Famous Nitrogen Fixers
Ann M. Hirsch 2008©
Farmers have known, probably since the time of the Egyptians, that legumes such as pea,
lentil, and clover are important for soil fertility.
Such practices as green manuring, crop rotation,
and intercropping have been known for millennia and were extensively described by the
Romans, but it was not until the 19
century that an explanation for the success of the legumes in
restoring soil fecundity, especially after a crop such as wheat had been grown, was uncovered.
In the 19
century, agriculture in Europe had progressed to the point that both green manuring
and intercropping using legume crops was standard procedure.
The leguminous plants were
known in German as “Stickstoffsammler” or nitrogen accumulators, whereas non-leguminous
crops such as wheat were called “Stickstofffreser” or nitrogen consumers.
Even though people observed “bumps” on legume roots as early as the 17
evidenced by a drawing published in 1679 by Malpighi (who thought they were insect galls), the
mechanism whereby legumes accumulated nitrogen was unknown.
It took a German scientist,
Hermann Hellriegel, in collaboration with Hermann Wilfarth, to recognize that the legume root
nodules themselves were responsible for the conversion of atmospheric nitrogen to ammonia
The organisms inside the nodule were thought by some to be vibrio-like or bacteria-like
organisms, but others were of the opinion that they were fungi.
The microorganisms were first
isolated and cultured from nodules of a number of different legume species by Martinus
Beijerinck (1888) of Holland.
Over time, modifications to the culture media were made to
ensure easy isolation and growth of the nodule bacteria, which were called
Martinus W. Beijerinck
Since that time, the original
found to consist of a number of
distinct genera, including
, and others.
Moreover, not only these alpha-
proteobacteria, but also
members of the beta-
proteobacteria have been described as being capable of nodulating legumes and eliciting nitrogen
Although not at all related to the alpha-rhizobia on the basis of 16S RNA, the beta-
genes have sequence similarity with the
genes of alpha-rhizobia, strongly
suggesting that horizontal gene transfer has taken place.
The structure of the nodules is identical
to that established by the alpha-rhizobia.
strain discovered (
was originally isolated from
(Rooibos tea plant) nodules in South Africa, but was
described as a
in the original publication; sequencing of the 16S RNA DNA region
showed that the isolate belong to the beta-rhizobia.
The team that discovered this novel