Cristiana Callieri

Freshwater picocyanobacteria along a trophic gradient and light quality range

Picocyanobacterial abundance and their contribution to the total phytoplankton biomass, estimated as chlorophyll a, was investigated in 32 deep and shallow lakes. The lake series covered a wide range of natural and artificial lakes, from high altitude clear lakes and deep, large subalpine lakes through large shallow lakes, small reservoirs and fish ponds. The chlorophyll a concentrations ranged between 0.2 and 390 µg l-1. No simple relation existed between trophic state and the abundance of picocyanobacterial, mainly represented by Synechococcus spp. Below 10 µg Chl a l-1 the percentage contribution of picocyanobacteria biomass to total phytoplankton biomass exceeded 70% in some cases, whereas above 100 µg Chl a l-1, the highest contribution was only 10%. At low chlorophyll a concentrations phycoerythrin-rich picocyanobacteria (PE) dominated almost exclusively but their contribution never exceeded 10% of the total picocyanobacterial abundance when chlorophyll a concentration was higher than 50 µg l-1. Above this value there was high light attenuation and a shift of the maximum light penetration from the blue-green towards the red portion of the spectrum. In this underwater light climate phycocyanin-rich picocyanobacteria (PC) dominated. In the lakes chosen for their representation of ranges in trophy and light quality, PC cells prevailed over the PE when vertical attenuation coefficient of PAR was higher than 2.25 m-1 and red light penetrated farthest. PE cells were 100% when the vertical attenuation coefficient of PAR was lower than 0.55 m-1 and the green and blue were the most penetrating lights.

Picophytoplankton in freshwater ecosystems: the importance of small-sized phototrophs

Abstract About 40 years have passed since the discovery of picophytoplankton; the present knowledge of the taxonomy physiology and ecology of these tiny photoautotrophic cells offers new perspectives on the importance of the microbial contribution to global biogeochemical cycles and food webs. This review focuses on the relationships among the components of picophytoplankton (picocyanobacteria and the picoplanktic eukaryotes) and biotic and abiotic environmental factors. The dynamics of picophytoplankton in aquatic ecosystems are strictly dependent upon basin size and trophy, temperature, and nutrient and light limitation, but they are also regulated by grazing and viral-induced lysis. The review considers: the pros and cons of the molecular approach to the study of the taxonomy of freshwater Synechococcus spp.; the importance of ecological aspects in understanding the puzzle of picophytoplankton phylogeny (genotype vs ecotype); and the role of biotic vs abiotic interactions in controlling picophytoplankton dynamics. Biotic, top-down control mechanisms are reviewed as well as knowledge of other biological interactions.

Picoplankton and Other Non-Bloom-Forming Cyanobacteria in Lakes

Studies on the picocyanobacteria, the major non-bloom forming group in lakes, have increased markedly in the past two decades.These cyanobacteria fall into two major groups, those that predominantly occur as solitary cells (here termed Pcy) and those that occur primarily as colonies (CPcy). The single-celled picoplankton have received most ecological study, though the colonial forms are better known taxonomically.The two groups are distributed worldwide and are ubiquitous in all types of lakes of varying trophic condition. The single-celled Pcy populations tend to be predominant in large, deep oligo-mesotrophic lakes, while the colonial species (CPcy) find optimal conditions in warmer, shallower and more nutrient rich (meso-eutrophic) lakes during the summer.The two groups are euryphotic and appear capable of adapting to a wide variety of light conditions, with CPcy more surface oriented and Pcy often reaching sub-surface peaks at irradiance levels 20–50% of surface values. Growth of the Pcy ranges from 0.1 d-1 to about 3.0 d-1 with doubling times from 7 h to 7 d.Their natural population growth rates are highly variable and appear in most cases to be in balance with loss rates, primarily from grazing. Pcy contribute substantially to total primary production within the euphotic zone, most significantly in ultra-oligotrophic lakes, but also in some meso- and eutrophic lakes, showing optimal growth at high N:P ratios (>20 molar) and more limitation by nitrogen than phosphorus.

