Adaptation and tolerance to low pH

The pH is a major environmental factor of freshwater aquatic ecosystems, ranging from <2 to 12. Biodiversity is greatly reduced in highly acidic environments (pH <3) such as acid mining lakes (AML). Although the reduction of species numbers with decreasing pH has been described in several protist and metazoan phyla, the patterns and processes of adaptation and tolerance to low pH are little understood for planktonic organisms. Presumably there are more acid tolerant species, which take refuge in low pH environments, than acidophil species which are specifically adapted to the high hydrogen ion concentration. We will design in vitro and in situ experiments within this project to test if acidotolerant species benefit from competitive release under conditions of acid stress. For the first time, we will study the interactive effect of temperature and pH stress at different food levels for planktonic ciliates, rotifers and crustacea. Since it has been assumed that tolerance towards temperature and pH stress may have a common denominator in the form of heat shock proteins, we will develop molecular markers for the detection of small heat shock proteins (HSPs) at the genomic DNA and mRNA level in our target species. Further, we will develop a flow cytometric protocol to measure the cytosolic pH and determine the external pH at which efficient internal pH regulation collapses in the study species. We aim to differentiate between genetically fixed, general physiological adaptation to acidic conditions (at the species level) and site-specific traits in similar habitats resulting from historic events, i.e. small-scale genetic shifts below the species level (microevolution). In the second part of the project we will, therefore, investigate the biomass spectrum and, using molecular methods, characterize the major players of three AML and one neutral reference lake. The four study lakes differ with respect to their pH, age and/or geographic location. Such lakes provide a rare case of suitable ecosystem models to test for the significance of strong habitat selection. We will design field and laboratory experiments challenging the assumption that the distribution of aquatic microorganisms (in AML) does not require any historical explanation, i.e. that the habitat acts strictly as a filter sensu Gleason`s `individualistic concept` of population ecology. If we find genetic and physiological differences at the sub-species level that cannot be explained by changing environments we will reject the hypothesis that the dispersal of free- living microorganisms is unlimited and that their distribution can be understood solely in terms of habitat properties. The conclusions that will emerge from this part of the project have far-reaching implications for general ecology and the ongoing debate on the biodiversity of free-living microbes.

The pH is a major environmental factor of freshwater aquatic ecosystems, ranging from <2 to 12. Biodiversity is greatly reduced in highly acidic environments (pH <3) such as acid mining lakes (AML). For instance, neither fish nor cladocerans (`water fleas`) dwell in such lakes. Although the reduction of species numbers with decreasing pH has been described in several protist and metazoan phyla, the patterns and processes of adaptation and tolerance to low pH were little understood for planktonic organisms. Within this project, we could experimentally demonstrate that, on the one hand, acid tolerant species benefit from the reduced pressure of predation and competition under conditions of acid stress. On the other hand, we found specifically adapted, acidophil species, which cannot survive under circumneutral conditions. We described, and will further describe for the first time, several of these species that were hitherto unknown (e.g. the rotifer Cephalodella acidophila). Furthermore, we investigated the as yet unknown combined effect of temperature and pH stress at different food supply with planktonic flagellates, ciliates and rotifers. The combination of these 3 key environmental variables narrows the realised (=actual) ph niche of all our study organisms. These results presumably can be applied in other habitats as well. We wanted to discriminate between genetically fixed, general physiological adaptations to acid conditions (at the species level) and locality-specific features in similar habitats, which result from small scale genetic changes of the organisms below the species level (microevolution). To this end, we investigated the biomass size spectrum and the dominant organisms using molecular methods in three acid mining lakes and one neutralised reference lake located in Austria (at Langau, Lower Austria) and East Germany (Lusatia). These four lakes differ with respect to their pH, age and/or geographic position. We tested experimentally the hypothesis that dispersal of microorganisms (in acid mining lakes) does not require a historical explanation, i.e. that the habitat acts as a filter that only preadapted organisms can pass. We found intraspecific genetic and physiological differences, as well as significant interactions between the organisms and their habitats, in which the organisms actually dwell or may exist. In other words, different strains of the same (microbial) species may behave differently in the same habitat; respectively the same strain can have a different fitness in similar (but not identical) habitats. We, therefore, reject the hypothesis that the distribution of free living microorganisms can be understood solely in terms of habitat properties. This conclusion is of general ecological relevance.