Nitrate and phosphate concentrations higher than those found in the natural environment slowed down growth of two strains of non-bloom-forming, phycoerythrin-rich Synechococcus spp. isolated from mesotrophic subalpine lakes. The results make clear why isolation of these picocyanobacteria in standard cultivation media was difficult. At low concentrations, closely related strains exhibited distinct growth characteristics with respect to these two nutrients, possibly explaining differences in their seasonal appearance in the natural environment.At low levels of nutrient loading, small red-pigmented phycoerythrin (PE)-rich cyanobacteria of the Synechococcus type dominate the autotrophic picoplankton in freshwater ecosystems (10,12,16,21). These non-bloom-forming Synechococcus spp. (19) belong to the same phylogenetic clade as marine Synechococcus spp. and Prochlorococcus spp. (7,8,20). Despite their ubiquity, they were discovered in the late 1970s only, when epifluorescence microscopy and flow cytometry were introduced as counting techniques (see references in reference 18) and the first isolates from marine and freshwater ecosystems became available (6, 23). The difficulty in cultivating red-pigmented freshwater Synechococcus spp. contrasts with the easy accessibility of blue-green, phycocyanin-rich species, particularly those of the closely related genus Cyanobium (8, 22; for strain histories, see reference 14).Recently, Becker et al. (3) reported that PE-rich picocyanobacteria from a biofilm of tiles deposited for 6 weeks in Lake Constance formed small colonies on agar plates amended with a mineral medium lacking nitrate, BG11 0 , although none of them was capable of nitrogen fixation. A similar inoculum spread on plates with nitrate (BG11) produced blue-green colonies only. The mineral medium BG11 was introduced by Stanier et al. (17) for the cultivation of coccoid cyanobacteria. In its original description, it is particularly rich in nitrate (1.5 g/ liter NaNO 3 , equivalent to 247 mg/liter NO 3 -N) and exhibits an N:P molar ratio of 100:1, which is far above the molar ratio of these elements in biomass (Redfield ratio, 16:1). Nevertheless, phosphate also is plentiful in BG11 compared to the environmental concentrations often limiting the growth of photoautotrophs in meso-and oligotrophic lakes.Cyanobacteria have developed highly efficient uptake and retention mechanisms for three nutrient anions: bicarbonate, nitrate, and phosphate (1,11,15). The apparent competitive disadvantage of PE-rich Synechococcus spp. on a cultivation medium rich in nitrate prompted us to examine the effect of high nutrient concentrations on the growth of two strains, Synechococcus rubescens strain SAG 3.81 and Synechococcus sp. strain BO 8807, isolated from Lake Zürich and Lake Constance, respectively.Growth inhibition by phosphate and nitrate. The two redpigmented Synechococcus strains BO 8807 and SAG 3.81 of subalpine cluster I (8) were raised in 96-well microtiter plates. Before growth experiments were conducted, aliquots of batch...

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With the radioactive probe atom "'In as a representative for shallow acceptors in Si the passivation of acceptors by H was studied by using the perturbed-yy-angular-correlation technique. It is shown that during passivation, close In-H pairs are formed and that the number of pairs exactly accounts for the number of deactivated acceptors. The formation of In-H pairs was investigated with use of different hydrogenation techniques and the stability of acceptor-H pairs was studied in isochronal annealing experiments.

The paper reports potassium diffusion measurements performed on gem quality single-crystal alkali feldspar in the temperature range from 1169 to 1021 K. Natural sanidine from Volkesfeld, Germany was implanted with 43 K at the ISOLDE/CERN radioactive ion-beam facility normal to the (001) crystallographic plane. Diffusion coefficients are well described by the Arrhenius equation with an activation energy of 2.4 eV and a pre-exponential factor of 5 × 10 −6 m 2 /s, which is more than three orders-of-magnitude lower than the 22 Na diffusivity in the same feldspar and the same crystallographic direction. State-of-the-art considerations including ionic conductivity data on the same crystal and Monte Carlo simulations of diffusion in random binary alloy structures point to a correlated motion of K and Na through the interstitialcy mechanism.

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