Freshwater Picocyanobacteria: Single Cells, Microcolonies and Colonial Forms

This chapter deals with some taxonomic and ecological aspects of picocyanobacteria (Pcy) single-cells, microcolonies and other colonial (CPcy), that are common in lakes throughout the world, and abundant across a wide spectrum of trophic conditions. We discussed phenotypic diversity of Pcy in conjunction with a genotypic approach in order to resolve whether a similar morphology also reflects a phylogenetic relationship. Microcolonies of different size (from 5 to 50 cells) constitute a gradient without a net separation from single-celled types and should be considered Pcy, as transition forms from single-cell to colonial morphotypes. The single-celled Pcy populations tend to be predominant in large, deep oligo-mesotrophic lakes, while the CPcy find optimal conditions in warmer, shallower and more nutrient rich lakes. The knowledge of Pcy diversity in pelagic and littoral zone habitats is a key to understand the dominance of certain genotypes in the water column and of their ubiquity. We devoted some paragraphs to analyse the factors (biotic and abiotic) which can influence the dynamics of the different Pcy forms and we have approached the study of their common ecology.

Lake level fluctuations boost toxic cyanobacterial "oligotrophic blooms".

Global warming has been shown to strongly influence inland water systems, producing noticeable increases in water temperatures. Rising temperatures, especially when combined with widespread nutrient pollution, directly favour the growth of toxic cyanobacteria. Climate changes have also altered natural water level fluctuations increasing the probability of extreme events as dry periods followed by heavy rains. The massive appearance of Dolichospermum lemmermannii ( = planktonic Anabaena), a toxic species absent from the pelagic zone of the subalpine oligotrophic Lake Maggiore before 2005, could be a consequence of the unusual fluctuations of lake level in recent years. We hypothesized that these fluctuations may favour the cyanobacterium as result of nutrient pulses from the biofilms formed in the littoral zone when the lake level is high. To help verify this, we exposed artificial substrates in the lake, and evaluated their nutrient enrichment and release after desiccation, together with measurements of fluctuations in lake level, precipitation and D.lemmermannii population. The highest percentage of P release and the lowest C∶P molar ratio of released nutrients coincided with the summer appearance of the D.lemmermannii bloom. The P pulse indicates that fluctuations in level counteract nutrient limitation in this lake and it is suggested that this may apply more widely to other oligotrophic lakes. In view of the predicted increase in water level fluctuations due to climate change, it is important to try to minimize such fluctuations in order to mitigate the occurrence of cyanobacterial blooms.

Bacteria, archaea, and crenarchaeota in the epilimnion and hypolimnion of a deep holo-oligomictic lake.

ABSTRACT In a deep, subalpine holo-oligomictic lake, the relative abundance of Archaea and Crenarchaeota, but not that of Bacteria, increases significantly with depth and varies seasonally. Cell-specific prokaryotic productivity is homogeneous along the water column. The concept of active Archaea observed in the deep ocean can therefore be extended to a deep oxic lake.

Constitutive presence of antibiotic resistance genes within the bacterial community of a large subalpine lake

The fate of antibiotic resistance genes (ARGs) in environmental microbial communities is of primary concern as prodromal of a potential transfer to pathogenic bacteria. Although of diverse origin, the persistence of ARGs in aquatic environments is highly influenced by anthropic activities, allowing potential control actions in well‐studied environments. However, knowledge of abundance and space–time distribution of ARGs in ecosystems is still scarce. Using quantitative real‐time PCR, we investigated the presence and the abundance of twelve ARGs (against tetracyclines, β‐lactams, aminoglycosides, quinolones and sulphonamides) at different sampling sites, depths and seasons, in Lake Maggiore, a large subalpine lake, and in the area of its watershed. We then evaluated the correlation between each ARG and a number of ecological parameters in the water column in the deepest part of the lake. Our results suggest the constitutive presence of at least four ARGs within the bacterial community with a high proportion of bacteria potentially resistant to tetracyclines and sulphonamides. The presence of these ARGs was independent of the total bacterial density and temperature. The dynamics of tet(A) and sulII genes were, however, positively correlated with dissolved oxygen and negatively to chlorophyll a, suggesting that the resistant microbes inhabit specific niches. These observations indicate that the lake is a reservoir of antibiotic resistances, highlighting the need of a deeper understanding of the sources of ARGs and the factors allowing their persistence in waters.

Microcolony Formation by Single-Cell Synechococcus Strains as a Fast Response to UV Radiation

ABSTRACT UV radiation (UVR) has different effects on prokaryotic cells, such as, for instance, filamentation and aggregation in bacteria. Here we studied the effect of UVR on microcolony formation in two freshwater Synechococcus strains of different ribotypes (group B and group I) and phycobiliprotein compositions (phycoerythrin [PE] and phycocyanin [PC]). Each strain was photoacclimated at two light intensities, low light (LL) (10 μmol m−2 s−1) and moderate light (ML) (100 μmol m−2 s−1). The cultures were exposed for 6 days to treatments with UVR or without UVR. PE-rich Synechococcus acclimated to LL had a low carotenoid/chlorophyll a (car/chl) ratio but responded faster to UVR treatment, producing the highest percentages of microcolonies and of cells in microcolonies. Conversely, the same strain acclimated to ML, with a higher car/chl ratio, did not aggregate significantly. These results suggest that microcolony formation by PE-rich Synechococcus is induced by UVR if carotenoid levels are low. PC-rich Synechococcus formed a very low percentage of microcolonies in both acclimations even with low car/chl ratio. The different responses of the two Synechococcus strains to UVR depend on their pigment compositions. On the other hand, this study does not exclude that UVR-induced microcolony formation could also be related to specific ribotypes.

