Indications fpr low net productivity of pelagic bacterioplankton

In contrast to the current view on the trophic role of bacteria in pelagic environments, the impression of a rather unproductive bacterioplankton arose from an example of the deep mesotrophic Lake Constance. Based on measurements of bacterial DNA, thymidine incorporation rates and grazing rates, turnover times of bacterial biomass exceeding 10 days were estimated during the growth period. Similarly, low productivity of bacterioplankton was indicated by low RNA/DNA ratios (< 1). Additional indications for a rather inactive bacterioplankton were provided by the observation of long lag phases in lake water cultures. Low bacterial productivity has also to be expected from energetic considerations. There is increasing evidence for low bacterial growth efficiencies under natural conditions. The presented material points to the possible need for reconsideration of current estimates of bacterial in situ growth

Vertical Distribution of Epibenthic Freshwater Cyanobacterial Synechococcus spp. Strains Depends on Their Ability for Photoprotection

Epibenthic cyanobacteria often grow in environments where the fluctuation of light intensity and quality is extreme and frequent. Different strategies have been developed to cope with this problem depending on the distribution of cyanobacteria in the water column. and either constant or enhanced levels of carotenoids were assayed in phycocyanin-rich strains collected from 1.0 and 0.5 m water depths. Protein analysis revealed that while the amount of biliproteins remained constant in all strains during light stress and recovery, the amount of D1 protein from photosystem II reaction centre was strongly reduced under light stress conditions in strains from 7.0 m and 1.0 m water depth, but not in strains collected from 0.5 m depth. spp. strains, depending on their genetically fixed mechanisms for photoprotection

Metagenomic and Metabolic Profiling of Nonlithifying and Lithifying Stromatolitic Mats of Highborne Cay, The Bahamas

BACKGROUND: Stromatolites are laminated carbonate build-ups formed by the metabolic activity of microbial mats and represent one of the oldest known ecosystems on Earth. In this study, we examined a living stromatolite located within the Exuma Sound, The Bahamas and profiled the metagenome and metabolic potential underlying these complex microbial communities. METHODOLOGY/PRINCIPAL FINDINGS: The metagenomes of the two dominant stromatolitic mat types, a nonlithifying (Type 1) and lithifying (Type 3) microbial mat, were partially sequenced and compared. This deep-sequencing approach was complemented by profiling the substrate utilization patterns of the mats using metabolic microarrays. Taxonomic assessment of the protein-encoding genes confirmed previous SSU rRNA analyses that bacteria dominate the metagenome of both mat types. Eukaryotes comprised less than 13% of the metagenomes and were rich in sequences associated with nematodes and heterotrophic protists. Comparative genomic analyses of the functional genes revealed extensive similarities in most of the subsystems between the nonlithifying and lithifying mat types. The one exception was an increase in the relative abundance of certain genes associated with carbohydrate metabolism in the lithifying Type 3 mats. Specifically, genes associated with the degradation of carbohydrates commonly found in exopolymeric substances, such as hexoses, deoxy- and acidic sugars were found. The genetic differences in carbohydrate metabolisms between the two mat types were confirmed using metabolic microarrays. Lithifying mats had a significant increase in diversity and utilization of carbon, nitrogen, phosphorus and sulfur substrates. CONCLUSION/SIGNIFICANCE: The two stromatolitic mat types retained similar microbial communities, functional diversity and many genetic components within their metagenomes. However, there were major differences detected in the activity and genetic pathways of organic carbon utilization. These differences provide a strong link between the metagenome and the physiology of the mats, as well as new insights into the biological processes associated with carbonate precipitation in modern marine stromatolites

The influence of spatial and temporal concentration gradients on phosphate partitioning between different size fractions of plankton: Further evidende and possible causes

Size-fractionated 32PO4-uptake experiments were done with mixed laboratory cultures of P-limited algae and their bacterial contaminants and with field samples from Lake Constance and three small lakes. In the mixed culture samples, tracer uptake was dominated by bacteria, but increasingly shifted toward algae when unlabeled orthophosphate was added. In samples from Lake Constance (2-m depth), the bacterial size fraction ( 1, >3, > 12 µm). We tested some possible causes for nutrient inhomogeneities.32P-labeled Daphnia appeared to release tracer in patchy fashion, favoring algal uptake in mixed culture samples, as well as in field samples from Lake Constance. In Lake Constance, the proportion of 32PO4 uptake by larger size classes (> 3, > 12, > 30 µm) was significantly higher in water from depths of 6 and 10 m than from 2 m. In samples from three small lakes, mixing with nutrient-rich hypolimnetic water favored 32PO4 uptake by algal size classes

Influence of metazoan zooplankton on the microbial community before and after the onset of the spring clear-water phase in Lake Constance (Bodensee)

Shortly before and shortly after the onset of the spring clear-water phase, we took plankton samples from the epilimnion of Lake Constance and incubated them in the laboratory under in situ conditions. In three duplicated treatments, mesozooplankton gt 100 mu-m were removed, left at natural densities or enriched nine-fold. Before the clear-water phase, cyclopoid copepods dominated the mesozooplankton, bacteria probably were controlled by high heterotrophic flagellate densities, and the most important phytoplankton grazers apparently were in the fraction lt 100 mu (ciliates, rotifers). In the copepod enrichments, these small grazers were decimated and chlorophyll a and autotrophic picoplankton reached the highest concentrations. Heterotrophic flagellates did not respond clearly to the different treatments. After the onset of the clear-water phase, Daphnia dominated the mesozooplankton and controlled most components of the microbial food web as well as the phytoplankton. All biomass parameters of the phytoplankton, autotrophic picoplankton, heterotrophic flagellates, and ciliates increased when Daphnia were removed. Bacterial abundances did not respond clearly to the removal of mesozooplankton, because protozoans became important bacterial grazers when they were released from the control by Daphni

The potential importance of grazing-resistant bacteria in planktonic systems

The paradigm of the 'microbial loop' has became increasingly important for understanding the structure and function of aquatic ecosystems. Most of the microbial loop studies have focused on energy flow and nutrient cycling. Much less is known, however, about the importance of grazing as a force shaping the structure and community composition of planktonic bacteria. Theoretical considerations of predator-prey interactions suggest that predator evasion mechanisms should have evolved for bacteria in the same way as in other predator-prey systems (e.g. zooplankton-phytoplankton). Consistent with this hypothesis, field data show that bacteria are often the most stable component of planktonic communities. Refuges from grazing are one of the possible mechanisms buffering bacterioplankton against strong seasonal fluctuations in abundance. Substantial direct and indirect evidence exists for the occurrence of grazing-resistant bacteria (GRB) in both marine and freshwater habitats. We summarize the potential mechanisms for grazing resistance, including morphological, chemical and behavioral defenses as well as growth in spatial refuges. Cell size appears to be an important factor influencing susceptibility to grazing, with a refuge at the lower and upper ends of the bacterial size range. Thus, a relative grazing resistance can be assumed for the large number of ultramicrobacteria as well as for morphologically complex growth forms such as filaments and aggregates. Besides morphological features, resistance may be achieved by other mechanisms for which, however, much less information is available. We describe how GRB can be included in conceptual models of the interactions among metazooplankton, bacterivorous protozoans and bacteria. It is suggested that the relative importance of GRB increases with increasing grazing pressure exerted by protozoans, whereas it decreases with increasing top-down control of protozoans by metazooplankton. GRB may reduce the productivity of planktonic systems through decreased trophic transfer efficiencies and reduced regeneration of bacterially bound nutrients