Effects of inorganic and organic nutrient addition on a coastal microbial community (Isefjord, Denmark)

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Abstract
Using flow cytometry (FCM), microbial populations were followed in a mesoscosm experiment manipulated with daily additions of mineral nutrients (as phosphates and nitrates in Red- field equilibrium), 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 microbial community. Five autotrophic groups (including Synechococcus spp. and cryptophytes), 2 groups of heterotrophic 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. abun- dance increased in the control, presumably because they experienced little competition from hetero- trophic bacteria (which were C-limited) and from larger phytoplankton (which were not as efficient in 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. When glucose 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, heterotrophic bacteria are the best competitors for mineral nutrients. The addition of both inorganic and organic nutrients also enhanced cryptophytes in contrast to all other autotrophs, probably because of their heterotrophic capacity. Our results agree with recent evidence suggesting that heterotrophic bac- teria are limited by both carbon and mineral nutrients, and demonstrate how this has important con- sequences for the success of their trophic neighbours in the microbial food web.

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