Light dependence of [3H]leucine incorporation in the oligotrophic North Pacific ocean

The influence of irradiance on bacterial incorporation of [H-3] leucine was evaluated at Station ALOHA in the oligotrophic North Pacific subtropical gyre. Six experiments were conducted on three cruises to Station ALOHA to examine how [H-3]leucine incorporation varied as a function of irradiance. Two experiments were also conducted to assess the photoautotrophic response to irradiance (based on photosynthetic uptake of [C-14] bicarbonate) in both the upper and lower photic zones. Rates of [H-3]leucine incorporation responded to irradiance in a photosynthesis-like manner, increasing sharply at low light and then saturating and sometimes declining with increasing light intensity. The influence of irradiance on bacterial growth was evaluated in both the well-lit (5 to 25 m) and dimly lit regions of the upper ocean (75 to 100 m) to determine whether the bacterial response to irradiance differed along the depth-dependent light gradient of the photic zone. [H-3] leucine incorporation rates were analyzed with a photosynthesis-irradiance model for a quantitative description of the relationships between [H-3] leucine incorporation and irradiance. Maximum rates of [H-3] leucine incorporation in the upper photic zone increased 48 to 92% relative to those of dark-incubated samples, with [H-3]leucine incorporation saturating at light intensities between 58 and 363 mumol of quanta m(-2) s(-1). Rates of [H-3]Ieucine incorporation in the deep photic zone were photostimulated 53 to 114% and were susceptible to photoinhibition, with rates declining at light intensities of >100 mumol of quanta m(-2) s(-1). The results of these experiments revealed that sunlight directly influences bacterial growth in this open-ocean ecosystem.