Trypsin as a coordinate regulator of N and P nutrients in marine phytoplankton

Trypsin is best known as a digestive enzyme in vertebrate animals, but its presence and function are unknown in phytoplankton, the major primary producers in the ocean. Here we report the wide distribution of trypsin genes in global ocean phytoplankton and significant influences of environmental nitrogen and phosphorus on their expression. Using CRISPR Cas9 mediated-knockout and overexpression analyses coupled with physiological measurements, we further demonstrate trypsins critical function in regulating nitrogen and phosphorus nutritional balance. Our results show that a trypsin in Phaeodactylum tricornutum PtTryp2 functions to repress N acquisition, but its expression decreases under N deficiency to promote N acquisition. On the contrary, PtTryp2 promotes Pi uptake per se, and its expression increases under P deficiency to further reinforce P acquisition. Furthermore, PtTryp2 knockout led to amplitude magnification of the NO3 and Pi uptake seesaw, whereas PtTryp2 overexpression dampened it. In addition, PtTryp2 expression responds to the variation of N P ratio and exhibits a tendency to stabilize the stoichiometry. All our data demonstrate in concert that PtTryp2 is a coordinate regulator of N P stoichiometric homeostasis. The novel function of trypsin and the N P nutrition coordinative regulatory mechanism in phytoplankton reported here open a window for deciphering how phytoplankton adapt to nutrient variable marine environments.