Response of Heterotrophic Bacteria Abundance and Community Structure to Asian Dust Addition in the Oligotrophic Northwest Pacific Ocean

The dust storms from the continent usually affect the abundance and diversity of planktons by supplying trace elements. As such, the response of heterotrophic planktonic bacteria to dusts, nutrients(i.e., nitrogen and phosphorus) or ferrous dosages was investigated in the Kuroshio Extension region of the Northwest Pacific Ocean(NWPO) through on-board incubation experiments during an oceanographic survey in spring 2014. The flow cytometry and 16S rRNA high-throughput sequencing methods were applied to explore the abundance and community structure of bacteria, and the percentage of high nucleic acid bacteria(HNA%). The results showed that the heterotrophic bacteria abundance was low(average 2.55×10^5 cells mL^-1) and subjected to both nitrogen(N) and ferrous(Fe) limitation. Sand-dust deposition observably promoted the activity of heterotrophic planktonic bacteria. The maximum abundance of heterotrophic bacteria was 6.98×10^5 cells mL^-1 in the dust-dosage group, which was 44% higher than the control(P < 0.05). The HNA% in the dust-dosage group was 1.37 times higher than the control(P < 0.05). The activation mechanism was mainly related to the dissolution of N and Fe in the dusts. The relative abundance of genus Winogradskyella was significantly increased by dust deposition while the relative abundance of the genera Tenacibaculum and Hyphomonas was decreased. These variations of bacterial community structure were ascribed to the dissolution of nutrients N and P. Comparing the results of different experimental groups, this study concluded that dust storm improved the abundance of heterotrophic bacteria by dissolution of N and Fe.

Winter cover crops alter methanotrophs community structure in a double-rice paddy soil

Methanotrophs play a vital role in the mitigation of methane emission from soils. However, the influences of cover crops incorporation on paddy soil methanotrophic community structure have not been fully understood. In this study, the impacts of two winter cover crops（Chinese milk vetch（Astragalus sinicus L.） and ryegrass（Lolium multiflorum Lam.）, representing leguminous and non-leguminous cover crops, respectively） on community structure and abundance of methanotrophs were evaluated by using PCR-DGGE（polymerase chain reaction-denaturing gradient gel electrophoresis） and real-time PCR technology in a double-rice cropping system from South China. Four treatments were established in a completely randomized block design： 1） double-rice cropping without nitrogen fertilizer application, CK; 2） double-rice cropping with chemical nitrogen fertilizer application（200 kg ha^（–1） urea for entire double-rice season）, CF; 3） Chinese milk vetch cropping followed by double-rice cultivation with Chinese milk vetch incorporation, MV; 4） ryegrass cropping followed by double-rice cultivation with ryegrass incorporation, RG. Results showed that cultivating Chinese milk vetch and ryegrass in fallow season decreased soil bulk density and increased rice yield in different extents by comparison with CK. Additionally, methanotrophic bacterial abundance and community structure changed significantly with rice growth. Methanotrophic bacterial pmo A gene copies in four treatments were higher during late-rice season（3.18×10^7 to 10.28×10^7 copies g^–1 dry soil） by comparison with early-rice season（2.1×10^7 to 9.62×10^7 copies g^–1 dry soil）. Type Ⅰ methanotrophs absolutely predominated during early-rice season. However, the advantage of type Ⅰ methanotrophs kept narrowing during entire double-rice season and both types Ⅰ and Ⅱ methanotrophs dominated at later stage of late-rice.