the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Effects of photosymbiosis and related processes on planktic foraminifera-bound nitrogen isotopes in South Atlantic sediments
Abstract. Foraminifera often form symbiotic relationships with photosynthetic algae, providing a host environment and inorganic nutrients in exchange for photosynthetic organic matter from the algal symbiont. To date, the history of this relationship has been studied in paleoceanographic records with the oxygen and carbon stable isotopes of foraminiferal calcite. More recently, photosymbiotic activity has been observed to impact the nitrogen isotope ratio (δ15N) of foraminiferal tissue and the organic matter incorporated into foraminiferal tests. Dinoflagellate symbiont-bearing species appear to be lower in δ15N than symbiont-barren species and more similar to their feeding sources, likely due to their retention of low-δ15N metabolic ammonium and thus a weaker amplitude for the “trophic enrichment factor,” the δ15N increase per trophic level that is widely observed in food webs. We report new glacial/interglacial foraminifera-bound δ15N (FB-δ15N) data from Deep Sea Drilling Program Site 516, located in the subtropical South Atlantic gyre, which contains multiple foraminifera species at adequately high abundance for inter-species comparison of foraminiferal nitrogen, carbon, and oxygen isotopes over a full glacial cycle. Our data show a conserved δ15N difference of 3–5 ‰ between dinoflagellate-bearing species and the other species, qualitatively consistent with, but greater in amplitude than, the δ15N difference observed in previous modern ocean and core-top studies. We propose that this greater amplitude is the result of lateral transport of symbiont-barren species into the South Atlantic subtropical gyre, which appears to represent a small region of low thermocline nitrate δ15N surrounded by regions with higher thermocline nitrate δ15N. The data point to FB-δ15N as the best available proxy for dinoflagellate symbiosis. However, they also suggest caution in regions with strong gradients, where species from contrasting environments occur in a single sediment sample.
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Status: open (until 21 Sep 2024)
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RC1: 'Comment on egusphere-2024-2291', Rocco Gennari, 05 Sep 2024
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Dear Authors and Editor,
I found the manuscript entitled "Effects of photosymbiosis and related processes on planktic foraminifera-bound nitrogen isotopes in South Atlantic sediments" by Auderset et al. very interesting as it shed light on potential bias in using the FB-∂15N in particular oceanographic areas. The manuscript also explain clearly how this method has the potential to discern among different type of symbiont hosted in foraminifera and non symbiont-bearing foraminifera. The manuscript is well written and clearly present data and discuss them. I just found several unclear aspect in some sentences, figures or references to figures, which were highlighted as comment or insert in the attached PDF. For this reasons I think that after the review of the authors the manuscript could be ready for publication, depending on the comments of other reviewers and of the editor.
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