AQUATIC MICROBIAL ECOLOGY, 45: 41-53

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

Matthew J. Church¹, Hugh W. Ducklow², Ricardo M. Letelier³, David M. Karl¹

¹University of Hawaii, Department of Oceanography, 1000 Pope Road, Honolulu, Hawaii 96822, USA

²School of Marine Science, College of William and Mary, P0 Box 1346, Gloucester Point, Virginia 23062, USA

³College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Administrative Building. Corvallis, Oregon 97331, USA

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 ³H-leucine (³H-leu) and [methyl-³H]-thymidine (³H-TdR) incorporation at Stn ALOHA (22 45' N, 158 W) in the oligotrophic North Pacific Ocean. Euphotic zone rates of ³H-leu and ³H-TdR incorporation were measured in situ under light and dark conditions on cruises to Stn ALOHA between April 2000 and August 2005. Rates of ³H-leu and ³H-TdR incorporation were elevated in the well-lit upper euphotic zone (<75 m) and declined with increasing depth. In the mid-eupliotic zone (75 to 125 m), where average PAR fluxes declined to 0.6-5 % of the surface irradiance, rates of ³H-leu were significantly greater when samples were incubated at in situ light levels (³H-leu_light) than paired in situ incubations kept in the dark (³H-leu_dark) (1-way ANOVA, p < 0.05). Average rates of photostimulated ³H-leu (δLeu = ³H-leu_light - ³H-leu_dark) in the upper 125 m varied between 4.2 and 11 pmol l^-1 h^-1, with δLeu approximately equal in magnitude to rates of ³H-leu_dark, between 75 and 125 m. In contrast, rates of ³H-TdR did not display a similar sunlight enhancement, with rates of ³H-leu_light and ³H-leu_dark not significantly different (1-way ANOVA, p > 0.05). Both ³H-leu and ³H-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 Prochorococcus 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 ot the oceanic ecosystem.