The occurrence of two genotypes of the planktonic foraminifer Globigerinoides ruber (white) and paleo-environmental implications

Planktonic foraminifera provide a record of the upper ocean environment in the chemical and isotopic composition of individual shells. Globigerinoides ruber is a common tropical–subtropical planktonic foraminifer, and this species is used extensively for reconstruction of the paleo-environment. The different stable isotopic compositions of two morphotypes, G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.), first identified in sediments, suggested that G. ruber s.s. was dwelling in the upper 30 m of the water column and G. ruber s.l. at greater depths. Plankton tows and sediment trap experiments provided additional evidence distinguishing the two morphotypes and their habitats and invited the question as to whether the two morphotypes could be distinguished genetically. In this study, using phylogenetic analysis of nuclear partial small subunit ribosomal DNA (SSU rDNA) gene sequences representing 12 new and 16 known sequences, we identified four genotypes within G. ruber white variation; one of which is a sister group of Globigerinoides conglobatus, whereas the three others were sister groups of the G. ruber pink variation. Moreover, these two major groups corresponded to morphological differences described as G. ruber s.l. and s.s., respectively. This genetic evidence corroborates differences between the two morphotypes in the isotope record, and it will contribute to a more precise reconstruction of the thermal structure of the water column.