IHB Significant Results

The influence of protists and viruses on bacterial assemblages in natural water environment

Various laboratory experiments with bacterial isolates have revealed a large amount of findings and observations but considerably less information is available on natural bacterial assemblages. Bacteria play a key role in the remimeralization of organic matter and considerably facilitate the flux of organic matter into the higher trophic levels. In natural aquatic environments, bacteria comprise the most dynamic and fast growing component of the ecosystem. Actively growing bacteria are simultaneously ingested mainly by protests, which are than consumed by larger organisms (large protozoa and zooplankton). During ingestion in every single chain of this food web a substantial part of energy is released back into the environment. In last decade, it has been documented that in natural waters bacteria are commonly influenced by viral infection, very often causing the lysis of the host cell. When studying various factors influencing the abundance, activity and phylogenetic composition of natural bacterial communities (in the cooperation with the French colleagues), an interesting feature has been observed: bacterial assemblage exposed to enhanced grazing pressure became more sensitive to viral infection. The ecological mechanism of this phenomenon is still not completely understood. We assume that during intensive protistan grazing the genetic diversity of bacteria is reduced and the only bacterial phylotypes resistant to grazing survive in the community. Viral lysis is a host specific factor infecting only the specific bacterial strains or groups. Under reduced bacteria phylogenetic diversity, viruses capable of infecting the remaining bacterial strains may easily reproduce. During these conditions, the higher probability of successful infection of the host bacteria occurs.

Fish in reservoirs

The main limitation of fish studies in deep reservoirs so far was the absence of sampling gear active in the open water, not just in the inshore areas. We put a lot of effort into methodological development of open water sampling. Sonar techniques were developed so they sample quantitatively nearly all volumes of the reservoirs. The sampling by Nordic multimesh gillnets of Scandinavian origin was modified for the same purpose. The most significant achievement was the introduction of inland modifications of marine techniques - pelagic trawling (see Fig.) and purse seining. Preliminary results show that the pelagic fish stock of the reservoirs is composed by the same fish species like inshore habitats (there are no obligatory pelagic species in our reservoirs) but the proportions of individual species differ significantly. Also the behaviour of juvenile and adult fish is very different. Adult bleak, bream, asp and white bream and juvenile roach, perch, pikeperch and bream prefer open water habitats. Sampling of the open water facilitated the first complex study of fish feeding in the open water of a Czech reservoir, the first estimates of the importance of different parts of the reservoir for fish and outlining of general features of fry distribution in the reservoirs. Despite significant achievements, the sampling of open water of freshwater systems is a young discipline. The problems persist with avoidance reactions, patchy distribution, migrations and other complications. In future research we concentrate on these and similar problems connected with understanding of fish role in water reservoirs.

A microscopic look on enzymes changes a view on competition of planktonic microbes for phosphorus

Overall availability of phosphorus (P) for organisms' growth controls trophic status of freshwaters. Cells of phytoplankton (algae and cyanobacteria) and bacteria are able to take up only orthophosphate, thus, its depletion in ambient water leads to a limitation of cells' growth. Micro-organisms use different strategies in competition for P. Among others, they release extracellular phosphatases, which hydrolyse organic P compounds by cleaving phosphate. Hitherto, we have been able to estimate only total phosphatase activity of all planktonic organisms and supposed that most phytoplankton species react on the P depletion in a similar way, i.e. they produce extracellular phosphatases. Recently, the novel fluorescent substrate (ELF®97 phosphate) enables a direct microscopic localisation of phosphatases on the algal, cyanobacterial, and bacterial cells. Combining the new method wit PC based image analysis, we are now able to measure cell-specific activity of particular micro-organism species. The method reveals surprising results - the particular phytoplankton species differ remarkably in their production of extracellular phosphatase, i.e. in its employment in their struggle for P. The use of this method allows for more detailed understanding of life strategy of the particular phytoplankton species, their seasonal development, driving forces of phytoplankton blooms, etc.

Acid rain changes phosphorus and nitrogen cycling in lakes

In contrast to the well known nutrient transformations in circum-neutral lakes, acidified water bodies exhibit significant changes in nutrient (P and N) cycles. These changes were evaluated in the strongly acidified Plešné Lake.
(1) Liberated orthophosphate from sedimenting seston is bound from a liquid to a particulate phase by colloidal Al hydroxides in the hypolimnion, and deposited. This abiotic P immobilization with Al reduces by ~20% the pool of potentially bio-available P and contributes to a severe P limitation of phytoplankton.
(2) The cessation of nitrification due to long-term water acidification leads to an atypical situation, where Plešné Lake becomes a net source of ammonium (NH₄⁺). NH₄⁺ entering the lake from terrestrial and atmospheric sources is assimilated by phytoplankton and transformed to organic nitrogen. After NH₄⁺ depletion, nitrate is utilized as an alternative nitrogen source. Organic nitrogen is mineralized and liberated as NH₄⁺ from the dead, sedimenting phytoplankton. Because the dissimilation occurs mostly below the productive layer, the liberated NH₄⁺ is not re-assimilated and accumulates in the hypolimnion. After lake overturns, this NH₄⁺ is exported from the lake. The acidified productive lakes, with dissimilative production of NH₄⁺ exceeding its assimilation, thus, become a net NH₄⁺ source.

Photochemical liberation of organically bound metals by solar radiation

HBI ASCR in co-operation with University of South Bohemia and University of Maine (US) identified a natural source of ionic metal species for surface waters: photochemical liberation of organically bound metals by solar radiation. This process explains high concentrations of aluminium and iron oxyhydroxides in lake sediments long before the onset of atmospheric acidification, since the beginning of soil formation in catchments. Oxyhydroxides of Al and Fe play an important ecological role, affecting ability of lake and marine sediments to bind phosphate and trace metals. We show that transport of Al and Fe oxyhydroxides to sediments occurs via the following sequence of processes: (i) Soil organic acids dissolve and bind metals, and export them from terrestrial to aquatic systems. (ii) Photochemical decomposition of organic-metal complexes liberates a significant portion of organically-bound aluminium and iron as ions. (iii) The liberated ions hydrolyse, precipitate as oxyhydroxide particles, and settle. The photo-liberation of ionic aluminium species represents a potential source of toxicity to fish and zooplankton.