Environment drives the co-occurrence of bacteria and microeukaryotes in a typical subtropical bay

The co-occurrence of bacteria and microeukaryote species is a ubiquitous ecological phenomenon,but there is limited cross-domain research in aquatic environments.We conducted a network statistical analysis and visualization of microbial cross-domain co-occurrence patterns based on DNA sampling of a typical subtropical bay during four seasons,using high-throughput sequencing of both 18S rRNA and 16S rRNA genes.First,we found obvious relationships between network stability and network complexity indices.For example,increased cooperation and modularity were found to weaken the stability of cross-domain networks.Secondly,we found that bacterial operational taxonomic units(OTUs)were the most important contributors to network complexity and stability as they occupied more nodes,constituted more keystone OTUs,built more connections,more importantly,ignoring bacteria led to greater variation in network robustness.Gammaproteobacteria,Alphaproteobacteria,Bacteroidetes,and Actinobacteria were the most ecologically important groups.Finally,we found that the environmental drivers most associated with cross-domain networks varied across seasons(in detail,the network in January was primarily constrained by temperature and salinity,the network in April was primarily constrained by depth and temperature,the network in July was mainly affected by depth,temperature,and salinity,depth was the most important factor affecting the network in October)and that environmental influence was stronger on bacteria than on microeukaryotes.

Methane and nitrous oxide emissions from a subtropical coastal embayment(Moreton Bay, Australia)

Surface water methane （CH4） and nitrous oxide （N20） concentrations and fluxes were investigated in two subtropical coastal embayments （Bramble Bay and Deception Bay, which are part of the greater Moreton Bay, Australia）. Measurements were done at 23 stations in seven campaigns covering different seasons during 2010-2012. Water-air fluxes were estimated using the Thin Boundary Layer approach with a combination of wind and currents-based models for the estimation of the gas transfer velocities. The two bays were strong sources of both CH4 and N2O with no significant differences in the degree of saturation of both gases between them during all measurement campaigns. Both CH4 and N2O concentrations had strong temporal but minimal spatial variability in both bays. During the seven seasons, CH4 varied between 500% and 4000% saturation while N2O varied between 128 and 255% in the two bays. Average seasonal CH4 fluxes for the two bays varied between 0.5 ± 0.2 and 6.0 ± 1.5 mg CH4/（m^2.day） while N20 varied between 0.4 ± 0.1 and 1.6 ± 0.6 mg N2O/（m^2-day）. Weighted emissions （t CO2-e） were 63%-90% N2O dominated implying that a reduction in N2O inputs and/or nitrogen availability in the bays may significantly reduce the bays＇ greenhouse gas （GHG） budget. Emissions data for tropical and subtropical systems is still scarce. This work found subtropical bays to be significant aquatic sources of both CH4 and N2O and puts the estimated fluxes into the global context with measurements done from other climatic regions.

Homogeneous selection is not always important in bacterial community in the eutrophic enclosed bay

Background:Previous studies have found that coastal eutrophication increases the influence of homogeneous selection on bacterial community assembly.However,whether seasonal changes affect the dominance of homogenous selection in bacterial community assembly in eutrophic bays remains unclear.Sansha Bay is an enclosed bay with ongoing eutrophication,located in the southeast coast of China.We investigated the bacterial community composition at two depths of the enclosed bay across seasons and the seasonal variation in community assembly processes.Results:Diversity analyses revealed that the bacterial community composition among seasons differed significantly.By contrast,there was little difference in the community composition between the two depths.The temperature was the key environmental factor influencing the community composition.The null model indicated that the relative importance of homogeneous selection decreased in the following order:spring>winter>autumn>summer.Homogeneous selection did not always dominate the community assembly among seasons in the eutrophic bay.The effects of pure spatial variables on the community assembly were prominent in autumn and winter.Conclusions:Our results showed the seasonal influence of eutrophication on bacterial community diversity.The seasonal variation in composition and structure of bacterial communities eclipsed the vertical variability.Eutrophication could enhance the importance of homogeneous selection in the assembly processes,but the seasonal environmental differences interfered with the steady-state maintained by ongoing eutrophication and changed the community assembly processes.Homogeneous selection was not always important in bacterial community in the eutrophic enclosed bay.The bacterial community was the most complex in summer,because the composition differed from other seasons,and the assembly process was the most intricate.These findings have contributed to understanding bacterial community composition and assembly processes in eutrophic coastal ecosystems.