Nucleic acid synthesis in oceanic microplankton from the drake passage,antarctica: Evaluation of steady-state growth

Differential uptake of [³H]adenine and [³H]thymidine, and incorporation of tritium label into proteins, RNA and DNA as a function of depth, indicate that surface microplankton incorporated most of the assimilated radioisotopes into nucleic acids. Growth processes for deep-sea microplankton were shifted towards higher rates of [³H]-adenine incorporation into RNA compared to rates of thymidine incorporation into DNA. Deep-sea microplankton also diverted a larger portion (up to 80%) of the assimilated tritium into biosynthetic pathways for amino acid and eventual incorporation into proteins. These results imply that protein synthesis is vital for deep-sea microplankton where populations may be stressed by low levels of available nutrients. The rates at which microplankton incorporated [³H]adenine into RNA and DNA, and [³H]thymidine into DNA, suggest that oceanic microplankton are in a transient state of balanced growth, i.e. between two steady-state growth conditions, irrespective of potential growth rates. Our results support the hypothesis that oceanic microplankton are in various growth states.