other aquatic environments and (2) if there was a pattern of distribution of phylotypes that reflected redox conditions, as well as (3) to identify populations that could be important in N dynamics in this nitrogen-limited lake. Mono Lake has been meromictic for almost a decade and has steep gradients in oxygen and reduced compounds that provide a wide range of aerobic and anaerobic habitats. We amplified a fragment of the nitrogenase gene (nifH) from planktonic DNA samples collected at three depths representing oxygenated surface waters, the oxycline, and anoxic, ammonium-rich deep waters. Forty-three percent of the 90 sequences grouped in nifH Cluster I. The majority of clones (57%) grouped in Cluster III, which contains many known anaerobic bacteria. Cluster I and Cluster III sequences were retrieved at every depth indicating little vertical zonation in sequence types related to the prominent gradients in oxygen and ammonia. One group in Cluster I was found most often at every depth and accounted for 29% of all the clones. These sequences formed a subcluster that contained other environmental clones, but no cultivated representatives. No significant nitrogen fixation was detected by the 15N2 method after 48 h of incubation of surface, oxycline, or deep waters, suggesting that pelagic diazotrophs were contributing little to nitrogen fluxes in the lake. The failure to measure any significant nitrogen fixation, despite the detection of diverse and novel nitrogenase genes throughout the water column, raises interesting questions about the ecological controls on diazotrophy in Mono Lake and the distribution of functional genes in the environment.
Steward
G F
GF
Department of Ocean Sciences, University of California, Santa Cruz, CA 95064, USA. grieg@hawaii.edu
Zehr
J P
JP
Jellison
R
R
Montoya
J P
JP
Hollibaugh
J T
JT
D001419:Bacteria / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D002140:California; D003001:Cloning, Molecular; D016000:Cluster Analysis; D017931:DNA Primers; D004553:Electric Conductivity; D005618:Fresh Water / Q000737:chemistry* / Q000382:microbiology*; D008969:Molecular Sequence Data; D009587:Nitrogen Isotopes / Q000378:metabolism; D009591:Nitrogenase / Q000032:analysis / Q000235:genetics*; D010100:Oxygen / Q000032:analysis; D010802:Phylogeny; D010933:Plankton / Q000235:genetics*; D016133:Polymerase Chain Reaction; D000644:Quaternary Ammonium Compounds / Q000032:analysis; D017422:Sequence Analysis, DNA; D013696:Temperature
D017931:DNA Primers; D009587:Nitrogen Isotopes; D000644:Quaternary Ammonium Compounds; D009591:Nitrogenase; D010100:Oxygen