Institut für Angewandte
Mikrobiologie, Justus-Liebig Universität Gießen, Heinrich-Buff-Ring
26-32, D-35392 Gießen GERMANY.
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Phosphorus
is an essential nutrient for plant growth and development and similar to
nitrogen it is one of the most important elements in crop production. Because
the availability of phosphorus to plants is restricted by various factors, it
seems to be reasonable to study microorganisms that are able to solubilize
phosphate from soil and promote its uptake by plants.
During the last ten
years knowledge about phosphate solubilizing microorganisms increased
signifcantly. Several strains of bacterial and fungal species have been
described and investigated in detail for their phosphate-solubilizing
capabilities.
Among the bacteria,
strains belonging to the rhizobia and related organisms have been investigated
most extensively until now.
In addition several
other organisms belonging to taxonomically different and phylogenetic largely
unrelated genera (e.g. Bacillus, Paenibacillus, Escherichia, Enterobacter, Rahnella, Pseudomonas, Burkholderia and some others)
and have a been characterized for their potential to solubilize phosphate.
It is surprising,
that the description of phosphate solubilizing bacteria is restricted to
relatively few bacterial genera, given the fact, that it can be expected that
phosphate solubilizing activity must be widespread among microorganisms.
A taxonomic
identification of phophate solubilizing bacteria is provided in the majority of
those publications, dealing with model studies and those studies in which few
isolates are investigated. In broader studies, the identification of single isolates
is often neglected, mainly due to the requirement of time- and material
consuming processes connected with taxonomic identification.
It is largely
accepted, that the diversity of bacteria in soil in very poorly known and it
has been estimated that less than one percent of the bacteria from natural
communities can be grown in the laboratory. Of course, this has caused us to
underestimate the bacterial diversity.
Diversity can be
defined as richness, or number of “types“. The term
“type“ may be replaced by the term “species“ however,
several ecologists have doubted that the present species definition in
bacteriology really reflects diversity.
Some microbial
ecologists favour a natural species concept for prokaryotes, which is mainly
based on the results of direct molecular analyses of natural microbial
populations.
But is it necessay
to adopt the obviously present molecular diversity into a species concept, with
all consequences (i.e. thousands or millions of new nomenspecies)?
The species
definition in bacteriology is a practical one. Especially the conclusions and recommendations of the Ad Hoc Committee on
Reconciliation of Approaches to Bacterial Systematics (Wayne et al., 1987, IJSB 37: 463-464) have provided
bacteriologists with a uniform definition of prokaryotic species that has been
widely used in systematic studies.
Since 1987 the introduction of new methods have provided new
opportunities for prokaryotic systematics, some of which have already been
realized. Developments of particular interest include:
Ø
The ability to order prokaryotic taxa
hierarchically among the ranks of genera and kingdoms has been relized by
routine 16S rDNA sequence analyses.
Ø
Determination of inter- and intra-species
relatedness has been facilitated by rapid DNA typing methods (AFLP, RAPD,
Rep-PCR, PFGE), gene clusters (ribotyping of rrn operons), individual genes (ARDRA of 16S rDNA), and intergenic 16S-23S
rDNA spacer regions (ISR).
Ø
Multilocus sequence typing (MLST) has brought a
new dimension into the elucidation of genomic relatedness at the inter- and
intraspecific level by sequence analyses of housekeeping genes subjected to
stabilising selection.
To date, this technique has been mainly used in epidemiology; but it
offers the opportunity to incorporate the insights available from population
genetics and phylogenetic approaches into bacterial systematics and, as already
recommended by Wayne et al. (1987), provides
microbiologists with the tools to search for phylogenetic markers independent
of ribosomal DNA genes.
The role of DNA sequence data, especially those of protein-coding genes,
in ecology and classification have been dealt with in theory and practice by
several publications and sequence analyses of complete genomes, starting with
the analysis of Haemophilus influenzae has
provided scientists with an immeasurable wealth of information, ranging from
sequences to chromosome architecture, including the position and nature of
episomal elements.
On the basis of
these developments, an Ad Hoc committee for the
Re-Evaluation of the Species Definition in Bacteriology met in February 2002 to
discuss these new developments. The recommendations of this meeting have been
published recently (Stackebrandt et al. 2002, IJSEM 52: 1043-1047) and will be elucidated in this presentation.
It was a major
outcome of the discussions, that the advantages of molecular biology will
influence our thinking and understanding of bacteral diversity and hence the
unit of diversity which may be defined as “species“.
Biodiversity of
phosphate solubilizing bacteria will be one of the major topics of the PSM
meeting in Salamanca and a dialog between taxonomists, populations geneticists
and microbial ecologists is essential to find a common language in the future.
It is anticipated that in the
future a better understanding of the overall processes of phosphate
solubilization is connected with a better understanding of the diversity and
interactions of these microorganisms (including the main players) and the
description of this diversity (preferably in taxonomical terms).