Measurement of Growth and Mortality Rates of Prochlorococcus and Synechococcus at Station ALOHA Using a New Selective Inhibitor Technique

Hong Bin Liu and Lisa Campbell

Abstract

Prochlorococcus, a new group of oxyphototrophic marine prokaryote containing divinyl chlorophyll a and b, is now recognized as a most important component of both microbial biomass and primary production at Sta. ALOHA as well as all tropic and subtropic open ocean. In order to estimate the growth rate and mortality due to grazing of Prochlorococcus and marine cyanobacteria Synechococcus, a new selective metabolic inhibitor method has been developed. The tests with increasing concentration of kanamycin in both laboratory culture and field samples (Kaneohe Bay) indicate that 1 mg ml^-1 of kanamycin prevents cell division of Prochlorococcus and Synechococcus. Applying this method to Sta. ALOHA, we found that the growth rate of Prochlorococcus is less than half of the growth rate of Synechococcus and the grazing mortality ranges from a few percent of the produced cells to surpass the growth rate. The lower growth rate of Prochlorococcus at Sta. ALOHA supports the hypothesis that Prochlorococcus are very sensitive to nitrogen depletion but have a better photoadaptive capacity than Synechococcus. The lower grazing rate on Prochlorococcus may be attributed to selective feeding of its grazer - heterotrophic nanoplankton.