We studied the effects of algal-and soil-derived dissolved organic matter (DOM) enrichments on the activity and community composition of bacterioplankton in an alpine lake. The effects of the DOM source on bacteria were tested by establishing dilution cultures amended with either an algal lysate or with a soil extract. Cultures were incubated for 6 days under close to in situ conditions, and changes in bacterial community composition and activity were tracked by micro-autoradiography ([ 3 H]-L-leucine) combined with fluorescent in situ hybridization and signal amplification by catalyzed reporter deposition. Heterotrophic bacterial production increased by 25-fold after the algal lysate addition. After 3 days, up to 90% of the bacteria were active in this treatment, and bProteobacteria, particularly the subgroup R-BT, represented 86% and 67% of the total bacterial counts, respectively. From day 3 onward, the percentage of active cells in this treatment decreased dramatically as did the relative abundance of subgroup R-BT. By the end of the experiment, b-Proteobacteria still dominated bacterial abundance (80%), but active cells were only 25% of total bacterial counts. In contrast, the addition of soil-derived DOM led only to a two-to three-fold increase in bacterial production. b-Proteobacteria was still the dominant group (50-60% of total bacterial counts and 50% of the cells positive for leucine incorporation), but Actinobacteria made a substantial contribution (15-23%). This pattern contrasted with that observed in the treatment receiving the algal lysate, where the relative abundance of the latter group rapidly decreased. In the treatment amended with algal-derived DOM, bacterial carbon production matched the observed decrease in dissolved organic carbon concentration. However, bacterial carbon produced on soil-derived DOM accounted only for 30% of the decrease in dissolved organic carbon concentration, suggesting a more inefficient utilization of this material. The expected climate-driven changes in DOM supply to alpine lakes will affect their bacterial community structure and activity.

No abstract

The peptidoglycan layer of the bacterioplankton cell wall contains four major amino acids (alanine, Ala; serine, Ser; aspartic acid, Asp; and glutamic acid, Glu) in a characteristic enantiomeric ratio (D/L ratio). It is assumed that bacterioplankton are the only biological source of significance for these four specific D-amino acid species in the ocean. The concentrations of these dissolved total enantiomeric amino acids were measured throughout the water column of the Faroe Shetland Channel (North Atlantic). Concurrently, the uptake of D-versus L-Asp and of Lleucine (as a measure of bacterial production) by bacterioplankton was determined. The D/L ratios of the dissolved total Ala, Asp, Glu, and Ser did not exhibit any particular trend with depth, averaging 0.49 for Ala, 0.42 for Asp, 0.15 for Glu, and 0.09 for Ser. The ratio of D-/L-Asp uptake by bacteria, however, increased from surface (D-/LAsp uptake ratio of ϳ0.03) to deeper layers reaching a D-/L-Asp uptake ratio of close to 1 at 1,000 m depth, indicating that mesopelagic bacteria utilize D-Asp almost as efficiently as L-Asp. Subsequent laboratory experiments with surface-water bacterioplankton assemblages incubated in nutrient-amended artificial seawater confirmed that bacterioplankton, in the absence of other utilizable organic carbon, efficiently utilize D-amino acids. In these laboratory experiments, the D-/L-Asp uptake ratio increased within 8 d to values similar to those obtained for mesopelagic bacteria. Furthermore, the presence of flagellates stimulated the uptake of D-Asp probably via enhanced release of D-amino acids during bacterivory. Thus, our results indicate that dissolved D-amino acids might be an important substrate for mesopelagic bacterioplankton. The efficient uptake of D-amino acids in the deeper layers of the ocean might indicate that mesopelagic bacterioplankton are utilizing bacterial cell wall-derived organic matter efficiently.

We investigated the development of cartilage canals to clarify their function in the process of bone formation.Cartilage canals are tubes containing vessels that are found in the hyaline cartilage prior to the formation of a secondary ossification centre (SOC). Their exact role is still controversial and it is unclear whether they contribute to endochondral bone formation when an SOC appears. We examined the cartilage canals of the chicken femur in different developmental stages (E20, D2, 5, 7, 8, 10 and 13). To obtain a detailed picture of the cellular and molecular events within and around the canals the femur was investigated by means of three-dimensional reconstruction, light microscopy, electron microscopy, histochemistry and immunohistochemistry [vascular endothelial growth factor (VEGF), type I and II collagen]. An SOC was visible for the first time on the last embryonic day (E20). Cartilage canals were an extension of the vascularized perichondrium and its mesenchymal stem cell layers into the hyaline cartilage. The canals formed a complex network within the epiphysis and some of them penetrated into the SOC were they ended blind. The growth of the canals into the SOC was promoted by VEGF. As the development progressed the SOC increased in size and adjacent canals were incorporated into it. The canals contained chondroclasts, which opened the lacunae of hypertrophic chondrocytes, and this was followed by invasion of mesenchymal cells into the empty lacunae and formation of an osteoid layer. In older stages this layer mineralized and increased in thickness by addition of further cells. Outside the SOC cartilage canals are surrounded by osteoid, which is formed by the process of perichondral bone formation. We conclude that cartilage canals contribute to both perichondral and endochondral bone formation and that osteoblasts have the same origin in both processes.

