| Marine photosynthesis accounts for approximately half of all global primary production. It is crucial in providing the base of the marine food chain and a critical component of the global carbon cycle. Because all but the upper few meters of the ocean is light-limited with respect to photosynthesis, photosynthesis and photosynthetic efficiency are often described using the photosynthesis-irradiance curve. This curve is convenient to measure and varies significantly with the environment. Nevertheless, the photosynthesis-irradiance relationship only provides an empirical description of photosynthetic efficiency and has little diagnostic and prognostic capacity. To understand modes of photosynthesis-irradiance variability, I focus on photosystem II-specific processes as a potentially dominant determinant of photosynthesis-irradiance magnitude and structure. I use the marine diatom Skeletonema costatum as a model marine phytoplankter and probe the role that photosystem II has in determining photosynthesis-irradiance relationships in the context of very low background light, photoacclimation and nitrogen limitation. I also use the fresh water green alga Chlamydomonas reinhardtii that is mutant in the xanthophyll cycle to investigate non-photochemical quenching. I relate photosystem II-specific (fluorescence) measures in the presence of a background irradiance gradient of conversion efficiency and cross-sectional area to observed carbon uptake and oxygen evolution-based measures of photosynthesis-irradiance, photosynthetic unit cross section, photosynthetic unit size and photosynthetic unit turnover. These results demonstrate that under a variety of environmental situations characteristic of natural oceanic variability, photosystem II properties describe well the relative structure of photosynthesis-irradiance curves. However, other processes not associated with photosystem II (or characterized by fluorescence) and unique to each environmental situation can significantly impact the magnitude of photosynthetic rates and efficiency. Combined, these results demonstrate the utility of photosystem II properties as a function of background irradiance in describing the irradiance structure of photosynthetic efficiency, but also demonstrate the limitations of fluorescence techniques. |
