Biocénose pélagique du lac oligomésotrophe de Poméranie (Pologne) : étude des communautés de la boucle mocrobienne et du pytoplancton

Cette étude présente des résultats concernant l'abondance et la biomasse des principales communautés de la boucle microbienne (bactéries hétérotrophes, picoplancton autotrophe, flagellés hétérotrophes, protistes ciliés) et des nano- et microalgues dans le grand (525 ha) et profond (48 m) lac oligomésotrophe Bobifcigskie Wielkie situé en Poméranie de l'ouest (Pologne). Les échantillons d'eau ont été prélevés en zone pélagique, en 1996. Dans la classe de taille du picoplancton, ce sont les bactéries hétérotrophes qui constituent le groupe trophique le plus abondant. Leur biomasse couplée à celle du picoplancton autotrophe beaucoup moins abondant (de 3,5 à 5,4 µg·C·L-1), constituait de 15 % à 22 % de la biomasse microbienne totale. La biomasse des protistes flagellés hétérotrophes et celle des protistes ciliés étaient proches (3,0-3,8 µg·C·L-1) et constituaient, en fonction de la profondeur de prélèvement, de 8 % à 24 % de la biomasse totale. Le phytoplancton de taille supérieure à 2,0 µm représentait la plus grande partie de la biomasse totale, à savoir de 50 % à 78 % (8,4-27,5 µg·C·L-1). Il était dominé, en ce qui concerne la taille des organismes, par le microplancton. Dans les couches supérieures du lac, les diatomées et les cryptophycées étaient les plus nombreuses, alors qu'en profondeur c'était les cyanobactéries filamenteuses du genre Planktothrix qui étaient les plus abondantes.This study presents results of investigations on abundance and biomass of the main communities of the microbial loop (heterotrophic bacteria, autotrophic picoplankton, heterotrophic flagellates, ciliated protozoa) as well as nano- and microalgae in the large (525 ha) and deep (48 m) oligo-mesotrophic lake Bobifcigskie Wielkie located in West Pomerania, Poland. Samples were taken from the pelagial zone in 1996. Within organisms of picoplankton size the heterotrophic bacteria were the most abundant trophic group. Their biomass, together with the much lower abundant autotrophic picoplankton (3.5 to 5.4 µg·C L-1), constituted 15% to 22% of the total microbial biomass. The sum of biomasses of heterotrophic flagellates and ciliated protozoa was similar at all depth (3.0-3.8 µg·C·L-1), ranging between 8-24% of total biomass. Phytoplankton organisms larger than 2.0 µm represented the biggest part of the total microbial biomass, i.e. from 50% to 78% (8.4-27.5 µg·C·L-1). The microplankton constitutes the largest part of the phytoplanktonic biomass. Whereas the diatoms and cryptomonads were the most abundant in the upper layers of the lake. The filamentous cyanobacteria of the genus Planktothrix dominated in the deeper waters

Distribution du picoplancton autotrophe dans la zone pélagique d'un lac méromictique (Lac Czane, Pologne)

La composition, l'abondance et la biomasse de la communauté du picoplancton autotrophe (PPA, 0,2-2 m) ont été examinées dans un lac situé en Pologne et récemment considéré comme méromictique, le lac Czarne. Les échantillons d'eau ont été prélevés tous les mètres dans la colonne d'eau, en mars et en juillet 1998. Pendant ces deux dates, le PPA se caractérisait par des changements significatifs de densité dans la colonne d'eau. Au printemps, l'abondance la plus forte a été observée à 9 m (2,1·105 cellules·mL-1) tandis qu'en été elle est observée à 5 m (3,1·105 cellules·mL-1). À toutes les profondeurs, le PPA était dominé par des Cyanobactéries. Au printemps, dominent les cellules libres de picocyanobactéries, contrairement à l'été où les cellules coloniales étaient prépondérantes. La contribution des cellules eucaryotes était faible en termes d'abondance (10 µg C·L-1 were recorded to the depth of 14 m, while during the second session biomass clearly concentrated in the upper part of the water column, to a depth of 7 m. Within PPA, usually Pcy dominated in terms of biomass. In March their contribution to the total PPA biomass varied from 35% to 88% (mean: 67%) and in July from 65% to 100% (mean: 88%). PEA biomass was higher in March than in July and mean values were respectively 3.0 µg C·L-1 and 0.3 µg C·L-1, accounting for 33% and 12% of the total PPA biomass. In July, PAE biomass was everywhere lower than Pcy biomass. In contrast, in March, PAE biomass exceeded Pcy biomass at some depths in the lower part of the water column. The contribution of PAE to the total PPA biomass was higher than to the total PPA abundance, especially in March, when PAE were the most numerous. Generally, the contribution of eukaryotic cells was smaller in terms of abundance and much higher in terms of biomass, because of the larger mean size of cells

Dynamics of autotrophic picoplankton communities in the epilimnion of a eutrophic lake (Strzeszyńskie Lake, Poland)

The results of 32 seasonal sampling sessions in 2000 were used to assess the dynamics of autotrophic picoplankton (APP), with special reference to the cyanobacterial component in Strzeszynskie Lake, a eutrophic lake in mid-western Poland. The abundance of picocyanobacteria (Pcy) fluctuated from 1.2x104 to 8.1x105cells mL-1 and the average abundance and biomass were 2.0x105cells mL-1 and 0.094 mg L-1, respectively. Pcy development was characterised by maxima in spring and summer. The summer maximum was twice the spring Pcy numbers. Eukaryotic APP (E-APP) were also present but in low numbers and their annual average contribution to the total APP community was only about 0.3% in abundance and about 3% in biomass. The spring Pcy community was dominated by single-celled cyanobacteria (S-Pcy) and in summer by colonial forms (C-Pcy). Throughout the sampling period, C-Pcy constituted on average 55% of the total Pcy and up to 90% in summer. C-Pcy abundance was significantly and positively correlated with water temperature (r=0.40, p=0.023, n=32). S-Pcy exhibited a distinct seasonal variation in FDC, ranging from 10% to 35%. Most FDC values above 30% were recorded from late May to mid July

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

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