Do mixotrophic protists make oligotrophic oceanic gyres sustainable ecosystems?

Lead Research Organisation: National Oceanography Centre
Department Name: Science and Technology


The aim of the proposal is to explain the functioning of the oligotrophic oceanic gyres, the Earth's largest ecosystems, which profoundly affect global biogeochemistry and climate. Compared to complex biogeochemical dynamics of C and N with their multiple inorganic pools that do not allow their complete microbial budgets to be accurately determined experimentally, a phosphate (P) budget in surface waters, particularly in the P depleted North Atlantic gyre, is easier to quantify because inorganic P has only one dissolved form in which it is taken up by microbes. The project proposes to experimentally test and to numerically model a new concept of a simplest ecosystem i.e. that: mixotrophic protists control the two dominant bacterioplankton populations, Prochlorococcus cyanobacteria and the SAR11 clade, competing for depleted inorganic P. Using a combination of outlined laboratory and oceanic cruise experiments the following hypotheses will be addressed (i) mixotrophic protists rather than heterotrophic protists dominate bacterial phagotrophy in oligotrophic waters; (ii) mixotrophic protists and not phototrophic protists or cyanobacteria dominate primary production in oligotrophic waters; (iii) protist phagotrophy rather than bacterioplankton senescent death / viral lysis dominate nutrient recycling in oligotrophic waters; (iv) an oligotrophic ecosystem controlled by mixotrophic protists is sustainable in terms of P recycling and C budget. The above hypotheses will be tested by employing and further developing methodology that combines multiple labelling of microorganisms with isotopic tracers and flow cytometric sorting, in combination with nutrient bioassay experiments.


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Hartmann M (2012) Mixotrophic basis of Atlantic oligotrophic ecosystems. in Proceedings of the National Academy of Sciences of the United States of America

Description Oligotrophic subtropical gyres are the largest oceanic ecosystems, covering >40% of the Earth's surface. Unicellular cyanobacteria and the smallest algae (plastidic protists) dominate CO2 fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidence from the surface mixed layer of the subtropical gyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplankton are fed on by plastidic and aplastidic protists at comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food web function in the open ocean, because plastidic protists should now be considered as the main bacterivores as well as the main CO2 fixers in the oligotrophic gyres.
Exploitation Route Through publications and data archived at BODC
Sectors Education,Environment