Utilization of bactericidal agents and light-dependent incubation to partition phytoplankton and bacteria nitrogen uptakes

Nitrogen uptake experiments provide important information on the nutrient cycling in the marine environment and on the growth of planktonic communities. Plankton assimilates nitrogen mainly for structural purposes and thus its uptake basically estimates the population growth, contrary to carbon and phosphorus that are also turned over in energetic cellular processes. Moreover, nitrogen uptake experiments allow the distinction between new (based on newly available nitrogen from outside the euphotic zone) and regenerated (based on recycled nitrogen inside the euphotic zone) primary production. This distinction has important implications for studies concerning the ocean carbon cycle and, in consequence, the carbon dioxide exchange with the atmosphere. The ratio of new to total production, integrated over a long time-scale, represents an estimate of the exportable production from the euphotic zone toward deeper waters. However, the analysis of nitrogen uptake data is complicated by the fact that they are not representative only of phytoplanctonic primary production as also bacteria assimilate several important nitrogen forms (i.e. nitrate, ammonium and urea). Bacteria production should not be accounted as exportable production since it basically remains in the euphotic zone being processed through the microbial loop. The partitioning of the total nitrogen uptake between phytoplankton and bacteria is usually obtained by size-fractionation experiments, but the serial filtration technique has several disadvantages. Size fractions do not exactly correspond to functional plankton classes, filters can retain bacteria because of their tightly association to large particles and high concentrations of suspended particulate matter alter filter porosity. The utilization of bacterial or algal inhibitors is another potentially powerful approach to the partitioning of the total nitrogen uptake that has not been largely used to date. In this work, we present preliminary results of two nitrogen uptake experiments carried out in August and October 2003 in the Gulf of Trieste (Northern Adriatic Sea, ITALY) in conditions of high and low primary production, respectively. The total (nitrate + ammonium + urea) nitrogen uptake was partitioned among some functional plankton classes with a combined utilisation of a treatment with bactericidal agents and light-dependent incubations. In the former experiment, phytoplankton, heterotrophic bacteria and photosynthetic bacteria contributed to the total nitrogen uptake (104 nmol-N/dm3 h) by 71, 7 and 22%, respectively. During the second experiment, they contributed to the total nitrogen uptake (24 nmol-N/dm3 h) by 94, 1 and 5%.