Articles | Volume 11, issue 14
Biogeosciences, 11, 3729–3738, 2014

Special issue: Current biogeochemical and ecosystem research in the Northern...

Biogeosciences, 11, 3729–3738, 2014

Research article 18 Jul 2014

Research article | 18 Jul 2014

Uptake of algal carbon and the likely synthesis of an "essential" fatty acid by Uvigerina ex. gr. semiornata (Foraminifera) within the Pakistan margin oxygen minimum zone: evidence from fatty acid biomarker and 13C tracer experiments

K. E. Larkin10,1, A. J. Gooday1, C. Woulds2, R. M. Jeffreys3, M. Schwartz4, G. Cowie5, C. Whitcraft6, L. Levin7, J. R. Dick8, and D. W. Pond9 K. E. Larkin et al.
  • 1National Oceanography Centre, Southampton, University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH, UK
  • 2School of Geography, University of Leeds, University Road, Leeds, UK
  • 3School of Environmental Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK
  • 4University of West Florida, Department of Environmental Studies, Pensacola, FL 32514, USA
  • 5Sir John Murray Laboratories, School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
  • 6California State University, Biological Sciences, Long Beach, 1250 Bellflower Blvd., MS 9502, Long Beach, CA 90840, USA
  • 7Center for Marine Biodiversity and Conservation, and Integrative Oceanography Division, Scripps, Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0218, USA
  • 8Institute of Aquaculture, University of Stirling, Stirling, FK9 4LA, UK
  • 9Scottish Association for Marine Science, Oban, Argyll, PA37 1QA, UK
  • 10European Marine Board, Wandelaarkaai 7/68, 8400, Oostende, Belgium

Abstract. Foraminifera are an important component of benthic communities in oxygen-depleted settings, where they potentially play a significant role in the processing of organic matter. We tracked the uptake of a 13C-labelled algal food source into individual fatty acids in the benthic foraminiferal species Uvigerina ex. gr. semiornata from the Arabian Sea oxygen minimum zone (OMZ). The tracer experiments were conducted on the Pakistan margin during the late/post monsoon period (August–October 2003). A monoculture of the diatom Thalassiosira weisflogii was 13C-labelled and used to simulate a pulse of phytoplankton in two complementary experiments. A lander system was used for in situ incubations at 140 m water depth and for 2.5 days in duration. Shipboard laboratory incubations of cores collected at 140 m incorporated an oxystat system to maintain ambient dissolved oxygen concentrations and were terminated after 5 days. Uptake of diatoms was rapid, with a high incorporation of diatom fatty acids into foraminifera after ~ 2 days in both experiments. Ingestion of the diatom food source was indicated by the increase over time in the quantity of diatom biomarker fatty acids in the foraminifera and by the high percentage of 13C in many of the fatty acids present at the endpoint of both in situ and laboratory-based experiments. These results indicate that . ex. gr. semiornata rapidly ingested the diatom food source and that these foraminifera will play an important role in the short-term cycling of organic matter within this OMZ environment. The presence of 18:1(n-7) in the experimental foraminifera suggested that U. ex. gr. semiornata also consumed non-labelled bacterial food items. In addition, levels of 20:4(n-6), a PUFA only present in low amounts in the diatom food, increased dramatically in the foraminifera during both the in situ and shipboard experiments, possibly because it was synthesised de novo. This "essential fatty acid" is often abundant in benthic fauna, yet its origins and function have remained unclear. If U. ex. gr. semiornata is capable of de novo synthesis of 20:4(n-6), then it represents a potentially major source of this dietary nutrient in benthic food webs.

Final-revised paper