In Situ Growth And Responses To Nitrogen And Phosphorus Starvation In Bloom-forming Cyanobacteria

Cyanobacterial blooms have caused notorious effects all over the world.Many efforts have been made to control cyanobacterial blooms.It is well established that cyanobacterial blooms result from complex and synergistic environmental factors,including temperature,nutrients and grazers.Among these factors,nitrogen(N)and phosphorus(P)over-enrichment have been demonstrated to play key roles in the occurrence of cyanobacterial blooms.Phosphorus has been traditionally implicated as playing a primary role in freshwater primary production and in the formation of cyanobacterial blooms.This paradigm has led to widespread reductions in inputs of phosphorus to control eutrophication in freshwater lakes.However,N is also reported to limit phytoplankton growth.To control cyanobacterial blooms,it is necessary to first identify those growth-limiting nutrients responsible for bloom development and proliferation.Beside these environmental factors,characteristics of blooming algae such as colony formation and strategies dealing with N deficiency and P deficiency also play vital roles in promoting bloom formation.In this study,1)we chose Lake Taihu as the representative large shallow eutrophic lake to systematically investigate the spatial and seasonal distribution of phytoplankton;2)then,in situ nutrient enrichment experiments were conducted to investigate the seasonal and spatial patterns of phytoplankton growth at two representative regions of Lake Taihu plagued by recurring Microcystis blooms.Differences in phytoplankton growth among size fractions were also examined;3)We investigated the proteomic and physiological responses of the bloom-forming cyanobacterium,Microcystis aeruginosa,to N starvation,P starvation and the simultaneous starvation of N and P;4)At last,we irrvestigated the recovery from nitrogen starvation and phosphorus starvation in Microcystis aeruginosa.Our results suggested that:1)The phytoplankton communities shifted from the dominance of eukaryotic algae and Microcystis to dominance of Microcystis from spring to summer.2)Growth rates were significantly higher in western Taihu than in Meiliang Bay,except in May and September.In western Taihu,phytoplankton growth was only slightly limited by phosphorus.In Meiliang Bay,phytoplankton growth was P limited during May-July,whereas it was N limited during August-September.Switching between P and N limitation closely followed the seasonal fluctuations in levels of these nutrients.Phytoplankton in the large-size fraction tended to dominate the phytoplankton communities during the Microcystis blooms,although the growth rate of the small-size fraction was significantly higher than that of the large-size fraction.Colonial Microcystis may be critical to maintain blooms under nutrient-limited conditions.3)M.aeruginosa could maintain nearly normal growth under P starvation for at least 7 days,whereas N deficiency significantly decreased M.aeruginosa growth.Orange carotenoid-binding proteins,metallothionein and several conserved exported hypothetical proteins were found differentially regulated with a similar manner responding to N starvation and P starvation,indicating general stress response roles for these proteins.Upon N starvation,the expression of several proteins involved in cellular carbon metabolism and carbon fixation were enhanced,which may enable their effective reuse of cellular substrates and accumulate glycogen for long-term survival.Upon P starvation,several proteins relating to protein synthesis and the assimilation of carbon and N were down-regulated,suggesting a general reduction in the cells metabolic rate,which could save energy and reducing power to maintain the basal growth.Though M.aeruginosa growth was significantly reduced when both N and P were unavailable,very limited significant changes in the proteomic composition were identified.4)Microcystis cells relieved from N starvation and P starvation resumed growth within 24 h and displayed significantly higher growth rates than no-starved-cells.However,N-starved cells can not resume their cellular activity to full capacity as no-starved cells suggested by the lower carbon production rate when N is available,whereas cellular activity of P-starved cells could recover to normal properties as no-starved cells within 24 h and retained a relatively higher growth rate when P was available.