N2 fixation by free-living and associative organotrophic
diazotrophs in marine and terrestrial habitats*
David Patriquin, Dalhousie University
I met Dr. Roger Knowles in 1970 while in the final
stages of my PhD on nutrient sources for tropical
seagrasses. I had concluded that there had to be high levels
of nitrogen fixation in the sediments in which they
Ruth & Roger Knowles, 2004
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are rooted. Roger encouraged me to work on the
problem in his lab and later I took up a postdoc there. A
vintage mass spec enabled 15N studies of N2 fixation,
while we used the latest GCs to analyze atmospheric
gases and apply the novel acetylene reduction
technique to probe sites and processes of N2 fixation.
On a memorable visit to DuPont labs, Ralph Hardy
talked optimistically about the potential for N2 fixation
and the newly discovered C4 photosynthetic pathway
to reduce fertilizer needs and increase crop production.
From Brazil came reports of high N2-fixing activity
associated with roots of tropical grasses including
maize. Amidst the excitement of these discoveries,
Roger instilled in us the value of stepwise, hypotheticodeductive
approaches for investigating questions in
microbial ecology and the importance of evaluating the
related methodologies. Research I began in Roger's lab
and carried on through the 70s and 80s with students at
Dalhousie University and collaborators at other
institutions added new genera to the list of marine
diazotrophs, (Vibrio and Campylobacter - C. nitrofigilis
later became the type species for the genus
Arcobacter), documented or suggested novel sites of
diazotroph activity (rhizosphere and sediments of
seagrasses, internal tissues of graminoid roots and
stems, sea urchin guts, saltmarsh detritus, sugarcane
litter) and revealed a fungal-diazotroph association
(Helicomyces roseus/Azospirillum brasilense) in
sugarcane litter that fixes nitrogen under fully aerobic
conditions. Following on the pioneering work by
Johanna Dobereiner, research by many investigators
has uncovered a high diversity of organotrophic
diazotrophs, including many new species, associated
with terrestrial plants. It has also provided detail at
molecular and higher levels about colonization of roots
and stems by diazotrophs. However, recent evaluations
of the contribution of N to major crops from associative
N2 fixation suggest it is small, with the possible
exceptions of rice, sugarcane and some perennial
forages under particular conditions. Efforts to
manipulate graminoid-diazotroph associations to
enhance N2 fixation have so far not succeeded. Recent
studies have increased the estimates of N2 fixation by
free-living and associative organotrohic diazotrophs in
marine systems, much of it attributed to sulfate
reducers. In association with seagrasses, sulfate
reducers fix N2 in excess of their own needs, there is
rapid transfer of newly fixed N to above-ground
biomass and as much as a third of N assimilated by the
plant can come from associative nitrogen fixation.
*Presentation in the Roger Knowles Tribute Symposium, Canadian Society of Microbiologists Annual Conference, Memorial University June 20-23, 2011. Slides (PDF) |