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

Abstract

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.

Does energy flux predict density‐dependence? An empirical field test

“…; Malerba et al. ) as well as reduced energy expended on growth, reproduction, and associated discretionary expenses (e.g., courtship, mating, territorial defense, exploratory behaviors; Grant and Porter ). Our model species (i.e., the bryozoan Bugula neritina ) is sessile and does not display obvious behaviors, but colonies can detect the presence of conspecifics in their surroundings (Gooley et al.…”

“…Conversely, organisms that rely on blooming resources can accelerate the rate at which energy is consumed to maximize consumption of these resources, while remaining small and responding quickly in population numbers (Kooijman 2013). The presence of conspecifics can, similarly, elicit changes in an individual's metabolic activity (Waters et al 2010, Nadler et al 2016 resulting in reduced metabolic rates (DeLong et al 2014;Malerba et al 2017) as well as reduced energy expended on growth, reproduction, and associated discretionary expenses (e.g., courtship, mating, territorial defense, exploratory behaviors; Grant and Porter 1992). Our model species (i.e., the bryozoan Bugula neritina) is sessile and does not display obvious behaviors, but colonies can detect the presence of conspecifics in their surroundings (Gooley et al 2010) and their density can alter the morphology of colonies and their feeding structures (Thompson et al 2015).…”

“…Research on metabolic theory has recently revealed density‐dependent effects on metabolism where the presence of conspecifics reduces per capita metabolic rates as a consequence of competitive effects (e.g., reduced food availability; DeLong and Hanson , DeLong et al. ; Malerba et al., ) or behavioral effects (e.g., lower stress levels in aggregated animals; Waters et al. , Nadler et al.…”

“…Not only the two processes are codependent, but they are both affected by conspecific density. Research on metabolic theory has recently revealed density-dependent effects on metabolism where the presence of conspecifics reduces per capita metabolic rates as a consequence of competitive effects (e.g., reduced food availability; Hanson 2009, DeLong et al 2014;Malerba et al, 2017) or behavioral effects (e.g., lower stress levels in aggregated animals; Waters et al 2010, Nadler et al 2016. Thus, the ways in which both energy intake (feeding) and energy expenditure (metabolism) are affected by density remains Predictions of how energy intake (feeding; green line) and expenditure (metabolism; orange line) might change with population density and the consequences for discretionary energy (gray shaded area).…”

Metabolic scaling across succession: Do individual rates predict community‐level energy use?

“…We suspect that a lower‐than‐expected metabolic rate may have been driven by metabolic suppression in response to a high density of conspecifics. This hypothesis is supported by recent studies, which have shown that per capita metabolic rates are reduced in dense populations (DeLong et al., ; Malerba, White, & Marshall, ), where conspecific density reduces energy use beyond the constraints of body size (Ghedini et al., ). Therefore, the overestimation of community metabolism could result from the high densities of Pyura reducing per capita metabolic rates in this species.…”