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A light-induced shortcut in the planktonic microbial loop

Ptacnik, Robert
Gomes, Ana
Royer, Sarah-Jeanne
Berger, Stella A.
Calbet, Albert
Nejstgaard, Jens C.
Gasol, Josep M.
Isari, Stamatina
Moorthi, Stefanie D.
Ptacnikova, Radka
Striebel, Maren
Sazhin, Andrey F.
Tsagaraki, Tatiana M.
Zervoudaki, Soultana
Altoja, Kristi
Dimitriou, Panagiotis D.
Laas, Peeter
Gazihan, Ayse
Martínez, Rodrigo A.
Schabhüttl, Stefanie
Santi, Ioulia
Sousoni, Despoina
Pitta, Paraskevi
Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.