Picophytoplankton responses to changing nutrient and light regimes during a bloom
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The spring bloom in seasonally stratified seas is often characterized by a rapid increase in photosynthetic biomass. To clarify how the combined effects of nutrient and light availability influence phytoplankton composition in the oligotrophic Gulf of Aqaba, Red Sea, phytoplankton growth and acclimation responses to various nutrient and light regimes were recorded in three independent bioassays and during a naturally-occurring bloom. We show that picoeukaryotes and Synechococcus maintained a “bloomer” growth strategy, which allowed them to grow quickly when nutrient and light limitation were reversed. During the bloom picoeukaryotes and Synechococcus appeared to have higher P requirements relative to N, and were responsible for the majority of photosynthetic biomass accumulation. Following stratification events, populations limited by light showed rapid photoacclimation (based on analysis of cellular fluorescence levels and photosystem II photosynthetic efficiency) and community composition shifts without substantial changes in photosynthetic biomass. The traditional interpretation of “bloom” dynamics (i.e., as an increase in photosynthetic biomass) may therefore be confined to the upper euphotic zone where light is not limiting, while other acclimation processes are more ecologically relevant at depth. Characterizing acclimation processes and growth strategies is important if we are to clarify mechanisms that underlie productivity in oligotrophic regions, which account for approximately half of the global primary production in the ocean. This information is also important for predicting how phytoplankton may respond to global warming-induced oligotrophic ocean expansion.
KeywordsPhytoplankton Synechococcus Phytoplankton Community Soluble Reactive Phosphorus Euphotic Zone
We thank our colleagues at the Interuniversity Institute for Marine Science in Eilat, Israel for assisting in data collection and providing laboratory space and equipment during the study. We also thank the anonymous reviewers who provided comments on the manuscript. M. Chernichovsky, Y. Chen, R. Foster, E. Grey, and J. Street assisted with sampling. D. Parks assisted with flow cytometry measurements at Stanford and I. Ayalon provided flow cytometry data analyzed at IUI. R. Labiosa helped develop experimental methods. This research was supported under the National Aeronautics and Space Administration (NASA) New Investigator Program NAG5-12663 to AP, the North Atlantic Treaty Organization (NATO) Science for Peace Grant SfP 982161 to AP and AFP, a grant from the Koret Foundation to AP, National Science Foundation (NSF) Oceanography grant OCE-0450874 to ARG., and Israel Science Foundation grant 135/05 to AFP. KRMM was supported through the National Science Foundation (NSF) Graduate Research Fellowship Program and the Department of Energy (DOE) Global Change Education Program. All experiments comply with the current laws of the countries in which the experiments were performed.
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