Picophytoplankton productivity: Perspectives from ecology, photophysiology, and optics.

摘要

Picophytoplankton (phytoplankton less than 2–3 μm in diameter) contribute significantly to the biomass and primary production in the world's oceans. The goal of this dissertation was to investigate picophytoplankton productivity using a multidisciplinary approach, addressing questions from ecology, photophysiology, and optics. Which environmental factors influence photosynthetic parameters in a region of the ocean dominated by picophytoplankton? Given the importance of irradiance in influencing photophysiology, how do the photosynthetic parameters of three picophytoplankton, Prochlorococcus , Synechococcus, and Pelagomonas calceolata , vary over a diurnal irradiance cycle typical of the surface layers of the ocean? What photoprotective mechanisms do they exhibit to minimize damage from excessive irradiance? How do the cellular optical properties of these picophytoplankton vary over a diel cycle? Do changes in optical measurements reflect changes in productivity? Three methods were employed to measure steps in the photosynthetic process. Variable fluorescence instruments were used to examine the efficiency of photosystem II, carbon-14 photosynthesis-irradiance incubations were performed to quantify carbon fixation, and optical measurements were used to monitor increases in cell size and population size. Results from a field study in an oligotrophic region of the Indian Ocean indicated that environmental factors, especially light and nutrients, were well correlated with photosynthetic parameters. However, changes in species composition also appeared to be an important factor influencing photosynthetic rates. The three picophytoplankton investigated in laboratory studies were able to maintain high photosynthetic rates midday by utilizing photoprotective mechanisms to minimize photodamage. P. calceolata exhibited xanthophyll cycling to dissipate excess energy, Synechococcus underwent a state transition, and Prochlorococcus showed evidence of a novel nonphotochemical quenching mechanism. Diel productivity resulted in variability in cell size and refractive index. Prochlorococcus and Synechococcus showed the potential for dynamic changes in refractive index as they acclimated to varying environmental conditions. The carbon-specific attenuation, important for interpreting diurnal changes in attenuation in terms of daily productivity, was found to increase with increases in size and increases in refractive index within the picophytoplankton size range. Picophytoplankton are taxonomically diverse suggesting the potential for a number of unique photosynthetic adaptations and continued research is needed to understand this diversity.