Experimental Study of Tolerance of Rhodomonas sp. (Cryptophyta, Pyrenomonadaceae) from the White Sea Lagoon to Hydrogen Sulfide

Using chlorophyll a fluorescence methods, the effect of H2S in various concentrations on light-dependent photosynthetic processes in cells of cryptophyte algae Rhodomonas sp. from a stratified lagoon at Cape Zeleny (White Sea) was studied. In short-term H2S exposure, the maximum quantum yield of PSII photochemistry (Fv/Fm), the quantum yield of electron transport in PSII (φEo), and the productivity index, i. e. , the index of PSII functional activity per the absorbed energy (PIABS), were found to decrease at concentrations above 2. 3 mg/L due to an increase in proportion of inactive PSII RCs. A decrease in active PSII RCs was accompanied by an increase in the energy dissipation per RC (DI0/RC). In microalgae treated by H2S concentration up to 8 mg/L the disruption of photosynthetic activity was reversible: the PSII activity was restored to almost initial values, although with some delay compared to the control. H2S concentrations starting from 18 mg/L dramatically increased the proportion of PSII RCs that are unable to transfer electrons to subsequent carriers of the photosynthetic electron transport chain due to both inhibition of electron transport on the acceptor side with accumulation of Q₀^ -, and inhibition on the donor side of PSII with accumulation of P₆₈₀^ +. Photosynthetic capacity under light-saturated conditions (rETRmax) was decreased due to impairment of electron transport in PSII while non-photochemical quenching (NPQ) in PSII antenna significantly increased. We observed recovery of photosynthetic activity of Rhodomonas sp. cells after high light treatment, indicating that H2S does not prevent PSII repair activity after photoinhibition. It is proposed to use of highly sensitive fluorescence methods to study the condition of phytoplankton in coastal meromictic water bodies with different H2S content.