CAN LOW ALLOMETRIC SCALING OF RESPIRATION RATES EXPLAIN GIGANTISM IN PELAGIC PROTISTS?

Planktonic foraminifera, an extant lineage of pelagic unicellular eukaryotes, have an extensive fossil record that sets them apart from most other plankton clades. Many planktonic foraminifera, like other taxa in Rhizaria, attain gigantic proportions relative to other protists (>600μm) in warm, oligotrophic open ocean habitats. Although patterns of planktonic foraminiferal size trends are well-studied in modern and ancient communities, the metabolic underpinnings of planktonic foraminiferal size have not been examined directly. We combined new and existing individual respiration rate measurements and biovolume estimates from lab-cultured planktonic foraminiferal specimens with an extensive dataset of test size measurements from core top samples to investigate the influence of metabolic scaling and mixotrophy in allowing planktonic foraminifera to reach large sizes. From the respirometry data, we found that respiration rate increases with foraminiferal biovolume with a slope that is lower than those reported in other marine plankton (and, indeed, most metazoans). This low allometric scaling potentially explains the relatively large sizes attained by pelagic Rhizarians.

We then used this relationship to explore the impact of gigantic individuals on foraminiferal community respiration and the role of mixotrophy in supporting populations of gigantic planktonic foraminifera in warm tropical seas. From this analysis, we found that the estimated metabolic impact of gigantic individuals at tropical to temperate latitudes far outstrips their relative abundance within the assemblages. We also found that mixotrophic species are the largest and most abundant at these localities. We hypothesize that low allometric scaling is the key factor allowing for gigantism 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. Foraminiferal size has been known to fluctuate throughout their fossil record in conjunction with extinction events and changes in global climates. Using the predictive equations presented here to examine foraminiferal size trends through the lens of metabolism could shed further light on their macroevolutionary history and future in the changing oceans.