Nitrite Physiological Characteristics Of Microcystis Aeruginosa

Phytoplankton account for approximately 50% of global primary production, and Microcystis aeruginosa blooms are dominant in fresh water. The increase of N and P is the main reason for the explosive growth of phytoplankton. During the water blooms, the concentration of CO₂ and dissolved O₂ decrease and pH increases, this could result in the nitrite accumulation by inhibiting oxidation of nitrite to nitrate or reduction of nitrite to ammonium. On the other hand, human activities and industrialization also lead to an increased accumulation of nitrite in the environment. In this study, the effects of nitrite on the growth and physiology of Microcystis aeruginosa were investigated when nitrite as sole nitrogen source, and under the present of nitrate condition or the ammonium condition; The impacts of intracellular nitrite content change on Microcystis aeruginosa growth were studied, and the competition of Microcystis aeruginosa and Scendesmus quadricauda under different nitrite concentrations was analysed..When nitrite as sole nitrogen source, Microcystis aeruginosa could take up nitrite for growth, and the growth of alga was not impacted in medium with nitrite concentration below 5mg/L. However, with nitrite concentrations increasing to 20mg/L, the growth of alga was inhibited and photosynthetic activity decreased. Meanwhile, the content of MDA and intracellular nitrite increased. These indicated nitrite inhibited the growth of Microcystis aeruginosa by decreasing photosynthetic activity and increasing the content of oxygen free radical. The increase of chlorophyll a content,CAT activity and nitrite reductase activity might be an adaptive mechanism of alga under high nitrite condition.When both nitrate and nitrite as nitrogen source, the Microcystis aeruginosa could grow under nitrite concentration of 30 mg/L. Furthermore, compared to medium with only nitrite, the alga had higher photosynthetic activity and less the content of MDA and intracellular nitrite in medium with both nitrate and nitrite under high nitrite concentration condition. The examination of nitrite uptake also showed the half-saturation constant of alga increased when nitrate appearance. These results revealed that nitrate could inhibit the nitrite uptake, which resulted in the decrease of the intracellular nitrite concentration and the inhibition of nitrite on alga growth weakening. In medium containing both nitrate and nitrite, the nitrite could be oxidized to nitrate, suggesting nitrite oxidoreductase has been conduced.When both ammonium and nitrite as nitrogen source, in medium containing nitrite concentration of 2 mg/L, the growth of alga increased, the photosynthetic activities and intracellular nitrite content also showed increase but MDA content decreased compared the medium of nitrite concentration of 0mg/L. With the nitrite concentrations further increasing to 10mg/L, the growth of alga slowed down and even ceased, the photosynthetic activity decreased but the content of intracellular nitrite and MDA increased. The examination of nitrite uptake showed the half-saturation constant of alga decreased when ammonium appearance, indicating ammonium could promote the uptake of nitrite and increase the intracellular nitrite content. In the range of low intracellular nitrite content, the increase of intracellular nitrite could promote the growth of alga, however, too high intracellular nitrite content could inhibit the growth of alga.At nitrate concentration of 50mg/L, the growth of alga hadn't significant change in high CO₂ group and low CO₂ group, the intracellular nitrite concentration,the photosynthetic activity and MDA content also remained constant. However, at nitrate concentration of 150mg/L and 300mg/L, the growth of alga tend to be inhibited, the photosynthetic activity of alga decrease but the intracellular nitrite concentration and MDA content were higher in low CO₂ group in contrast with high CO₂ group respectively. In the same way, in the medium with only nitrite, the growth of alga also increased with intracellular nitrite content decreasing. These results showed the increase of intracellular nitrite content was the main reason of the inhibition of growth of alga. By observing the change of intracellular nitrite content and the nitrite concentration in medium containing 0 mg NO₂^--N L^-1 :240mg NO₃^--N L^-1 and in medium with 30mg NO₂^--N L^-1 :240mg NO₃^--N L^-1, we found there existed nitrite moving in alga and nitrite discharging from alga simultaneously. When the nitrite concentration being high in medium, the nitrite moving in alga was more than nitrite discharging from alga, as a result, the intracellular nitrite content increased. In contrast, the nitrite concentration of medium increased.Compared to the sole culturing, under the co-culturing condition the growth of M. aeruginosa and Scendesmus quadricauda were inhibited, indicated that there existed competition between M. aeruginosa and S. quadricauda. In co-culturing condition, with nitrite concentration increasing, S. quadricauda became more dominant in competition, and when nitrite concentration of 30mg/L, the growth of M. aeruginosa was inhibited entirely. The examination of mutual allelopathy showed mutual allelopathy was intensified with the increase of nitrite. Therefore, the result of competition of M. aeruginosa and Scendesmus quadricauda was due to under high nitrite concentration (20mg/L,30mg/L) the worse damage of M. aeruginosa and more intense allelopathy would be revealed.