Do Acid-tolerant Cyanobacteria Exist?

Acidification of freshwater ecosystems changes phytoplankton biomass and reduces species composition. However, there are contradictory statements with respect to the occurrence of cyanobacteria below pH 4.5. Textbooks have not reported cyanobacteria in acid and very acid environments, whereas only a few papers on acidification of lakes through acid precipitation noted the occurrence of cyanobacteria in those environments. In a phytoplankton survey of 10 lakes in the Bavarian Forest as well as the lignite mining districts of Bavaria (Upper Palatine) and Lusatia, covering a pH gradient from 8.0 to 2.8, we demonstrate that acid-tolerant cyanobacteria do exist. Most strikingly, one of the most acid lakes (pH 2.9), Lichtenauer See (Lusatia), was inhabited by two populations of filamentous cyanobacteria, resembling Oscillatorial/Limnothrix and Spirulina spp. Eukaryotic phytoplankton was almost absent in this lake at the time. In contrast to filamentous cyanobacteria, picoplanktic ones were totally lacking where pH < 4.5. This indicates that members of coccal picoplanktic cyanobacteria and filamentous cyanobacteria have different acid tolerances. At present, it is not known how the acid-tolerant cyanobacteria described here maintain a strong transmembrane pH gradient.     

Präzipitation von Aluminium und Phosphat in Gewässern unter dem Einfluss von Versauerung

Precipitation of Aluminium and Phosphate Affected by Acidification Acidified waters often show elevated concentrations of Al (with up to 6 mg L^–1 being not unusual). A pH increase resulting e.g. from mixing with non‐acidified water or from biological activities may be linked with Al precipitation. Up to now, this phenomenon was described for acid mine drainages. This investigation focuses on a whitish precipitate naturally formed in a brook of an atmospherically acidified catchment in the Ore Mountains, Germany. Based on infrared spectra the precipitate was identified as an Al‐hydroxosulfate with crystal water. A simulation of natural conditions in the laboratory showed that Al precipitated only if sulfate or phosphate ions were added to the solution. In the case of sulfate being added, the infrared spectrum of the precipitate was similar to the natural precipitate. ²⁷Al NMR spectroscopy revealed tetrahedrally coordinated Al in some precipitates which evidences the participation of the tridecameric [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ cation beside other polymeric Al cations. Precipitation experiments subjected to the given conditions showed that the phosphate elimination from solution with Al was much higher than with Fe. With Al and Fe added together, the P elimination rate was likewise high, and phosphate was bound onto Al in the precipitate. This was demonstrated by SEM‐EDX spectroscopy. Based on these results we present a possible reaction mechanism. The precipitation of Al together with P allows a significant retention of both elements in sediments because in contrast to Fe, Al immobilizes phosphate even under anoxic conditions.     

Groundwater Inflow Controls Acidity Fluxes in an Iron Rich and Acidic Lake

To ascertain the influence of hydrological boundary conditions on acidity fluxes in lakes influenced by acid mine drainage, acidity budgets were developed for two sediments in areas of differential groundwater inflow (approx. 1 L m^?2 d^?1 and 10 L m^?2 d^?1). In both sediments iron was deposited as schwertmannite leading to iron(III) enriched sediments (3.9…6.2 mmol g^?1, referred to dry weight). Compared to the surface water, the inflowing groundwater had higher pH (4.5 vs. 3), ferrous iron (6…20 mmol L^?1 vs. 0.8…2.0 mmol L^?1), and sulfate (5…60 mmol L^?1 vs. 8…13 mmol L^?1) concentrations. The inflow changed the sediment pore water chemistry and triggered a further increase in pH to above 5.5. In both sediments acidity generation in the surface water (10…30 mol m^?2 a^?1) strongly prevailed over acidity consumption in the sediments (> ?0.6 mol m^?2 a^?1). With advective groundwater inflow, however, more acidity was consumed due to TRIS formation (?0.12 mol m^?2 a^?1 vs. ?0.017 mol m^?2 a^?1), iron carbonate burial (upper estimate: ?0.14 mol m^?2 a^?1 vs. ?0.022 mol m^?2 a^?1), and unspecific ferrous iron retention (?0.39 mol m^?2 a^?1 vs. ?0.08 mol m^?2 a^?1). Also, less acidity was generated due to schwertmannite transformation (?2.4 mol m^?2 a^?1 vs. ?0.11 mol m^?2 a^?1). The acidity balance of internal processes in the sediment with groundwater inflow was negative, whereas it was positive in the other sediment. The study demonstrates that in acidic and iron rich lakes the hydrological boundary conditions strongly affect geochemical processes as subsumed in acidity fluxes.     

Acid Rain and Natural Organic Matter (NOM)

Twentyfive years of research on the effects of acid rain on rivers and lakes has, to a very small extent, documented changes in the nature and properties of natural organic matter (NOM). In Western Norway, a "whole-watershed-artificial-acidification-experiment" took place in the period 1988–1996. The goals of this long-term experiment were to study the role of NOM in acidification of surface water and the effects of acid precipitation on the quality and properties of NOM. In the HUMEX project (Humic Lake Acidification Experiment) one half of a lake and the corresponding catchment was artificially acidified with H₂SO₄ and NH₄NO₃ over a period of 5 years. The other half of the lake and catchment served as a control. In addition to monitoring of the general chemical composition of the water from the two lake halves, a number of other chemical and biological characteristics were studied. Here, we report the results related to changes in the nature and chemical properties of NOM. During the first few years of acidification, a significantly lower concentration of NOM was recorded in the acidified half of the catchment, compared with the control. However, statistical analyses of all data (covering a 2-years pre-treatment period and 5 years of treatment) related to the concentration of NOM (TOC, colour, and UV absorbance) did not suggest any significant effect on the quantity of NOM. This apparent discrepancy between the initial decrease in the concentration of NOM and no effect when the whole 5-years period is considered, may be due to the results of two different simultaneous processes. The results suggest that there first was a reduction of TOC and colour, as a consequence of the acidification, followed by an increase, perhaps owing to increased fertilisation (nitrogen) and in addition to a general temperature increase during the period. In addition, short-term studies of the aquatic surface microlayers, lipophilicity of the NOM, content of organic sulfur, and molecular size indicate differences in the quality of the NOM between the two lake halves, which could affect light absorption.