No abstract

We examined the ability of different freshwater bacterial groups to take up leucine and thymidine in two lakes. Utilization of both substrates by freshwater bacteria was examined at the community level by looking at bulk incorporation rates and at the single-cell level by combining fluorescent in situ hybridization and signal amplification by catalysed reporter deposition with microautoradiography. Our results showed that leucine was taken up by 70–80% of Bacteria-positive cells, whereas only 15–43% of Bacteria-positive cells were able to take up thymidine. When a saturating substrate concentration in combination with a short incubation was used, 80–90% of Betaproteobacteria and 67–79% of Actinobacteria were positive for leucine uptake, whereas thymidine was taken up by < 10% of Betaproteobacteria and by < 1% of the R-BT subgroup that dominated this bacterial group. Bacterial abundance was a good predictor of the relative contribution of bacterial groups to leucine uptake, whereas when thymidine was used Actinobacteria represented the large majority (> 80%) of the cells taking up this substrate. Increasing the substrate concentration to 100 nM did not affect the percentage of R-BT cells taking up leucine (> 90% even at low concentrations), but moderately increased the fraction of thymidine-positive R-BT cells to a maximum of 35% of the hybridized cells. Our results show that even at very high concentrations, thymidine is not taken up by all, otherwise active, bacterial cells.

1. Oligotrich ciliates are an important part of most marine plankton communities. Mixotrophic (chloroplast‐sequestering) oligotrichs, a common component of marine oligotrich communities, obtain fixed carbon from both photosynthesis as well as the ingestion of particulate food. Mixotrophy, in general, is often considered an adaptation permitting exploitation of food‐poor environments. We examined the hypothesis that, among oligotrichs, mixotrophs may be at a disadvantage relative to heterotrophs in food‐rich conditions in a nutrient‐enrichment experiment. We compared growth responses of mixotrophic and heterotrophic oligotrichs in natural communities from the N.W. Mediterranean Sea in microcosms with daily nutrient additions resulting in increases in nanoflagellates and Synechococcus populations. The results indicated that both mixotrophic and heterotrophic oligotrichs respond to prey increases with rapid growth (μ=1.2 d−1). 
2. To examine the hypothesis that the proportion of mixotrophic to heterotrophic oligotrichs changes with the trophic status of a system, increasing with oligotrophy, we examined data from a variety of marine systems. Across systems ranging in chlorophyll concentration from about 0.1 to 40 μg L−1, oligotrich cell concentrations are correlated with chlorophyll concentrations, and mixotrophs are a consistent component of oligotrich communities, averaging about 30% of oligotrich cell numbers. 
3. We discuss the costs, benefits and possible uses of mixotrophy in marine oligotrichs and suggest that mixotrophy in marine oligotrichs is not closely linked to the exploitation of food‐poor environments, but probably serves a variety of purposes.

SummaryWe studied the interactive effects of dissolved organic matter (DOM) and solar radiation on the activity and community structure of bacteria from an alpine lake. Activity was assessed both at the community level as leucine incorporation rates and at the single-cell level by microautoradiography. Fluorescent in situ hybridization and signal amplification by catalysed reporter deposition (CARD-FISH) was used to track changes in the bacterial community composition. Bacteria-free filtrates of different DOM sources (lake, algae or soil) were incubated either in the dark or exposed to solar radiation. Afterwards, the natural bacterial assemblage was inoculated and the cultures incubated in the dark for 24-48 h. Bacterial activity was enhanced in the first 24 h in the soil and algal DOM amendments kept in the dark. After 48 h, the enhancement effect was greatly reduced. The initial bacterial community was dominated by Betaproteobacteria followed by Actinobacteria. The relative abundance (expressed as a percentage of DAPIstained cells) of Betaproteobacteria increased first in dark incubated DOM amendments, but after 48 h no significant differences were detected among treatments. In contrast, the relative abundance of Actinobacteria increased in pre-irradiated DOM treatments. Although Betaproteobacteria dominated at the end of the experiment, the relative abundance of their R-BT subgroup differed among treatments. Changes in bacterial community activity were significantly correlated with those of the relative abundance and activity of Betaproteobacteria, whereas the contribution of Actinobacteria to the bulk activity was very modest. Our results indicate a negative effect of DOM photoalteration on the bulk bacterial activity. The magnitude of this effect was time-dependent and related to rapid changes in the bacterial assemblage composition.