Benthic foraminifera of dysoxic sediments: chloroplast sequestration and functional morphology

Our recent surveys of dysoxic and anoxic sites reveal that many of the comm on foraminiferal inhabitants sequester chloroplasts. Such species include: Nonionella stella, which dominates the laminated sediments of the silled Sa nta Barbara Basin and comprises up to 82% of the living assemblage when [O- 2] is below 2 mu M (similar to 0.04 ml/l); the closely related species Noni onellina labradorica, which occurs in oxygen-depleted, silled fjords of Swe den; Stainforthia fusiformis, which dominates dysoxic sediments of Norwegia n fjords; and Bulimina elegantissima, which is abundant in a shallow-water oil seep site supporting the filamentous, sulfide-oxidizing bacteria Beggia toa. The literature contains examples of at least eight Elphidium species a nd one species from each of three other foraminiferal genera (i.e., Haynesi na, Nonion, Reophax) that are known to sequester chloroplasts. These forami nifera are typically infaunal and/or may live under dysoxic conditions. Pho tosynthetic activity of the sequestered chloroplasts might provide oxygen t o the host foraminifera, thereby enabling them to inhabit anoxic pore water s. 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 t hat the host employs an as yet unidentified biochemical pathway associated with the sequestered chloroplasts. Additionally, these foraminifera have ex ternal test ornamentations that may facilitate separation of the chloroplas ts from their algal prey. We discuss potential uses for these morphological features in interpreting the fossil record. (C) 1999 Elsevier Science B.V. All rights reserved.