The Apoplast of Spruce Needles as a New Habitat for Autotrophic Nitrifiers: Evidence and Ecological Consequences

Dr. Rainer Gasche; Institute for Meteorology and Climate ResearchAtmospheric Environmental Research Division; Forschungszentrum Karlsruhe GmbH Kreuzeckbahnstr. 19; D-82467 Garmisch-Partenkirchen, Germany Tel: +49 8821 183 132; Fax: +49 8821 183 294; URL: http://www..fzk.de eMail: [email protected] 1. Abstract: Hitherto identified habitats of chemolithoauotrophic ammonia oxidizers (CAO) and chemolithoautotrophic nitrite oxidizers (CNO) are the pedo-, hydroand lithosphere. The phyllosphere, especially the apoplast of tree leaves, had not yet been considered to be a potential habitat of these ecologically important bacteria. Though preliminary evidence based on physiological studies did strongly suggest that these bacteria are most likely present inside (the needle apoplast) rather outside the spruce needles (needle surface) of a 90 year old spruce plantation exposed to long year chronic atmospheric nitrogen input, final prove of the exact location of these bacteria was lacking. In order to unequivocally demonstrate the presence of autotrophic nitrifiers in the apoplast of spruce needles, a combination of FISH and confocal laser scanning microscopy (cLSM) was used. These experiments clearly demonstrate for the first time that CAO and CNO indeed colonize the apoplast of spruce needles. These findings are further supported by in situ NH3-exchange experiments, in which spruce twigs were fumigated in situ with NH3 in order to quantify NH3 uptake from the atmosphere into the plants. This uptake was reduced by 25-30% when 10 Pa acetylene -a powerful inhibitor of ammonia monooxigenase of CAOwas added to the fumigation mixture. This result shows for the first time that besides plant physiological activity –prior to these new findings believed to represent the single process of NH3 uptake from the atmosphere into the plants – autotrophic nitrifiers (CAO) inside the needles contribute significantly to the observed in situ NH3-uptake into the spruce needles of the N-affected spruce forest studied. Furthermore, since nitrifiers are important biological sources and sinks of primarily and secondarily climate relevant trace gases (N2O and NO), the new finding of the presence of CAO and CNO within the apoplast of spruce needles can be expected to have additional ecological implications, e.g. for our understanding about the basic processes involved in canopy-atmosphere exchange of these radiatively important N-trace gases. .