Physiological and morphological scaling enables gigantism in pelagic protists

Planktonic foraminifera are pelagic protists frequently used to study paleoenvironmental change. Many planktonic foraminifera, like other taxa in Rhizaria, reach gigantic proportions relative to other pelagic protists (> 600 μm), placing them in a size class dominated by metazoans. Here, we combine new and existing respiration rate measurements, micro-CT scans, and test size measurements to investigate allometric scaling of metabolic rates, relative biomass density, and mixotrophy in contributing to the ability of planktonic foraminifera to reach large sizes. Respiration rate increases with foraminiferal biovolume with a slope of 0.51 ± 0.18. This allometric scaling slope is lower than those reported in other plankton. Further, the basal respiration rates for planktonic foraminifera exceed those of other organisms in their size class when probable biomass, rather than test volume, is considered. Using the allometric regression on a published database of modern planktonic foraminifera from the Atlantic Ocean, we estimate that gigantic individuals account for 15.3–26.1% of foraminiferal community respiration in temperate and tropical/subtropical latitudes, despite making up only 4.5–8.3% of individuals. We hypothesize that shallow scaling of test size with metabolism and of test size to actual biomass is the key factor allowing for gigantism in planktonic foraminifera. Having a large test and broadcasting rhizopodial networks increases the functional volume of the organism, allowing higher passive prey encounter rates to support the elevated metabolic rates in planktonic foraminifera. Mixotrophy may act as a mitigating factor for metabolic challenges at low latitudes, accounting for the presence of large populations of giant, predominately mixotrophic Rhizarians in these assemblages.