Trophic interactions in the microbial food web in a coastal upwelling system off central Chile (∼36°C)

Coastal upwelling areas are highly productive systems and were initially characterized by having a short food chain, being ecologically efficient in the trophic transfer. Large micro-phytoplankton, predominating under high availability of nutrients in the mixed layer, are grazed by large herbivorous zooplankton, and they, in turn, are consumed by planktivorous fishes. Under this scheme, little attention was paid to the role of micro-organisms in these areas. This thesis provides an assessment of the temporal variability in the structure of micro-organism assemblages and of the trophic interactions in microbial food webs in the Humboldt Current System (HCS) off Concepcion, central Chile, as a basis to understand the relevance of the carbon flow through the microbial food web in this coastal upwelling area

Chemical microenvironments and single-cell carbon and nitrogen uptake in field-collected colonies of Trichodesmium under different pCO2

Gradients of oxygen (O2) and pH, as well as small-scale fluxes of carbon (C), nitrogen (N) and O2 were investigated under different partial pressures of carbon dioxide (pCO2) in field-collected colonies of the marine dinitrogen (N2)-fixing cyanobacterium Trichodesmium. Microsensor measurements indicated that cells within colonies experienced large fluctuations in O2, pH and CO2concentrations over a day–night cycle. O2 concentrations varied with light intensity and time of day, yet colonies exposed to light were supersaturated with O2 (up to ~200%) throughout the light period and anoxia was not detected. Alternating between light and dark conditions caused a variation in pH levels by on average 0.5 units (equivalent to 15 nmol l−1 proton concentration). Single-cell analyses of C and N assimilation using secondary ion mass spectrometry (SIMS; large geometry SIMS and nanoscale SIMS) revealed high variability in metabolic activity of single cells and trichomes of Trichodesmium, and indicated transfer of C and N to colony-associated non-photosynthetic bacteria. Neither O2 fluxes nor C fixation by Trichodesmium were significantly influenced by short-term incubations under different pCO2 levels, whereas N2fixation increased with increasing pCO2. The large range of metabolic rates observed at the single-cell level may reflect a response by colony-forming microbial populations to highly variable microenvironments