Microbial glucose uptake and growth along a horizontal nutrient gradient in the North Pacific

The quantitative role of carbon-rich compounds for bacterial carbon demand can vary with nutrient availability. This study is the first to investigate the effect of nutrient concentrations on bacterial uptake of a carbon-rich compound (glucose) along a naturally occurring nutrient gradient. Seawater was collected from varying depths at six stations across the nutrient gradient between the nutrient-poor North Pacific gyre and the nutrient-rich sub-Arctic gyre. Ambient concentrations of free glucose, glucose uptake, bacterial production, and nutrient concentrations were determined across the gradient. Incubations were carried out to determine the effect of nutrient additions on glucose uptake and bacterial production. Samples from the northern, nutrient-rich end of the transect had high glucose uptake rates and high bacterial production in surface waters. The northern stations also showed rapid turnover of the glucose pool. At the southern stations, glucose uptake rates, bacterial growth rates, and fractional glucose turnover were lower. At these stations, glucose uptake increased when inorganic nutrients and amino acids were added. The variations in glucose uptake indicate that microbial uptake of nitrogen- and phosphorus-poor organic material depends on the availability of nitrogen and phosphorus from other sources.