Marine diazotroph diversity, distribution and activity


A major research focus in the Zehr lab is the examination of the biogeography of major nitrogen-fixing cyanobacteria as well as the biochemistry and molecular biology of isolated strains. Our focus is on the major groups of unicellular nitrogen-fixing cyanobacteria (Crocosphaera watsonii and the uncultivated symbiont of Braarudospahera bigelowii, UCYN-A), which early reasearch in the Zehr lab revealed to be important in nitrogen fixation in the open ocean.  We are also studying the heterocyst-forming cyanobacteria (Richelia and Calothrix) and unicellular cyanobacteria (unidentified) that are commonly living in assocaition with a variety of diatoms (RhizosoleniaHemiaulusChaetocerosClimacodium). Although one of these groups has cultivated isolates (C. watsonii) from the Atlantic and Pacific Oceans, the ecophysiology of these microorganisms remains not well understood. 

Phylogenies of major open ocean nitrogen-fixing cyanobacteria. Left: a nitrogenase (nifH) amino acid phylogenetic tree and photomicrographs of major open ocean nitrogen-fixing cyanobacteria showing representatives of the major clades (photomicrographs by R.Foster). Right: DNA distance trees of (a) Trichodesmium, (b) UCYN-A and (c) Crocosphaera (UCYN-B) show that these groups have little DNA sequence divergence (short branch lengths within a genus indicate that the different strains differ by only a few nucleotides in DNA sequence) within their respective clades. Abbreviations for the ocean regions where the sequences were obtained: AS, Arabian Sea; BS, BalticSea; GoM, Gulf of Mexico; Med, MediterraneanSea; NA,North Atlantic; NP,North Pacific; SP, SouthPacific; SCS, South China Sea. From Zehr et al., (2011).