HOT-75 COMMEMORATIVE SCIENCE SYMPOSIUM

Prochlorococcus Growth Rate and Daily Variability at Station ALOHA

Hongbin Liu¹, Lisa Campbell² and Hector Nolla¹

¹School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

²Department of Oceanography, Texas A&M University, College Station, TX 77843-3146

DNA synthesis and cell division of Prochlorococcus are tightly synchronized to the daily light cycle -- DNA replication occurred in late afternoon and cell division occurred at night, therefore, cell division rates can be estimated from the fraction of cells in each cell cycle stage during a 24-h sampling period. Growth rates of Prochlorococcus were investigated during several cruises at Station ALOHA in the subtropical North Pacific Ocean. Growth rate of Prochlorococcus remained high during all cruises and displayed no seasonal variation. The maximum growth rate (up to one doubling per day) usually occurred between 45 and 75 m. Prochlorococcus production was estimated to contribute more than 60% of the total gross primary production in subtropical North Pacific Ocean at Station ALOHA. Our results suggest Prochlorococcus are not severely nutrient-limited in the oligotrophic environment. Rapid nutrient recycling by grazing activity permits Prochlorococcus to contribute a significant fraction of the total primary production.

Diel patterns in cell concentration and cellular chlorophyll fluorescence for Prochlorococcus were also observed. Maximum abundances occurred at midnight following the peak in cell division. Cell concentration then decreased and began to increase again around 1600 hr (1900 hr in upper 30 m of the water column). Cellular chlorophyll fluorescence reached a minimum at midday, then increased to a maximum immediately prior to cell division at sunset. Chlorophyll fluorescence decreased again during cell division and increased through the second half of the night to sunrise.