Response Of Rhizosphere Microbial Communities Of Mangrove Plant And Spartina Alterniflora To Tidal Elevation Gradient

Mangrove ecosystems are inter-tidal wetlands spreading along tropical and sub-tropical coastlines worldwide,and they are ecologically important in cycling of carbon(C),nitrogen(N),sulfur(S)and phosphorus(P)at the interfaces of sea and land.With the effect of global climate change,severe anthropogenic disturbances and Spartina alterniflora invasion,mangrove ecosystems are facing serious threats.In this study,we slected mangrove wetlands at the Yunxiao National Mangrove Reserve,planted Kandelia obovata and Spartina alterniflora,and analyzed their rhizosphere microbial communities under the background of global climate change(e.g.,sea level rise).The progress of sea-level-rise(0-250 cm)was simulated by setting‘Marsh Organ' in the field,which can control the water level.We not only observed the growth of Kandelia obovata and Spartina alterniflora,but also determined the physical and chemical parameters of the rhizosphere microenvironment.At the same time,functional genes involved in C,N,P and S cycling were analyzed.In addition,we examined correlations of rhizosphere microbial communities and environmental factors as well as the relationships among plants,rhizosphere environments and microorganisms.This study provides insights into potential ecological effects of sea-level-rise and S.alterniflora invasion on sedimental microbial communities.The main results obtained are as follows:1.K.obovata grew better at 0-200cm,while S.alterniflora grew better at 0-150 cm after one-year experiment;soil water content decreased gradually with the decrease of flooding time;the contents of nitrate and phosphate in the rhizosphere soil had significantly(p<0.05)positive correlations with the water level,while the sulfate content showed a significant negative correlation with water levels.2.Functional genes involved in C,N,P and S cycling(18,21,9 and 5,respectively)were analyzed by high-throughput real-time quantitative PCR,showing that most of functional genes had a lower abundance at 0 cm and 250 cm,and a higher abundance at 100-200 cm with a parabola distribution.The relative gene abundances of K.obovata and S.alterniflora at 50 cm and 100 cm were significantly(p<0.05)different.K.obovata rhizosphere microbial communities appeared to play a more active role in sulfur reduction and P cycling,while S.alterniflora rhizosphere microbial communities seemed to be more actively in the Calvin cycle and nitrite reduction.Metabolic flow analysis indicted that rhizosphere microbial communities could play a main part in N mineralization at the low and middle tides,and denitrification was dominant at the high tide level.Phosphonate uptake and poly-pi dissolving were obvious at the low and middle tide levels,while P dissolving capacity increased at the high tide.The sulfur reduction process was dominant in the sulfur cycle at low and middle tide levels,and sulfur reduction and sulfur oxidation processes were weakened at the high tide level.3.The composition of rhizosphere microbial communities was investigated by high-throughput sequencing of 16S rRNA gene amplicons.The results showed that the rhizosphere microbial diversity decreased with the increase of water level,and they were very sensitive to water level variation for both K.obovata and S.alterniflora.At the phylum level,34 phyla responded to water level variation;at the genus level,58-62%of genera responded to water levels variation,including Bacillus,Paenisporosarcina,Woeseia,and Longispora,and they are known to play a role in sulfur reduction,nitrogen fixation,and phosphate dissolution.Also,correlation analysis showed that rhizosphere microbial communities were related to nutrient content,and N and S cycling genes.Through functional predictions with.16S rRNA gene sequences,we found that most of rhizosphere microorganisms were affilitated with heterotrophic bacteria,and were largely involved in the process of S reduction and N fixation at the low and middle tides,which is consistent with the quantification of functional genes.However,there was little difference in the rhizosphere microbial community between K.obovata and Spartina alterniflora at 0 cm and 250 cm,and the main difference of genera at 100 cm was Psychrobacter,Desulfatiglans and Desulfobulbus;signifcantly(p<0.05)different genera at 200 cm included Bacillus,Thiohalophilus and SBR1031,which are known to be largely involved in sulfur reduction,indicating that K.obovata and S.alterniflora rhizosphere microbial communities may function differently for sulfur cycling.