Comparing phytoplankton carbon to volume relationships using three-dimensional (3D) and two-dimensional (2D) imaging

Phytoplankton are a taxonomically and morphologically diverse group that plays a critical role in the cycling of carbon within marine ecosystems. The carbon-biomass of oceanic phytoplankton is often inferred from cell size distributions because cell carbon content scales with cell size. The current understanding in cell volume (V) to carbon content of phytoplankton comes from cells grown in replete conditions where volume has been calculated from a 2D image. To understand how carbon (C) to volume (V) relationships for 2D and 3D image analysis change in response to abiotic factors, cell size, carbon and nitrogen content and chlorophyll concentration were measured for six species in high and low light treatments. Carbon content ranged from 15 to 17,284 pg C cell^-1 and nitrogen ranged from 2 to 6,455 pg N cell^-1. Light treatment had no significant impact on C density (pg C cell ^-1 or C μm^-3 (p>0.05)). However, cellular carbon and nitrogen content decreased over time in both high and low light treatments, indicating a potential effect from nutrient stress. To relate our C: Vol measurements to established 2D relationships we compared calculated 2D volumes to 3D volumes that were measured using confocal-microscopy. 2D volumes that were geometrically determined were generally slightly higher than measured 3D volumes although the slope of the relationship between C and V was similar. The relationships for 2D calculated volumes were, pg C cell^-1 = 0.6 *volume ^0.725 and pg N cell^-1 = 0.033* volume^0.88 while the relationships for 3D measured volumes were, pg C cell^-1 = 1.3*volume ^0.647, and pg N cell^-1= 0.081 x volume ^0.798. The lower volume measurement from the high-resolution 3D image indicates that estimations of cell carbon when using a 2D image are on average 20% higher. The consistence of C:V between light treatments suggests that the same relationship can be used to convert phytoplankton size to carbon throughout the euphotic zone. These results help constrain C:V relationships enabling deeper understanding of carbon cycling in the euphotic zone.