In an alpine lake, the lion's share of the lakes metabolic activity in the investigated layer was accomplished by a tiny part of the bacterial community. Such minorities have often been neglected in ecosystem studies. However, they appear to be of crucial importance in some ecosystems, the authors point out.
Microorganisms are all over the place - but who does what, and when?
An international group of scientists around Niculina Musat from Max-Planck-Institute for Marine Microbiology in Bremen, Germany, managed to determine simultaneously the metabolism and identity of single bacterial cells. At Lake Cadagno, an alpine lake in Switzerland, the scientists compared the metabolic activity of three species of bacteria. They now publish their surprising results in the "Proceedings of the National Academy of Science" (PNAS): The lion's share of the lakes metabolic activity in the investigated layer was accomplished by a tiny part of the bacterial community. The species constituting only 0.3 percent of all bacterial cells was responsible for more than 40 percent of the ammonium- and 70 percent of the carbon uptake.
Opposite to most inland waters, Lake Cadagno is permanently stratified (meromictic). The transition zone between an upper, oxic and a lower, anoxic layer is the habitat of Chromatium okenii, Lamprocystis purpurea and Chlorobium clathratiforme - all of these microorganisms are living photosynthetically in the absence of oxygen. Chlorobium clathratiforme, being the most abundant species, accounts for up to 80 percent of all cells in the investigated layer. Nevertheless, C. clathratiforme contributes only about 15 percent of the total ammonium and carbon uptake. Lamprocystis purpurea, an abundant, small species, took up less than 2 percent of the investigated nutrients. On the contrary, the comparatively large cells of Chromatium okenii, comprising a tiny part of the bacterial community, contributed the major part to the uptake of ammonium and carbon.
"Most studies on the ecology of microbial communities deal with the abundant organisms. This is also true for genetical analyses of environmental samples. Groups of microorganisms with a frequency of less than one percent, however, are often neglected and regarded as minor or of no importance. However, our results clearly show that exactly those minorities can be essential for the understanding of an ecosystem. Neglecting them can easily lead to erroneous conclusions", underlines co-author Marcel Kuypers.
Comparing cells within one species, Musat and her colleagues found even more surprises: Metabolic rates vary greatly between individual cells of the same species, showing that microbial populations in the environment are heterogeneous, being comprised of physiologically distinct individuals. The scientists suspect genetic reasons for this heterogeneity. Differences between individual cells probably result from minor variability within the genome, springing from mutations during evolution.
The results at hand are available thanks to the so-called NanoSIMS-Technology. The scientists from the Max-Planck-Institute in Bremen operate their NanoSIMS since mid 2008 and have optimized this special mass spectrometer for ecological issues. This allows to analyze the distribution of various labelled carbon and nitrogen compounds within single cells. At the same time, single microbial cells are identified by the scientists applying molecular techniques. "This method will revolutionize ecological investigations", Marcel Kuypers is confident.Manfred Schlösser
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