Effects of temperature on the growth and competition between Microcystis aeru-ginosa and Chlorella pyrenoidosa with different phosphorus availabilities

The frequent outbreak of harmful algal blooms (HABs) is one of the most serious outcomes of eutrophication, and temperature plays a critical role in the species succession; therefore, a better understanding of the impact of temperature is essential for mitigating HABs. In this study, a broadly distributed cyanobacterium Microcystis aeruginosa and a Chlorophyta species Chlorella pyrenoidosa were mono-cultured and co-cultured at three different temperatures (15 degrees C, 25 degrees C and 35 degrees C) with different phosphorus (P) availability, to explore algal growth patterns and competition results. In the mono-cultures, M. aeruginosa had a poor adaptation for its growth at low temperatures and it grew better at higher temperatures, but C. pyrenoidosa had flexibility for its growth in a wider range of temperatures and it showed the best growth at 25 degrees C. However, high P availability could alleviate the negative effects of low temperatures on M. aeruginosa growth. Meanwhile, algal P utilization behaviors could have a close relationship with their growth patterns at different temperatures, when higher P availability enhanced the growth advantages of two species at the optimal temperature in the mono-cultures. In the co-cultures, the mutually beneficial effects of two algal species for their growth were observed at low temperatures. But at warmer temperatures, the growth of M. aeruginosa was promoted and the growth of C. pyrenoidosa was inhibited when compared to monocultures. The superior P utilization of M. aeruginosa and its allelopathic effects on C. pyrenoidosa could be the main causes. Consistent with the batch experiment in flasks, water temperature in different seasons had a great impact on the growth strategies of algal species, and P enrichment affected algal succession in the natural waters. Overall, our results indicated the significant interactive effects of temperature and P availability on algal growth, and the strong competitive advantages of M. aeruginosa in eutrophic waters at high temperatures.