Phytoplankton size-scaling of net-energy flux across light and biomass gradients.

Many studies examine how body size mediates energy use, but few investigate how size simultaneously regulates energy acquisition. Furthermore, rarely energy fluxes are examined while accounting for the role of biotic and abiotic factors in which they are nested. These limitations contribute to an incomplete understanding of how size affects the transfer of energy through individuals, populations, and communities. Here we characterized photosynthesis-irradiance (P-I) curves and per-cell net-energy use for 21 phytoplankton species spanning across four orders of magnitude of size and seven phyla, each measured across six light intensities and four population densities. We then used phylogenetic mixed models to quantify how body size influences the energy turnover rates of a species, and how this changes across environments. Rate-parameters for the P-I curve and net-energy budgets were mostly highly correlated and consistent with an allometric size-scaling exponent of <1. The energy flux of a cell decreased with population density and increased with light intensity, but the effect of body size remained constant across all combinations of treatment levels (i.e. no size×populationdensity interaction). The negative effect of population density on photosynthesis and respiration is mostly consistent with an active downregulation of metabolic rates following a decrease in per-cell resource availability, possibly as an adaptive strategy to reduce the minimum requirements of a cell and improve its competitive ability. Also, because an increase in body size corresponds to a less-than-proportional increase in net-energy (i.e. exponent<1), we propose that volume-specific net-energy flux can represent an important cost of evolving larger body sizes in autotrophic single-cell organisms.