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