Factors influencing test porosity in planktonic foraminifera

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.

Extensive morphological variability in asexually produced planktic foraminifera

“…Planktic foraminiferal morphology is plastic in response to environmental manipulations in culture (6)(7)(8), with the extent of this plasticity epitomized by our observations. We show that the full range of morphologies in one species (N. pachyderma) and part of another (N. incompta) occurs in genetically similar individuals (i.e., clones), without manipulation of major macroenvironmental variables.…”

Intraspecific size variation in planktonic foraminifera cannot be consistently predicted by the environment

“…Planktonic foraminifera shell size is the result of various processes such as respiration, calcification, symbiont photosynthesis, and feeding. Temperature influences the rates of these processes differently among species (Burke et al 2018; Lombard, Erez, et al, 2009; Weinkauf, Kunze, Waniek, & Kucera, 2016). In addition, models show that planktonic foraminifera species have different temperature–growth relationships depending on their trophic strategy (Lombard, Labeyrie, Michel, Spero, & Lea, 2009).…”

“…Planktonic foraminifera grow by sequential addition of chambers until reproduction, when the cell dies (semelparity; Hemleben, Spindler, & Anderson, 1989). Seawater temperature affects their growth directly through biochemical reaction rates or indirectly by influencing oxygen availability and the abundance of prey and symbionts (Caron, Faber, & Bé, 1987; Bijma, Faber, & Hemleben, 1990; Bijma, Hemleben, Oberhaensli, & Spindler, 1992; Burke et al 2018; Lombard, Erez, Michel, & Labeyrie, 2009; Takagi et al., 2019). On a global scale, planktonic foraminifera assemblages increase in size with increasing sea‐surface temperature, and the largest species occur in the tropics (Schmidt, Renaud, et al, 2004).…”

Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations

“…Scans were made at 90 kV using 20× optical magnification and were reconstructed using the Zeiss software. Micro-CT scans were processed and analyzed in VG Studio, with volumes assessed by creating a mesh wrap in the MeshLab software (Cignoni et al, 2008) as described in Burke et al (2020).…”