Benthic foraminifera of dysoxic sediments: chloroplast sequestration and functional morphology

Abstract

Our recent surveys of dysoxic and anoxic sites reveal that many of the common foraminiferal inhabitants sequester chloroplasts. Such species include: Nonionella stella, which dominates the laminated sediments of the silled Santa Barbara Basin and comprises up to 82% of the living assemblage when [O 2] is below 2 μM (∼0.04 ml/l); the closely related species Nonionellina labradorica, which occurs in oxygen-depleted, silled fjords of Sweden; Stainforthia fusiformis, which dominates dysoxic sediments of Norwegian fjords; and Bulimina elegantissima, which is abundant in a shallow-water oil seep site supporting the filamentous, sulfide-oxidizing bacteria Beggiatoa. The literature contains examples of at least eight Elphidium species and one species from each of three other foraminiferal genera (i.e., Haynesina, Nonion, Reophax) that are known to sequester chloroplasts. These foraminifera are typically infaunal and/or may live under dysoxic conditions. Photosynthetic activity of the sequestered chloroplasts might provide oxygen to the host foraminifera, thereby enabling them to inhabit anoxic pore waters. However, given that most of the surveyed sites occur in the aphotic zone where light levels are too low to fuel photosynthesis, it is more likely that the host employs an as yet unidentified biochemical pathway associated with the sequestered chloroplasts. Additionally, these foraminifera have external test ornamentations that may facilitate separation of the chloroplasts from their algal prey. We discuss potential uses for these morphological features in interpreting the fossil record.