Diel nucleic acid synthesis and particulate DNA concentrations: Conflicts with division rate estimates by DNA accumulation l

The diel rate of microbial nucleic acid synthesis and the concentration of particulate DNA were measured at various locations in the oligotrophic Pacific Ocean. Net synthesis of RNA and DNA was observed over 24-h periods at all locations investigated, but the actual rate sometimes varied with time of day. It is concluded that whenever possible, daily rates of synthesis should be determined from a series of consecutive incubations conducted over a complete 24-h period. In addition, nonreplicating DNA comprised 7%90% of the total particulate DNA at the locations investigated. These results appear to preclude the use of light-dependent DNA accumulation as a measure of phytoplankton growth rate in the oligotrophic oceans. Nucleic acid synthesis rates measured with [3H]adenine have been used to quantify microbial production and cell division in various marine environments (Karl 1979; Karl et al. 198 1 b; Karl and Winn 1984; Winn and Karl 1984~). In recent field investigations, Karl and Winn (1984) and Winn and Karl (1984a) multiplied average hourly rates of DNA synthesis, measured over incubation periods of 6-l 2 h, by 24 to approximate daily rates of synthesis. This calculation assumes that DNA is synthesized at a relatively constant rate over 24-h periods, at least in the oligotrophic ocean. These extrapolations appeared to be justified because previous studies had shown that rates of nucleic acid synthesis in mixed populations of phytoplankton and bacteria were equivalent in light and dark incubations (Karl et al. 198 1 b). However, Falkowski and Owens (1982), in describing a new method to estimate phytoplankton growth rate, presented data indicating that particulate DNA (p-DNA) concentrations increase only in the l This work was supported, in part, by National Science Foundation grants OCE 80-05 180, OCE 8216673, and OCE 83-5 175 1 (awarded to D.M.K.). light. Although Falkowski and Owens conducted their experiments in continental shelf waters, their data might be interpreted as evidence that the assumption of a relatively constant diel rate of nucleic acid synthesis in more oligotrophic environments is inappropriate. In view of the data presented by Falkowski and Owens we have conducted several experiments using an algal culture and natural microbial assemblages to test the hypothesis that rates of microbial nucleic acid synthesis are independent of time of day in the oligotrophic ocean. The results of these experiments are summarized herein. The data presented by Falkowski and Owens also indicate that little or no “detrital” DNA was present in the seawaters they investigated. We include a discussion of our field data which lead us to conclude that significant quantities of nonreplicating p-DNA exist in the oligotrophic Pacific Ocean. The term “nonreplicating DNA” as used here refers to DNA that cannot be accounted for from measurements of microbial biomass or activity. This DNA may be associated with dead, dormant, or debilitated cells or may be adsorbed onto nonliving particulate materials. The p-DNA fraction that cannot be accounted for by the presence of living biomass has been termed detrital DNA (Holm-Hansen 1969a; Falkowski and Owens 1982). However, we prefer to use the term nonreplicating DNA, since some of this DNA may be associated with viable but nongrowing (i.e. dormant) cells and therefore may not be truly detrital. We acknowledge the technical assistance of D. B. Craven and U. M. Magaard, and thank the scientists, crew, and officers of the RV Wecoma, RV Kana Keoki, RV Cayuse, and RV Kila for assistance in sample collection. Participation in the PRPOOS ex-