Allelopathic Inhibitory Effects of the Dried Macroalga <i>Ulva pertusa</i> on the Photosynthetic Activities of Red Tide-Causing Microalga <i>Skeletonema costatum</i>

Abstract

Allelopathic effects of dried macroalgaUlva pertusa(Chlorophyta) on photosynthesis of the microalgaeSkeletonema costatum(Bacillariophyta) were evaluated using coexistence culture systems.S. costatumwas cultured with different biomass of driedU. pertusaunder controlled laboratory conditions for three days. The chlorophylla(Chla) fluorescence transient O-J-I-P curve coupled with its specific parameters inS. costatumwas established. The oxygen evolution rate was measured, the Chlafluorescence transients were recorded in vivo at high time resolution and analyzed according to the JIP-test which can quantify photosystem II activity. A clear dose-dependent relationships were observed between the dried biomass ofU. pertusaand its inhibitory effect onS. costatum.The rate of light-saturated maximum photosynthetic oxygen evolution (Pmax) was markedly decreased by different biomass of driedU. pertusa, while dark respiration rate (Rd) was increased. A decrease in the O-J-I-P curve expressed as Chlafluorescence intensity along with its specific parameters were observed, which was also time-dependent. The main photosynthetic inhibitory targets of the macroalga on the microalga, according to oxygen evolution rate and the JIP-test, can be expressed as, the damage to the oxygen-evolving complex, a decrease in the number of active reaction centers and the blocking of the electron transport chain. The results of the present study suggest that dried fragments ofU. pertusaeffectively inhibit photosynthesis inS. costatum.