| Global warming is considered as the major cause to ocean warming.As a crucial part of the global marine ecosystem,Chinese coastal areas are expected to reach 5℃by the end of the 20th century,which makes it to be one of the most severely warming areas among the whole world.Marine carbon cycle is assumed to be the core of the marine biogeochemical cycling.Based on the theory of Microbial Carbon Pump(MCP),in coastal waters,warming will change the abundance,growth,metabolism and community composition of different types of marine heterotrophic bacteria(FreeLiving and Particle-Attached),thus affecting the production and consumption of Dissolved Organic Matter(DOM),and changes the bioavailability of DOM as a consequence.Previous studies mainly focused on the dynamic changes of DOM concentration in the heterotrophic bacteria utilizing process under different temperatures,and the changes of spatiotemporal structures of microbial communities in different sea areas with temperature changes.However,under the framework of MCP,few studies focus on the potential effects of warming on the coupling of DOM transformation and heterotrophic bacterial community structure under long period and multiple time scales along Chinese coastal waters.To understand the process and mechanism,further studies on microbial utilization of DOM is of vital significance under future climate conditions.In this study,we collected seawater among seven different months(April,May,July,August and October in 2019,November and December in 2020)in Aoshan Bay,Qingdao,respecti vely.We used classic Bioassay method for 30-day culture experiment,which was set up with an in-situ group and two warming groups(+3℃ and+6℃)under dark condition.Meanwhile,the concentration of organic carbon,the abundance and community structure of heterotrophic bacteria within different particle sizes,and the optical characteristics of chromophoric DOM(CDOM)were studied during warming incubation.Previous results from our lab suggested that warming would increase the Bioavailable Total Organic Carbon(BTOC)proportion when the in-situ seawater temperature was lower than 25℃ but would reduce the BTOC ratio when the in-situ water temperature was higher than 25℃.However,whether the change of heterotrophic bacterial community structure also has a temperature threshold remains unknown.The main conclusions of this paper were as follows:1.The results of 7-times warming incubation experiments showed that the existence of 25℃ as the temperature threshold based on the changes of BTOC ratio had been re-affirmed.When the in-situ water temperature was higher than 25℃,the BTOC ratio decreased with warming;ΔBTOC was inversely proportional to warming.Warming decelerated the bacterial utilization of organic carbon,while the degradation coefficient was remarkably uncorrelated to temperature(p>0.05).When the in-situ water temperature was lower than 25℃,the proportion of BTOC increased with warming;ΔBTOC was positively proportional to warming.Warming accelerated the bacterial utilization of organic carbon,while the degradation coefficient was significantly positive correlated to temperature(p<0.0001).At the same time,when the in-situ temperature was lower than 10℃,warming also decelerated the bacterial utilization of organic carbon,while the degradation coefficient was also significantly uncorrelated to temperature(p>0.05).Above results imply that there is an optimal temperature range for the bacterial utilization of TOC,meanwhile,both high and low temperatures will inhibit the marine heterotrophic bacterial carbon metabolism.2.When the in-situ seawater temperature was lower than 25℃(April,May and October),the diversity and richness of PA bacterial community were higher than that of FL.With the extension of incubation,the diversity and richness of PA bacterial community showed an upward trend,which was mainly related to the increase of phylogenetic diversity and evenness of bacterial community.When the in-situ seawater temperature was higher than 25℃(July and August),the diversity of PA and FL bacterial community was not stable,which was related to the decrease of bacterial community richness.Multivariate analysis of variance showed that the month had the largest explanation for the variance of the community composition,richness and diversity.Meanwhile,with the increasing of temperature differences,the heterogeneity of the bacterial community increased.These results suggested that in addition to temperature,the differences in the molecular composition and concentration of DOM,other environmental factors(such as nutrients,Chla and pH,etc.),grazing by protozoa and effects of viral lysis could be the other potential reasons for the differences in bacterial communities.3.With the increasing of temperature in the costal seawater,there were two subcommunities,which had positive and negative responses,and the temperature thresholds of total heterotrophic bacteria,PA bacteria and FL bacteria were different,respectively.Specifically,among three types of heterotrophic bacteria(total bacteria,PA bacteria and FL bacteria)shared the same temperature threshold concerning negative-responsing subcommunities,which was 16℃.In terms of positive-responsing subcommunities,the total heterotrophic bacteria and PA bacteria had the identical temperature threshold,which was 29.5℃,whereas the temperature threshold of FL bacteria was 26℃.These results indicated that cumulative responses of both positive and negative responsing bacterial subgroups were not 25℃.Our findings showed that the reason why the 25℃ temperature threshold of BTOC emerged,under the warming conditions,could be the result of heterotrophic bacterial carbon metabolism processes,rather than the change of heterotrophic bacterial community structure. |
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