A Novel Diatoxanthin ─ Nonphotochemical Quenching ─ Fluorescence (DNF) Model For The Daytime Change Of In Vivo Fluorescence of Phytoplankton In Coastal Ecosystems

Estuarine/coastal diatoms are a group of phytoplankton that has evolved a rapidly-regulated, nonphotochemical quenching (NPQ) mechanism as an adaptation to variable light. These diatoms dominate in the coastal ecosystems that provide valuable ecosystem services (e.g., fisheries) to human beings. NPQ is known to depress in vivo fluorescence (IVF) of phytoplankton, especially on sunny days. The objective of this work is to develop a computational tool to improve chlorophyll a quantification based upon IVF in coastal ecosystems. Thalassiosira pseudonana (CCMP1335), a coastal diatom, was cultured under a sinusoidal bright white light regime while IVF was recorded continuously with Cyclops-7F sensors. Minimum fluorescence, maximum fluorescence, and fluorescence-irradiance curve were measured with the FIRe fluorometer at appropriate time intervals. The results displayed a 24-hr cycle of NPQ-regulated minimum and maximum fluorescence in which both were depressed during midday. The most intriguing mechanism we discovered was that the subsequent change of photochemical quenching by increasing NPQ deepened the IVF decline. Accordingly, a Diatoxanthin ─ Nonphotochemical quenching ─ Fluorescence (DNF) model was developed to predict the diurnal change in IVF. This model has two parts: the first is based upon the assumption that NPQ correlates linearly with the diatoxanthin-to-chlorophyll a ratio, which itself is a function of growth irradiance; the second model component is novel in that it adjusts for the asymmetric NPQ-to-irradiance relationship, and a subroutine resolves the dynamic half-saturation light constant of photochemical quenching. The model result is in good agreement with the IVF of Thalassiosira pseudonana. Similar work will be conducted with larger-sized diatoms, and the model will be expected to become a cell-size-associated calculation describing the relationship between IVF and true chlorophyll a concentration.