Temporal and vertical dynamics in picoplankton photoheterotrophic production in the subtropical North Pacific Ocean

Heterotrophic microbial production is a fundamental determinant in the flow of bioelements and energy within the pelagic ecosystems of the open ocean. To characterize the temporal dynamics in rates of heterotrophic picoplankton production (HPP), we examined vertical profiles of 3 H-leucine ( 3 H-leu) and [methyl-3 H]-thymidine ( 3 H-TdR) incorporation at Stn ALOHA (2245' N, 158W) in the oligotrophic North Pacific Ocean. Euphotic zone rates of 3 H-leu and 3 H-TdR incorporation were measured in situ under light and dark conditions on cruises to Stn ALOHA between April 2000 and August 2005. Rates of 3 H-leu and 3 H-TdR incorporation were elevated in the well-lit upper euphotic zone (< 75 m) and declined with increasing depth. In the mid-euphotic zone (75 to 125 m), where average PAR fluxes declined to 0.6-5% of the surface irradiance, rates of 3 H-leu were significantly greater when samples were incubated at in situ light levels ( 3 H-leu light ) than paired in situ incubations kept in the dark ( 3 H-leu dark ) (1-way ANOVA, p < 0.05). Average rates of photostimulated 3 H-leu (Leu = 3 H-leu light -3 H-leu dark ) in the upper 125 m varied between 4.2 and 11 pmol l -1 h -1 , with Leu approximately equal in magnitude to rates of 3 H-leu dark between 75 and 125 m. In contrast, rates of 3 H-TdR did not display a similar sunlight enhancement, with rates of 3 H-TdR light and 3 H-TdR dark not significantly different (1-way ANOVA, p > 0.05). Both 3 H-leu and 3 H-TdR displayed similar temporal variability, but neither proxy for HPP was correlated to measured rates of primary production. These observations provide the first examination of the temporal dynamics in HPP at Stn ALOHA, and lend insight into the potential importance of photoheterotrophic growth by Prochlorococcus spp. Although organic matter utilization by Prochlorococcus spp. has been documented previously, this is the first study to evaluate their potential role in secondary production of the oceanic ecosystem.