Abstract

Abstract. The clustering of mitochondria near pores in the test walls of foraminifera suggests that these perforations play a critical role in metabolic gas exchange. As such, pore measurements could provide a novel means of tracking changes in metabolic rate in the fossil record. However, in planktonic foraminifera, variation in average pore area, density, and porosity (the total percentage of a test wall that is open pore space) have been variously attributed to environmental, biological, and taxonomic drivers, complicating such an interpretation. Here we examine the environmental, biological, and evolutionary determinants of pore characteristics in 718 individuals, representing 17 morphospecies of planktonic foraminifera from 6 core tops in the North Atlantic. Using random forest models, we find that porosity is primarily correlated to test surface area, test volume, and habitat temperature, key factors in determining metabolic rates. In order to test if this correlation arose spuriously through the association of cryptic species with distinct biomes, we cultured Globigerinoides ruber in three different temperature conditions, and found that porosity increased with temperature. Crucially, these results show that porosity can be plastic: changing in response to environmental drivers within the lifetime of an individual foraminifer. This demonstrates the potential of porosity as a proxy for foraminiferal metabolic rates, with significance for interpreting geochemical data and the physiology of foraminifera in non-analog environments. It also highlights the importance of phenotypic plasticity (i.e., ecophenotypy) in accounting for some aspects of morphological variation in the modern and fossil record.

Highlights

  • Geochemical data from foraminiferal calcite often differ among species living in the same habitat due to biological factors collectively known as “vital effects” (Erez, 1983; Spero et al, 1991; Ezard et al, 2015)

  • We found that measurements of elliptical or top-half surface area paired with volume always produced better-fitting models than the two-dimensional measurements

  • Test porosity in planktonic foraminifera from core-top samples is primarily explained by test size and temperature

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Summary

Introduction

Geochemical data from foraminiferal calcite often differ among species living in the same habitat due to biological factors collectively known as “vital effects” (Erez, 1983; Spero et al, 1991; Ezard et al, 2015). Vital effects are often attributed, at least in part, to differences in metabolic processes such as respiration and photosynthesis (e.g., WolfGladrow et al, 1999). Though, these factors have not been directly measured in the vast majority of species, leaving this idea largely untested (e.g., Ravelo and Fairbanks, 1995). Various aspects of foraminiferal test morphology have been observed to respond directly and measurably to metabolically relevant conditions in laboratory culture. Food quality and abundance can affect the terminal test size of an adult

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