Seasonal dynamics, composition and feeding patterns of ciliate assemblages in oligotrophic lakes covering a wide pH range

Seasonal changes in the structure of ciliate assemblages in eleven oligotrophic mountain lakes at different altitudes, covering a wide pH range from 4.9 to 6.9, were studied. Seven mountain lakes lay above the timberline (0vre NeÅdalsvatn and Stavsvatn, Norway; Lochnagar, Scotland; Starolesnianske pleso and Nizne Terianske pleso, Slovak Republic; Chuna ozero, Russia; Lago Paione Superiore, Italy) and four acidified lakes (Cerne, Certovo, Plesne and Prasilske jezero, Sumava Mountains, Czech Republic) in the mountain forest. Additionally, thr tropical high latitude Lago de Alchichica (Mexico) was analysed. Ciliate taxons were identified using the quantitative protargol staining approach, and feeding patterns were tentatively detected by using fluorescence microscopy methods. Nano- to microphytoplankton hunters and/ or mixotrophic ciliates prevailed in all acidic lakes. Numbers of ciliates were very low (seasonal lake mean below 200 cells/I) except in two Tatra lakes (to 21000 cells/I). Prostomes of the genera Urotricha, Holophrya and Prorodon dominated in most of the samples, both numerically as well as in biomass (seasonal lake mean from 13.6 to 100% and 4.6 to 99.7%, respectively), particularly in acidified water lakes (Starolesnianske, Certovo and Prasilske jezero). Among mixotrophs, oligotrichs of genera Pelagostrombidium and Limnostrombidium were the most prominent (up to 49.9 and 64.6%, respectively). The picoplankton-feeders (minute oligotrichs, peritrichs and scuticociliates) were found to be an important component even in the oligotrophic environment but this ecological type never dominated within the water column (up to 22.2 and 30.3 %, respectively). Major food sources of gymnostomes, prevailing only in Lochnagar (82.5 and 87.5 %, respectively; Mesodinium sp.), remained unclear; large Askenasia spp. were apparently mixotrophic. Using a cluster analysis, the lakes were grouped according to the total numbers of ciliates and contributions of distinct ecological groups with different feeding patterns. Within low acidic lakes, Lochnagar differed from all others. The rest of the lakes was divided into two subgroups: remote pristine lakes of the Northern transect (the Norwegian lakes and Chuna ozero), and the others, geographically affiliated to the south transect (Pyreneans, Alps, Tatra Mts.). Such a differentiation is in good agreement with the clustering based on chemical parameters reported for the lakes. No marked differences in the ciliate distribution were detected when the above timberline- and forest surrounded mountain lakes were compared.

Grazing-induced Synechococcus microcolony formation: experimental insights from two freshwater phylotypes

Freshwater cyanobacteria of the genus Synechococcus are ubiquitous and organized either as single cells of diverse morphology or as microcolonies of different size. We studied the formation of microcolonies induced by the mixotrophic nanoflagellate Poterioochromonas sp. grazing on two Synechococcus strains belonging to phylotypes with different content of phycobiliproteins (PE: phycoerythrin-rich cells, L.Albano Group A; PC: phycocyanin-rich cells, MW101C3 Group I). The quantitative variations in cell abundance, morphological and physiological conditions were assessed on short-term incubations in semi-continuous cultures, single culture (PE, PC) and co-culture (PE+PC), with and without predators, by flow cytometry, and PhytoPAM. Under grazing pressure, we observed that (i) the abundance of PE single cells decreased over time with a concomitant formation of PE microcolonies; (ii) in PC single cultures, no significant variation in single cells was found and microcolonies did not form; (iii) both PE and PC formed monoclonal microcolonies in co-culture; (iv) PC cells increased the photosynthetic efficiency of the PSII (higher Fv/Fm) in co-culture. In the aftermath of microcolony formation as a predation-induced adaptation, our findings indicated a different response of Synechococcus phylotypes potentially co-existing in natural environment and the importance of their interaction.