Growth rates and production of heterotrophic bacteria and phytoplankton in the North Pacific subtropical gyre

In field work conducted at 26°N, 155°W, in the North Pacific subtropical gyre, phytoplankton growth rates μp estimated from 14C labeling of chlorophyll a (chl a) averaged approximately one doubling per day in the euphotic zone (0–150 m). Microbial (microalgal plus heterotrophic bacterial) growth rates μm calculated from the incorporation of 3H-adenine into DNA were comparable to or exceeded phytoplankton growth rates at most depths in the euphotic zone. Photosynthetic rates averaged 727 mg C m−2 day−1 Phytoplankton carbon biomass, calculated from 14C labeling of chl a, averaged 7.2 mg m−3 in the euphotic zone. Vertical profiles of particulate DNA and ATP suggested that no more than 15% of particulate DNA was associated with actively growing cells. Heterotrophic bacterial carbon biomass was estimated from a two-year average at station ALOHA (22°45′N, 158°W) of flow cytometric counts of unpigmented, bacteria-size particles which bound DAPI on the assumption that 15% of the particles were actively growing cells and that heterotrophic bacterial cells contained 20 fg C cell−1 The heterotrophic bacterial carbon so calculated averaged 1.1 mg m−3 in the euphotic zone. Heterotrophic bacterial production was estimated to be 164 mg C m−2 day−1 or 23% of the calculated photosynthetic rate. Estimated heterotrophic bacterial growth rates averaged 0.97 day−1 in the euphotic zone and reached 4.7 day− at a depth of 20 m. Most heterotrophic bacterial production occurred in the upper 40 m of the euphotic zone, suggesting that direct excretion by phytoplankton, perhaps due to photorespiration or ultraviolet light effects, was a significant source of dissolved organic carbon for the bacteria.