1. National Oceanography Centre, Southampton, Hampshire SO14 3ZH, UK
2. Plymouth Marine Laboratory, Plymouth, Devon PL1 3DH, UK

Correspondence to: Mikhail V. Zubkov¹ Correspondence and requests for materials should be addressed to M.V.Z. (Email: mvz@noc.soton.ac.uk).

Abstract

Planktonic algae <5 m in size are major fixers of inorganic carbon in the ocean¹. They dominate phytoplankton biomass in post-bloom, stratified oceanic temperate waters². Traditionally, large and small algae are viewed as having a critical growth dependence on inorganic nutrients, which the latter can better acquire at lower ambient concentrations owing to their higher surface area to volume ratios^{3, 4}. Nonetheless, recent phosphate tracer experiments in the oligotrophic ocean⁵ have suggested that small algae obtain inorganic phosphate indirectly, possibly through feeding on bacterioplankton. There have been numerous microscopy-based studies of algae feeding mixotrophically^{6, 7} in the laboratory^{8, 9, 10} and field^{11, 12, 13, 14}, as well as mathematical modelling of the ecological importance of mixotrophy¹⁵. However, because of methodological limitations¹⁶ there has not been a direct comparison of obligate heterotrophic and mixotrophic bacterivory. Here we present direct evidence that small algae carry out 40–95% of the bacterivory in the euphotic layer of the temperate North Atlantic Ocean in summer. A similar range of 37–70% was determined in the surface waters of the tropical North-East Atlantic Ocean, suggesting the global significance of mixotrophy. This finding reveals that even the smallest algae have less dependence on dissolved inorganic nutrients than previously thought, obtaining a quarter of their biomass from bacterivory. This has important implications for how we perceive nutrient acquisition and limitation of carbon-fixing protists as well as control of bacterioplankton in the ocean.

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