Summary Microcystis sp. are major players in the global intensification of toxic cyanobacterial blooms endangering the water quality of freshwater bodies. A novel green alga identified as Scenedesmus sp., designated strain huji (hereafter S. huji), was isolated from water samples containing toxic Microcystis sp. withdrawn from Lake Kinneret (Sea of Galilee), Israel, suggesting that it produces secondary metabolites that help it withstand the Microcystis toxins. Competition experiments suggested complex interaction between these two organisms and use of spent cell‐free media from S. huji caused severe cell lysis in various Microcystis strains. We have isolated active metabolites from the spent S. huji medium. Application of the concentrated allelochemicals interfered with the functionality and perhaps the integrity of the Microcystis cell membrane, as indicated by the rapid effect on the photosynthetic variable fluorescence and leakage of phycobilins and ions. Although some activity was observed towards various bacteria, it did not alter growth of eukaryotic organisms such as the green alga Chlamydomonas reinhardtii.

Summary Toxic Microcystis spp. blooms constitute a serious threat to water quality worldwide. Aeromonas veronii was isolated from Microcystis sp. colonies collected in Lake Kinneret. Spent Aeromonas media inhibits the growth of Microcystis aeruginosa MGK isolated from Lake Kinneret. The inhibition was much stronger when Aeromonas growth medium contained spent media from MGK suggesting that Aeromonas recognized its presence and produced secondary metabolites that inhibit Microcystis growth. Fractionations of the crude extract and analyses of the active fractions identified several secondary metabolites including lumichrome in Aeromonas media. Application of lumichrome at concentrations as low as 4 nM severely inhibited Microcystis growth. Inactivation of aviH in the lumichrome biosynthetic pathway altered the lumichrome level in Aeromonas and the extent of MGK growth inhibition. Conversely, the initial lag in Aeromonas growth was significantly longer when provided with Microcystis spent media but Aeromonas was able to resume normal growth. The longer was pre‐exposure to Microcystis spent media the shorter was the lag phase in Aeromonas growth indicating the presence of, and acclimation to, secondary MGK metabolite(s) the nature of which was not revealed. Our study may help to control toxic Microcystis blooms taking advantage of chemical languages used in the interspecies communication.

Summary Various approaches have been proposed to control/eliminate toxic Microcystis sp. blooms including H2O2 treatments. Earlier studies showed that pre‐exposure of various algae to oxidative stress induced massive cell death when cultures were exposed to an additional H2O2 treatment. We examined the vulnerability of exponential and stationary‐phase Microcystis sp. strain MGK cultures to single and double H2O2 applications. Stationary cultures show a much higher ability to decompose H2O2 than younger cultures. Nevertheless, they are more sensitive to an additional H2O2 dose given 1–6 h after the first one. Transcript analyses following H2O2 application showed a fast rise in glutathione peroxidase abundance (227‐fold within an hour) followed by a steep decline thereafter. Other genes potentially engaged in oxidative stress were far less affected. Metabolic‐related genes were downregulated after H2O2 treatments. Among those examined, the transcript level of prk (encoding phosphoribulose kinase) was the slowest to recover in agreement with the decline in photosynthetic rate revealed by fluorescence measurements. Our findings shed light on the response of Microcystis MGK to oxidative stress suggesting that two consecutive H2O2 applications of low concentrations are far more effective in controlling Microcystis sp. population than a single dose of a higher concentration.

No abstract

Vermetid reefs and rocky shores are hot spots of biodiversity, often referred to as the subtropical equivalent of coral reefs. The development of the ecosystem depends on the activity of several reef builders, including red crustose coralline algae (CCA) such as Neogoniolithon brassica‐florida. Despite its importance, little is known about Neogoniolithon sp. acclimation to rapid changes in light intensity and corresponding photosynthetic activity. To overcome the large spatial variability in the light field (due to location and the porous nature of the rocks) we grew Neogoniolithon sp. on glass slides and characterized its photosynthetic performance in response to various light intensities by following O2 exchange and fluorescence parameters. This was also performed on rock‐inhabiting thalli collected from the east Mediterranean basin. Generally, maximal photosynthetic rate was reached when Neogoniolithon sp. thalli grown under low illumination (such as in protected niches where the light intensity can be as low as 1% of surface illumination) were examined. When exposed to light intensities higher than those experienced during growth, Neogoniolithon sp. activates adaptive/protective mechanisms such as state transition and nonphotochemical fluorescence quenching and increases the dark respiration thereafter. We find that the Fv/Fm parameter (variable/maximal fluorescence) is not suitable to assess photosynthetic performance in Neogoniolithon sp. and propose using instead an alternative parameter recently developed. Our findings help to clarify why Neogoniolithon sp. is usually observed in shaded niches along the reef surfaces.