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

Gradients of oxygen (O) and pH, as well as small-scale fluxes of carbon (C), nitrogen (N) and O were investigated under different partial pressures of carbon dioxide (pCO) in field-collected colonies of the marine dinitrogen (N)-fixing cyanobacterium Trichodesmium. Microsensor measurements indicated that cells within colonies experienced large fluctuations in O, pH and CO concentrations over a day-night cycle. O concentrations varied with light intensity and time of day, yet colonies exposed to light were supersaturated with O (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 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 O fluxes nor C fixation by Trichodesmium were significantly influenced by short-term incubations under different pCO levels, whereas N fixation increased with increasing pCO. 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.