海洋蓝细菌与异养细菌相互作用:以原绿球藻为例

Interactions between marine cyanobacteria and heterotrophic bacteria:A case study of Prochlorococcus

蓝细菌(cyanobacteria)是海洋最主要的初级生产者,贡献了海洋净初级生产力的25%.海洋异养细菌具有多种代谢路径,能够吸收和利用蓝细菌的光合作用产物.海洋蓝细菌和异养细菌之间的相互作用关系影响海洋食物网、固碳和储碳,具有重要的生态效应和生物地球化学意义.原绿球藻(Prochlorococcus)作为蓝细菌的典型代表类群,是海洋中体积最小、数量最大的光合自养原核微生物.原绿球藻的基因组高度精简,从而减少了细胞复制中所需的物质和能量,同时减小了细胞体积,因此在寡营养大洋表层环境中具有竞争优势.然而,基因组减小同时使得原绿球藻单个细胞基因多样性下降,从而使其适应环境的潜能降低.因此,相比于其他蓝细菌,原绿球藻更加依赖于海洋环境中其他微生物的协助,以维持自身生存的需求.本文根据国内外近期的相关研究成果,从原绿球藻与异养细菌之间基因的互补关系和生理特性等角度,归纳了两者之间存在的互利共生、偏利共生等多种相互作用关系及其生态效应,并提出了未来的研究重点.

Cyanobacteria are the key primary producers in the global ocean and account for 25%of its net productivity.They play a vital role in the ocean by driving the circulation of matter and the flow of energy via the absorption of considerable amounts of carbon dioxide and the release of dissolved organic matter.This dissolved organic matter is converted into biomass by marine heterotrophs,which keeps the cycling of elements in oligotrophic waters at equilibrium.Prochlorococcus,an ecologically important genus of cyanobacteria,is the smallest and most abundant oxygenic phototroph in the global ocean.Prochlorococcus has highly streamlined genomes,which can provide a competitive advantage in the oligotrophic surface waters;however,the streamlined genomes also force Prochlorococcus to adapt to a narrow range of environmental parameters.Owing to these genomic characteristics,Prochlorococcus often establishes interactions with heterotrophic bacteria to survive in the challenging marine environment.Because cyanobacteria are abundant in the ocean,the cyanobacteria-heterotrophic bacteria relationship can significantly affect marine carbon sequestration and storage,which in turn has important ecological implications.Previous studies have reported on the relationships between Prochlorococcus and various heterotrophic bacteria.For example,Prochlorococcus establishes positive interactions with Alteromonas sp.that enhance its growth rate and environmental adaptability.In this review,we discuss the underlying mechanisms of the interaction patterns between Prochlorococcus and heterotrophic bacteria,as well as their genetic,physiological,and ecological significance.Prochlorococcus and heterotrophic bacteria tend to form mutualistic relationships in which they directly or indirectly exchange metabolites or recycle organic carbon through complementary excretion and crosstalk between pathways,such as citrate,glycolate,and malate pathways.Prochlorococcus and heterotrophic bacteria also commonly establish commensal relationships.For instance,heterotrophic bacteria protect Prochlorococcus by scavenging hydrogen peroxide,which Prochlorococcus is sensitive to.Although many heterotrophic bacteria cannot synthesize vitamin B12,they require it as a cofactor for essential functions.Their genetic characteristics suggest that most cyanobacteria have the pathways for synthesizing vitamin B12,but that they may provide vitamin B12 to heterotrophic bacteria to promote the growth of"helper"in the oligotrophic ocean.Therefore,the interactions between cyanobacteria and heterotrophic bacteria help maintain the stability and diversity of marine ecosystems.Global climate change may influence cyanobacteria-heterotrophic bacteria interactions,which could directly affect the structure and dynamics of the microbial community in the oligotrophic ocean,and therefore,primary productivity and element cycling in the ocean.Here,we suggest future research priorities and potential applications based on newfound knowledge of the subject.First,using the known"heterotrophic bacteria-cyanobacteria"relationships,the cultivation of marine heterotrophic bacteria,cyanobacteria and their viruses should be promoted to build a more comprehensive"cyanobacteria-heterotrophic bacteria-virus"ecological model for marine environments.Second,the interaction mechanisms between cyanobacteria and heterotrophic bacteria should be revealed by new biotechnologies,such as(meta)genomics,(meta)transcriptomics,(meta)proteomics,and(meta)metabolomics.Third,cyanobacteria-heterotrophic bacteria interactions should be studied against the backdrop of climate change using both ex situ(i.e.,laboratory)and in situ investigations to predict changes in marine microbial community and its potential impact on biogeochemical cycling in oceans.