Photosynthetic Energy Allocation Strategies of Thalassiosira pseudonana During Light\ud Limited Growth

Graduation date: 2016Access restricted to the OSU Community, at author's request, from July 7, 2015 - January 7, 2016Photosynthetic energy allocation strategies were investigated in the marine\ud diatom, Thalassiosira pseudonana, grown under a wide range of light limitation. Steady-state,\ud continuous cultures were established at three light-limited growth rates. Simultaneous measurements of photosynthetic activity were made that targeted different\ud points in photosynthetic energy flow from gross to net photosynthesis. Cells maintained\ud high photosynthetic efficiencies across all growth rates by increasing chlorophyll content\ud and PSII reaction centers as light became increasingly limited. Across all growth rates\ud 65-80% of gross photosynthesis (GPP*₀₂), was maintained as net primary production\ud (NPP*[subscript c]). A constant 5% of GPP*₀₂ was allocated for the direct reduction of nitrate and\ud sulfate across all light limited growth rates. At low light limited growth rates, a higher\ud fraction of energy was allocated to light dependent respiration and mitochondrial\ud respiration, reflecting greater requirements for maintenance energy in the form of ATP.\ud In contrast, fast growing light limited cells allocated a greater fraction of GPP₀₂ to\ud reductant (NADPH) needed for biosynthesis of biochemically reduced macromolecules.\ud Lipids were a small fraction across all light limited growth rates for short-term\ud biofractionation measurements but a higher fraction for only high and medium light\ud limited growth rates in biomass biofractionation measurements. However, these\ud behaviors were not reflected in the biochemical reduction state of biomass. The nearly\ud four-fold greater C[subscript r]/C[subscript o] ratio in cells acclimated to the lowest light level was driven by a\ud predominance of protein relative to carbohydrates. Significant differences in rates of\ud mitochondrial respiration (MR) were observed in the light and dark. Lower rates of MR\ud in the light may reflect a form of pathway gating that preserves carbon storage\ud compounds in the light by switching to non-carbon pathways for ATP generation. This\ud strategy may be linked to their ecological success particularly following prolonged\ud periods of darkness following deep mixing events. Behaviors in photosynthesisirradiance\ud (PE) relationships that are characteristic of photoacclimation [namely, constant\ud light limited slopes (α*) and variable maximal rates of photosynthesis (P[subscript max])] were\ud observed at the level of PSII but not when PE curves were generated from short-term ¹⁴Cuptake\ud rates. These unexpected growth rate-dependent shifts in α* add complexity to\ud interpretations of PE curves that are commonly used for describing phytoplankton\ud responses to environmental variables. Our results also demonstrate that photosynthetic\ud metabolism is markedly different depending on the limiting resource (e.g., light vs.\ud nutrients). These data are discussed in the context of the adaptive growth strategies thus\ud far understood for diatom species