Selective Utilization Of Autochthonous/allochthonous Carbon By Lake Bacterioplankton

The production of lake bacterioplankton is mainly supported by lake dissolved organic carbon source?DOC?.Lake bulk DOC originates from two sources:autochthonous DOC?mainly derived from phytoplankton?and allochthonous DOC?mainly derived from terrestrial plants?.It is well accepted that autochthonous carbon has the characteristics of low carbon/phosphorus ratio,high fatty acid concentration,and small molecular weight of compounds,hence is high-quality and main source of bacterioplankton production.To the contrary,the allochthonous carbon contains low carbon/phosphorus ratio and refractory carbohydrates with long-chain compounds,and is regarded as difficult to be assimilated by bacteria.However,field investigations reveal a remarkably high incorporation of allochthonous carbon to lake food webs in recent years.In that case of contradiction,we propose that when autochthonous and allochthonous carbon coexist,bacterioplankton can use allochthonous and autochthonous carbon indifferently by adjusting the community structure,according to the previous theory that when a new substrate was added to the environment,bacteria in situ can use it through community structure changes.To testify the hypothesis,this paper set up 4 microcosms,by using stable isotopes and Illumina Mi Seq sequencing techniques,to comparatively study the selective utilization and bacterial community structure changes when using the mixture of allochthonous carbon and autochthonous carbon.The main results are as follows:1.At the end of the experiment,compared with the Control group?CK?,the concentration of dissolved inorganic carbon?DIC?in the water of the corn stalk group?YM group?,phytoplankton group?FY group?and mixture?HH group?increased by 22.83%,47.59%and26.32%,respectively;the CO₂ concentration of planktonic bacteria respiration in the YM group,FY group and HH group increased by 73.33%,90.48%and 69.23%,respectively;the respiration rate of planktonic bacteria in YM group,FY group and HH group increased by 54.11%,85.08%and 23.86%,respectively.The DIC and CO₂ concentration and bacterial respiration rate in the FY group were significantly higher than those in the HH group and YM group.2.A two end-member mixing model showed that the contribution of allochthonous DOC to the growth and respiration of bacterioplankton were 58.45±33.17%and 42.56±0.02%,respectively.The contribution of algae DOC to the growth and respiration of bacterioplankton were 41.55±33.17%and 57.44±0.02%,respectively.3.Compared with the CK group,the diversity and richness of bacterioplankton in the YM group,FY group,and HH group were significantly reduced,and the diversity of planktonic bacteria in the HH group was significantly higher than that in the FY group.Bacteria communities were dominated by Proteobacteria,Actinobacteria,Deinococcus-Thermus,Bacteroidetes and Firmicutes phyla.Principal coordinate analysis?PCo A?and analysis of similarities?ANOSIM?analysis showed that the community structure of bacterioplankton in different carbon source groups was significantly different,and the community structure of the HH group was similar to that in the FY group.Species difference analysis showed that the different species between the FY and YM groups were present in the HH group,and their relative abundance was between the former two.They were Aeromonas,Vogesella,Burkholderiaceae,Acinetobacter,Exiguobacterium,Acidovorax,Chryseobacterium and Enhydrobacter.Based on the above results,this paper draws the following conclusions:bacterioplankton in the mixed group assimilated both autochthonous and allochthonous carbon both with no significant differences.However,they tend to respire more autochthonous carbon.Planktonic bacteria use both the carbon through community structure changes.The preference of different species for autochthonous and allochthonous carbon may be the reason for bacteria community structure changes.This research will help us understand the selective utilization strategies of different carbon resources by bacterioplankton and the related mechanisms,and ultimately deepen our understanding of freshwater food web structure,as well as the interaction between aquatic and terrestrial ecosystems.