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Comunicación oral Bacterial biomass production is a key parameter for evaluating the role of bacterioplankton in ocean carbon cycling. However, bacterial production cannot be directly measured and is typically estimated from the incorporation rates of radiolabelled leucine. The conversion of leucine uptake rates into bacterial carbon production rates requires the use of conversion factors (CFs) which must be empirically determined. Despite the empirical leucine-to-carbon CFs vary widely across environments very little is known about its potential controlling factors. We conducted a set of 10 surface seawater cultures experiments where the growth of the natural bacterial assemblage was promoted by filtration (removal of grazers) or by both filtration and dilution. Sampling stations were located between 30 ºN and 30 ºS, including the Atlantic, Pacific and Indian oceans. CFs varied from 0.13 to 1.47 Kg C mol Leu-1, being higher in the filtrated than in the filtrated and diluted treatment. The abundance of picocyanobacteria explained 60% of the observed variability. Our results further suggest that the composition of bacterioplankton, as assessed by ARISA fingerprinting, may partially explain the observed variation in CFs.

The dark ocean is one of the largest biomes on Earth, with critical roles in organic matter remineralization and global carbon sequestration. Despite its recognized importance, little is known about some key microbial players, such as the community of heterotrophic protists (HP), which are likely the main consumers of prokaryotic biomass. To investigate this microbial component at a global scale, we determined their global abundance and biomass in meso- and bathypelagic waters in samples from the Malaspina-2010 circumnavigation using a combination of epifluorescence microscopy and flow cytometry. HP were clearly ubiquitous in the global deep ocean, even at the deepest 4000 m samples investigated. Their abundances decreased with depth, from an average of 72 ± 19 cells mL-1 in mesopelagic waters to 11 ± 1 cells mL-1 in bathypelagic waters, whereas their global biomass decreased from 280 ± 46 to 50 ± 14 pg C mL-1. The parameters that better explained the variance of HP abundance were depth and prokaryote abundance, and to lesser extent oxygen and the abundance of large viruses. Different signs suggested active grazing of HP on prokaryotes, such as the presence of flagella in most cells, and the generally good correlation with prokaryote abundance. On a finer scale, the prokaryote:HP ratio in abundance varied at a regional scale, and sites with the highest ratios appear related to a larger contribution of osmotrophy to flagellate nutrition. Our study is a step forward towards determining the relationship between HP and their environment, unveiling their importance as players in the dark ocean's microbial food web.

The application of molecular methods to marine ecology in the last decades has completely changed our view of the patterns of diversity and distribution of microorganisms in the ocean (Giovannoni et al. 1990, Zinger et al. 2012). However, these methods are expensive and time-consuming when applied on a large number of samples. Flow-cytometry, on the other hand, allows an efficient and rapid processing of a large number of samples. In this sense, the use of single-cell measurements by flow-cytometry for diversity purposes would be a great advance. In marine ecosystems, this concept has been introduced by Li 1997 as `cytometric diversity'. OBJECTIVES: In this study we evaluated the power of cytometric diversity to detect changes in the composition of bacterioplankton communities: Cant 1) By comparing changes in bacterial composition of 3.5 years surface samples obtained by cytometric diversity and molecular approaches. 2) Analysing the cytometric diversity patterns of a set of 10-years monthly bacterioplankton flow-cytometry samples for 3 coastal stations.