Photosynthetic Inhibition on the Microalga <i>Phaeodactylum tricornutum</i> by the Dried Macroalga <i>Gracilaria tenuistipitata</i>

Abstract

The effects of the dried macroalga, Gracilaria tenuistipitata (Rhodophyta), on photosynthesis of a highly stress-resistant microalga, Phaeodactylum tricornutum were studied. P. tricornutum was cultured with dried G. tenuistipitata under controlled laboratory conditions for three days. All oxygenic photosynthetic organisms tested so far exhibit a polyphasic rise of fluorescence transients during the first second of illumination. These phases are labeled as O, J, I, P. The chlorophyll a (Chl a) fluorescence transient O-J-I-P curve coupled with its specific parameters in P. tricornutum was established. The Chl a fluorescence 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-and time-dependent relationships were observed between the dried biomass of G. tenuistipitata and its inhibitory effect on Pyramimonas sp. A decrease in the O-J-I-P curve expressed as Chl a fluorescence 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 the JIP-test, can be expressed as 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 of G. tenuistipitata effectively inhibit photosynthesis in P. tricornutum and could thus be potential candidates for use in the control and mitigation of harmful algal blooms.