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 (N 2)-fixing cyanobacterium Trichodesmium. Microsensor measurements indicated that cells within colonies experienced large fluctuations in O₂, pH and CO₂ concentrations 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 N 2 fixation 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.