Bacterial utilization of glucose in the water column from eutrophic to oligotrophic pelagic areas in the eastern North Atlantic Ocean

Vertical profiles of glucose utilization rates were compared at three sampling stations in the eastern part of the tropical North Atlantic Ocean. The investigation area was along 20–21 °N and the three sampling sites, characterised by differences in their primary productivity, were located at 18 °W, 21 °W and 31 °W. In the superficial waters, maximum (Vmax) glucose utilization (respiration plus incorporation) depended on the nutritional load being 20-fold higher in the eutrophic, compared to the oligotrophic zone. Due to these variations, natural turnover times for this labile compound were approximately 1 day in the eutrophic area, and up to 435 days in the oligotrophic area. Bacterial activity showed a steep decline immediately below the mixed layer in the mesotrophic and eutrophic areas and below the deep chlorophyll maximum in the oligotrophic area. Discrepancies between microbial activities in the three areas decreased with increasing depth: at depths below 250 m potential utilization rates of glucose were similar whatever the nutrient richness of the photic layer. Nevertheless, the distribution of microbial activities through the whole water column depended greatly on the productivity of superficial waters. In nutrient-rich areas 73% of glucose utilization activity was realized in the productive upper layer, whereas only 4% was metabolized at depths below 250 m. Conversely, in the oligotrophic area, more than 40% of the glucose utilized in the whole water column was processed in the intermediate and deep-water masses. Integration of Vmax values for the whole water column, suggested potential carbon fluxes due to bacterial utilization of glucose of 6 and 34 mg C m−2 d−1 in the oligotrophic and eutrophic areas, respectively. The fate of the metabolised carbon depended on the nutrient availability. In the mixed-water layer the glucose respiration percentage (%R) increased from 30% in nutrient-rich areas to 60% under oligotrophic conditions, moreover %R increased with depth. This infers that at lower nutritional loads, a greater proportion of highly labile compounds is used for energetic purposes, and therefore return to the inorganic carbon pool, but with very low turnover rates.