Physiological and molecular responses of invasive cyanobacterium Raphidiopsis raciborskii to ambient phosphorus deficiency

Raphidiopsis raciborskii can cause harmful cyanobacterial blooms when concentrations of environmental phosphorus (P) are very low, thus the physiological and molecular mechanisms involved in the acclimation to P need to be characterized better. The growth, chlorophyll fluorescence, alkaline phosphatase, and expression of genes directly involved in P assimilation were compared in the R. raciborskii FACHB 1496 strain grown with and without inorganic P. The specific growth rate (mu), Chl a, and six fluorescence parameters (minimal fluorescence (F-0), maximal fluorescence (F-m), maximal variable fluorescence (F-v), electron transport flux (further than Q(A)) per RC (ET0/RC), quantum yield of the electron transport in PSII (o(E0)), and the probability that an electron from a trapped exciton is moved into the electron transport chain beyond QA(-) (psi(0))) markedly decreased in R. raciborskii in response to experimental P-deficiency. In contrast, the relative variable fluorescence at the J-step (V-J), trapped energy flux (leading to Q(A) reduction) per RC (TR0/RC), and alkaline phosphatase activity significantly increased. In addition, gene expressions involved in the alkaline phosphatase (phoA1 and phoA2), high-affinity inorganic P transporter (pstS1), phosphonate transporter and metabolism (phnD and phnM), and nucleotidase (nucH) were significantly upregulated under P deficiency. However, physiological and molecular responses were resumed rapidly after P re-supplementation following P-deficient conditions. Our results highlight that R. raciborskii can perform coordinated and complex cellular and physiological responses to cope with P deficiency, reflecting R. raciborskii's multi-faceted machinery to respond to environmental P fluctuations.

Automated summary

Under phosphate deficiency, R. raciborskii showed coordinated and complex physiological and molecular responses, which were rapidly resumed after phosphate re-supplementation.