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Bacterioplankton and bacterioperiphyton composition was analyzed using the CARD-FISH technique in three shallow lakes of the Pampean Plain (Argentina) with contrasting regimes: clear vegetated, turbid due to phytoplankton and turbid inorganic, due to inorganic particles. We postulated that these differences would influence the proportion of the main bacterial groups both in periphyton and in plankton. The turbid lake due to phytoplankton presented the highest total abundances in both communities. Alphaproteobacteria was the dominant group in the three lakes in both communities. Redundancy analysis (RDA) evidenced that the bacterioplankton structure was different among lakes and mainly influenced by dissolved inorganic nitrogen and conductivity. On the other hand, for the bacterioperiphyton, RDA showed that bacterial group abundances increased with higher periphytic chlorophyll-a values. In the clear lake the relative abundance of Betaproteobacteria and Cytophaga increased in the bacterioperiphyton towards the end of the colonization. Our study suggests that the lake regime (clear or turbid) influence the structure of bacterioplankton and bacterioperiphyton. Se analizó la composición de bacterioplancton y bacterioperifiton utilizando la técnica de CARD-FISH en tres lagunas de la llanura pampeana (Argentina) con regímenes contrastantes: laguna clara vegetada, laguna turbia por fitoplancton y laguna turbia inorgánica. Se postula que estas diferencias podrían influir sobre la proporción de los principales grupos bacterianos, tanto en el bacterioperifiton como en el bacterioplancton. La laguna turbia por fitoplancton presentó la mayor abundancia total de bacterias en ambas comunidades. El grupo de Alfaproteobacteria fue dominante en ambas comunidades y en las tres lagunas. El análisis de redundancia (RDA) evidenció que la estructura del bacterioplancton fue diferente entre las lagunas y estuvo principalmente influenciado por el nitrógeno inorgánico disuelto y la conductividad. Por otra parte, para el bacterioperifiton el RDA mostró que la abundancia de los grupos bacterianos se incrementó a mayores valores de clorofila-a perifítica. En la laguna clara las abundancias relativas de los grupos Betaproteobacteria y Citofaga aumentaron hacia el final del período de estudio. Este estudio sugiere que el régimen (claro o turbio) de cada cuerpo de agua influye sobre la estructura del bacterioplancton y bacterioperifiton.

19 pages, 5 figures, 3 tables, 1. We analysed the latitudinal variation of bacterioplankton in 45 freshwater environments (lakes, shallow lakes and ponds) across a transect of more than 2100 km stretching from Argentinean Patagonia (45°S) to Maritime Antarctica (63°S), to determine the factors that mainly determine bacterioplankton community structure. 2. Bacterioplankton community composition (BCC) was assessed by a fingerprinting method (denaturing gradient gel electrophoresis) followed by band sequencing, whereas the abundances of total bacteria and picocyanobacteria were estimated by epifluorescence microscopy. 3. Bacterioplankton community composition was controlled by a combination of spatial (latitude and longitude) and environmental [e.g. phosphate, light diffuse attenuation coefficient (Kd) and dissolved organic carbon] factors. Total bacterioplankton abundance declined with latitude. A multiple regression analysis showed that phosphate, Kd and latitude had significant effects on total bacterioplankton abundance. 4. Of 76 operational taxonomic units identified in the studied lakes, 45 were shared between Patagonian and Antarctic water bodies, 28 were present only in Patagonian lakes and three were restricted to the Antarctic lakes. Significant differences were found in BCC between Patagonia and Antarctica. Among the sequences, 54% were similar (>97% sequence similarity) to others reported from cold habitats elsewhere on the planet (glaciers, high mountain lakes, Arctic). 5. Our results provide new evidence that supports the hypotheses of biogeographic patterns of bacterial assemblages and suggest that both spatial and environmental factors control bacterioplankton community structure, The Antarctic expeditions were supported by the ‘Instituto Anta´rtico Argentino (DNA)’ in the framework of a cooperative project between this institution, UBA and ICM-CSIC. This work was financed by the Argentinean Funds for Technical and Scientific Investigation (FONCYT, PICT 32732), the Spanish Project MIXANTAR (REN 2002-11396-E ⁄ANT) and the CSICCONICET joint project PROBA (2007AR0018, CSIC)