Jacquet S, Havskum H, Thingstad TF, Vaulot D
carbon, Competition, Flow cytometry, Food web, mesocosms, picoplankton, Nutrients, virus, Isefjord, Denmark, 3 m3
Using flow cytometry (FCM), microbial populations were followed in a mesoscosmexperiment manipulated with daily additions of mineral nutrients (as phosphates and nitrates in Redfieldequilibrium), of degradable organic carbon (as glucose-C), or with the 2 treatments combined.Intensive sampling was performed in order to assess the short time-scale variability of the microbialcommunity. Five autotrophic groups (including Synechococcus spp. and cryptophytes), 2 groups ofheterotrophic bacteria, and 2 groups of viruses could be discriminated by FCM. The control enclosure(no addition) revealed that heterotrophic bacteria were carbon-limited. Synechococcus spp. abundanceincreased in the control, presumably because they experienced little competition from heterotrophicbacteria (which were C-limited) and from larger phytoplankton (which were not as efficientin nutrient uptake at low nutrient concentration and could not, therefore, sustain high growth rates).When N and P were added, however, larger-celled autotrophic populations were favoured. Whenglucose was added, alone or together with inorganic elements, the abundance of Synechococcus spp.and small eukaryotes was reduced, suggesting that, when released from C-limitation, heterotrophicbacteria are the best competitors for mineral nutrients. The addition of both inorganic and organicnutrients also enhanced cryptophytes in contrast to all other autotrophs, probably because of theirheterotrophic capacity. Our results agree with recent evidence suggesting that heterotrophic bacteriaare limited by both carbon and mineral nutrients, and demonstrate how this has important consequencesfor the success of their trophic neighbours in the microbial food web.