Articles | Volume 11, issue 4
Biogeosciences, 11, 995–1005, 2014

Special issue: The impact of anthropogenic perturbations on open ocean carbon...

Biogeosciences, 11, 995–1005, 2014

Research article 20 Feb 2014

Research article | 20 Feb 2014

The role of mixotrophic protists in the biological carbon pump

A. Mitra1, K. J. Flynn1, J. M. Burkholder2, T. Berge3, A. Calbet4, J. A. Raven5, E. Granéli6, P. M. Glibert7, P. J. Hansen3, D. K. Stoecker7, F. Thingstad8, U. Tillmann9, S. Våge8, S. Wilken10, and M. V. Zubkov11 A. Mitra et al.
  • 1Centre for Sustainable Aquatic Research (CSAR), Swansea University, Swansea SA2 8PP, UK
  • 2Center for Applied Aquatic Ecology, North Carolina State University, 620 Hutton Street – Suite 104, Raleigh, NC 27606, USA
  • 3Centre for Ocean Life, Marine Biological Section, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark
  • 4Institut de Ciències del Mar (CSIC). Passeig Marítim de la Barceloneta, 37-49, 08003 Barcelona, Spain
  • 5Division of Plant Sciences, University of Dundee at the James Hutton Institute, Invergowrie, Dundee DD2 5DA, UK
  • 6Marine Sciences Centre, Linnaeus University, 39182 Kalmar, Sweden
  • 7University of Maryland Center for Environmental Science, Horn Point Laboratory, P.O. Box 775, Cambridge, MD 21613, USA
  • 8Department of Biology, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway
  • 9Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
  • 10Monterey Bay Aquarium Research Institute (MBARI), 7700 Sandholdt Road, Moss Landing, CA 95039-0628, USA
  • 11National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK

Abstract. The traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic – combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e.g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e.g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic structure affects microbial trophic dynamics and the biological pump. The mixotroph-dominated structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.

Final-revised paper