A new automated flow cytometer for high frequency in situ characterisation of heterotrophic microorganisms and their dynamics in aquatic ecosystems

Heterotrophic microorganisms represent the main consumers and mineralisers of the organic matter in the ocean as well as in inland waters. These organisms are in strong interaction with phytoplankton and higher trophic levels and may react quickly to environmental changes. In contrast with phytoplankton, due to the lack of photosynthetic pigments they do not naturally emit fluorescence, which makes more complex their analysis by flow cytometry, and consequently, their dynamics at short time and spatial scales are poorly documented. To fill this gap and extend high frequency observation capacity of flow cytometry (FC) to heterotrophic microorganisms, an automated staining module was designed and now equips a Cytosense flow cytometer to perform in situ automated sampling and analysis. Here we describe the first results got with this device and demonstrate the capacity of this new FC instrument to analyse heterotrophic prokaryotes dynamics at high frequency. In a successful experiment, this prototype yielded an extensive data set generated by automated analyses run every 2 hours over 7 days in the frame of a mesocosm investigation addressing natural marine communities.