Growth and mortality rates of Prochlorococcus and Synechococcus measured with a selective inhibitor technique

Abstract

A selective metabolic inhibitor method has been developed to estimate growth rates and mortalities due to protozoan grazing of the photoautotrophic prokaryotic picoplankton Prochlorococcus and Synechococcus. Laboratory and field experiments show that 1 mg m l l (final concentration) kanamycin inhibits the growth of Prochlorococcus and Synechococcus effectively and does not significantly affect protozoan grazing At Station ALOHA (22' 45' N, 158' W) 100 km north of Oahu, Hawaii, USA, growth rates of Prochlorococcus ranged from 0.4 to 0.5 d 1 within the surface mixed layer to about 0.1 d 1 at the bottom of the euphotic zone. Synechococcus grew faster, with a daily growth rate of up to 1.0 d 1 in surface waters. Grazing mortalities varied for Prochlorococcus and Synechococcusfrom 20 to 116 % and 43 to 87 % of growth rates, respectively. Growth generally exceeded grazing. Because of its high abundance (up to 2 x lo5 cells m l l in the upper 100 m), Prochlorococcus contributes significantly to phytoplankton biomass and primary production in the subtropical North Pacific Ocean. At Station ALOHA in October 1993, integrated (0 to 175 m) carbon production due to Prochlorococcus was 382.2 mg C m 2 d l . In contrast, Synechococcus produced only 14.6 mg C m 2 d l .