Inhibition of photosynthesis in the microalga Chaetoceros curvisetus (Bacillariophyta) by macroalga Gracilaria lemaneiformis (Rhodophyta)

We investigated the effects of dried macroalga Gracilaria lemaneiformis (Rhodophyta) on photosynthesis of the bloom-forming microalga Chaetoceros curvisetus. C. curvisetus was cultured with different amounts of dried G. lemaneiformis under controlled laboratory conditions. We measured the photosynthetic oxygen evolution rate and established the chlorophyll a fluorescence transient (OJIP) curve coupled with its specific parameters. We observed concentration-dependent and time-dependent relationships between dried G. lemaneiformis and inhibition of photosynthesis in C. curvisetus. Co-culture with dried G. lemaneiformis also resulted in a decrease in the light-saturated maximum photosynthetic oxygen evolution rate (P-max) in C. curvisetus, and a decrease in the OJIP curve along with its specific parameters; the maximum photochemical efficiency of PSII (F-v/F-m), the amount of active PSII reaction centers per excited cross section at t=0 and t=t(FM) (RC/CS0 and RC/CSm, respectively), the absorption flux per excited cross section at t=0 (ABS/CS0), and the efficiency with which a trapped exciton moves an electron into the electron transport chain (psi(0)). The dark respiration rate (R-d) increased in C. curvisetus co-cultured with dried G. lemaneiformis. The JIP-test and the oxygen evolution results indicated that dried G. lemaneiformis decreased the number of active reaction centers, blocked the electron transport chain, and damaged the oxygen-evolving complex of C. curvisetus. This result indicated that dried fragments of G. lemaneiformis could effectively inhibit photosynthesis of C. curvisetus, and thus, could serve as a functional product to control and mitigate C. curvisetus blooms.