Effects of iron availability on competition between Microcystis and Pseudanabaena or Chlorella species

Abstract

Iron availability has been suggested to be one of the key factors limiting the formation of Microcystis sp. blooms in eutrophic lakes. However, the significance of competition of Microcystis sp. with other phytoplankton species under iron limitation is still unclear. Here, the competitive ability of Microcystis aeruginosa is reported from mixed cultures with Pseudanabaena sp. or Chlorella pyrenoidosa at two iron concentrations (pFe 21.4 and 20.3). In monoculture, low iron conditions caused a smaller decrease in maximal relative electron transport rate, PSI/PSII fluorescence ratio and phycocyanin content in M. aeruginosa than in Pseudanabaena sp. The same iron concentration resulted in a significant decrease in chlorophyll a content, PSI/PSII fluorescence ratio, and maximal PSII photochemical yield in M. aeruginosa but there was no change in these factors in C. pyrenoidosa. All three species were more vulnerable to photoinhibition under low iron conditions. Iron limitation also decreased their cellular volume and increased their functional absorption cross-section of PSII. When mix-cultured with Pseudanabaena sp. for 6 days, M. aeruginosa cell abundance decreased by 18.4% under high iron conditions but increased by 23.7% under low iron conditions. When mix-cultured with C. pyrenoidosa, M. aeruginosa cell abundance showed decreased percentages in both low (25.7%) and high (13.8%) iron conditions. In comparison to monoculture, C. pyrenoidosa showed significantly increased growth rate when mix-cultured with M. aeruginosa under low iron conditions. In such low iron conditions, Chrome Azurol-S (CAS)- positive molecules were secreted by M. aeruginosa, which were likely to promote the growth of C. pyrenoidosa. In summary, M. aeruginosa had a competitive advantage compared with Pseudanabaena sp. and a disadvantage compared with C. pyrenoidosa under low iron or iron-limited conditions.