Effects Of Humic Acid Organic Compounds On Physiology Of Microcystis And Its Competition With Scenedesmus

Cyanobacterial blooms occurrs frequently worldwide,leading to a decline of water quality and biodiversity in aquatic ecosystems.Microcystis is the common strain that forms cyanobacterial blooms.Microcystis must outcompete other phytoplankton species to produce enough biomass for a bloom.Thus,studying the competitive relationships between Microcystis and other algae and its mechanisms is essential for controlling the occurrence of Microcystis blooms.Most researchers considered that allelochemicals are the main factors affecting algae competition,but these researchers only analyzed the possible allelochemicals and not verified the inhibitory effect on cyanobacteira growth or competition with other phytoplankton.Beisdes,humic acid-like substances secrected by algae or phytoplankton could complex iron in waters and then reduce iron bioavailability,so the nutrient of iron may also be the important factor affecting cyanobacterial growth.In this study,the allelopathy of the competition between Microcystis and Scenedesmus was assessed,after which the effect of iron and humic acid was also analyzed.Microcystis growth is affected by natural dissolved organic matter?DOM?through allelopathy or altering the iron bioavailability was also investigated,because that DOM contains certain allelochemicals and humic acid-like substances.The primary conclusions are as follows:?1?The results demonstrated that both filtrate and dialytic filtrate of mono and mixed cultures inhibited growth of Microcystis and Scenedesmus.However,the inhibition rate dramatically decreased when iron was added.The potential allelochemical,2-butyl-octanol,was found to have a little inhibitory effect to the two strains.Thus,previously reported allelopathy to Microcystis would be overestimated.Additionally,we identified a new competition mechanism between M.aeruginosa and S.obliquus.M.aeruginosa and S.obliquus secrete humic acid-like substances reducing the bioavailability of iron and thereby resulting in growth inhibition to each other.?2?The biomass of Microcystis and Scenedesmus could be calculated using zeaxanthine and lutein,respectively.Thus,the competition between cyanobacteria and green algae may be also determined in this way.In mono culture,humic acid significantly inhibited Microcystis and Scenedesmus growth at low iron concentration,with the maximum inhibition rate was 67.84%and 38.31%,respectively.The maximum inhibition ratio of humic acid to mono-cultured Microcystis and Scenedesmus was reduced to 5.82%and 23.06%at high concentration.Scenedesmus was the dominant competitor in mixed culture.The inhibition of Microcystis by Scenedesmus was reduced with the increasing of humic acid at low iron concentration,But the inhibition was increased when humic acid increased at high iron concentration.?3?The dissolved organic matter derived from paddy soil and rice husks increased Microcystis growth by promoting photosynthesis.Biochar at low carbon concentrations(1,3,and5 mg L^-1)also improved Microcystis growth by increasing the maximum photosynthesis II quantum yield.However,biochar at high concentration decreased the protein and RNA concentrations in Microcystis and therefore reduced Microcystis biomass.Pectin did not affect Microcystis photosynthesis,protein concentration,RNA concentration,or growth.The results suggested that Microcystis growth was improved by the amino acids tryptophan and tyrosine,decreased by abundant humic-acid-like substances,and unaffected by polysaccharides.