Project: Optical and Nutrient Dependence of Quantum Efficiency

The control of photosynthetic quantum yield of phytoplankton
by light intensity and diapycnal nutrient flux

Primary production in the ocean is probably the least known part of the ocean's
carbon cycle. One reason that primary production is little known is the lack of
understanding of the geographical and temporal variability in phytoplankton physiology.
For example it is only recently that the importance has been revealed, of the
so-called photoprotectant pigments, pigments that, in effect, shield the photosynthetic
apparatus from too much sunlight. This project will investigate the geographic and
temporal variability of a fundamental property of oceanic photosynthesis: the quantum
yield, or the ratio of the available light to the amount of carbon fixed in photosynthesis.
The PIs propose an hypothesis based on earlier measurements, that in the lower parts
of the euphotic zone in the stratified ocean, the upward flux of nutrients regulates
the value of the quantum yield, while in the upper parts, irradiance governs its value,
through the pigment composition of the phytoplankton. This hypothesis will be tested
by making estimates of the quantum yield's maximum value through very careful and
comprehensive measurements of the bio-optical properties and species composition of
the phytoplankton, as well as the submarine light environment, hydrography, and nutrients.
These measurements will be along both temporal and spatial gradients in the ocean to
create the basis for environmental regulation of quantum yield. These measurements will
be used to establish precisely how the maximum value of the quantum yield is regulated
by solar flux and plant nutrients. This research provides a mechanism to understand
how the processes of nutrient supply and light affect the physiology of natural populations
of phytoplankton, a long-standing problem in biological oceanography. It also provides a
means for improving the modeling primary productivity, including estimating productivity
in the global ocean from space.