Preliminary Study On The Effects Of Warming On Two Typical Diatoms And Their Associated Bacteria

Phytoplankton drives the carbon cycle and other related nutrient cycles by converting inorganic carbon into organic carbon via the process of photosynthesis.Meanwhile,heterotrophic bacteria convert the organic carbon released by phytoplankton to CO2 and regulates the incorporation of DOC in the marine microbial food web.Therefore,the interactions between phytoplankton and heterotrophic bacteria play a vital role in the marine biogeochemical cycles and marine food web.Ocean warming due to global warming may significantly influence the interactions between marine phytoplankton and heterotrophic bacteria.The effects of ocean warming on phytoplankton and heterotrophic bacteria have been studied extensively.However,the effects of ocean warming on the interactions between phytoplankton and heterotrophic bacteria were not well studied.In this thesis,we first studied the interactions of Phaeodactylum tricornutum with six bacteria and then focused investigating on the interactions between P.tricornutum and Alteromonas sp.,Bacillus pumilus.Furthermore,we studied the effects of warming on the interaction between P.tricornutum and Alteromonas sp.as well as on the interactions between Skeletonema costaum and related heterotrophic bacteria and bacterial community composition.The main results are as follows:1.We used six bacterial species to co-cultured with P.tricornutum,and found that different species and initial concentrations had various effects on P.tricornutum.Alteromonas sp.with different initial concentrations promoted P.tricornutum's growth,and the higher bacterial initial concentration,the more significant promoting effects was observed.Low initial concentration of B.pumilus has no effects on P.tricornutum,while high concentration of B.pumilus inhibited the growth of P.tricornutum.The DOC concentration of P.tricornutum with low initial concentration of Alteromonas sp.co-culture was lower than in axenic P.tricornutum culture at both exponential and stationary phase,however,DOC concentration of P.tricornutum with high initial concentration of Alteromonas sp.co-culture was higher than axenic culture.A positive relationship between DOC concentration and P.tricornutum/Alteromonas sp.concentration ratio was observed.2.Our results showed that elevated temperature inhibited specific growth rate of P.tricornutum but promoted Alteromonas sp.'s growth.The promoting effects of Alteromonas sp.on the growth of P.tricornutum was not observed under warming conditions.At exponential phase,DOC concentration of co-culture under 24? was significantly lower than axenic culture under 24?.Whereas the DOC concentration of co-culture under 20? was significantly lower than axenic culture under 24? at stationary phase.3.The specific growth rate of S.costatum and bacterial abundance was increased under warming conditions.The dominant species Alteromonas and Marinobacter were influenced by elevated temperature that the relative abundance of Alteromonas and Marinobacter hydrocarbonoclasticus were increased but the relative abundance of Marinobacterium stanieri was decreased under warming conditions.The relative abundance of rare OTUs were more significantly influenced by warming compared to abundant OTUs.The relative abundance of rare OTUs declined along with the rising temperature and some rare OTUs were disappeared under ? and 30°C.The interactions between bacteria,mainly positive interactions,were enhanced when temperature increased from 20?to 30?.Furthermore,Proteobacteria were dominated under 30?and had extensively connected between each other.DOC concentration increased with the increase of S.costatum's cell concentration.DOC concentration was highest at day 8 under 30°C and there was a significant difference between 30? and 20? or 25?,but there was no significant difference between these three temperatures at day 4 and day 14.In summary,different initial concentrations and bacterial taxon has various effects on P.tricornutum's growth and the interaction between P.tricornutum and Alteromonas sp.may be enhanced under warming conditions.The growth of S.costatum and bacterial community composition were significantly influenced by warming and the carbon flux may speed up under warming condition.Some abundant and rare OTUs and interactions among bacteria were also significantly influenced by warming.