Background
Purple phototrophic bacteria or PPB, in the words of the authors, "include genetically and evolutionarily remote prokaryotic microorganisms that are capable of performing photosynthesis without releasing oxygen (Bryant and Firgaard, 2006)," and as the microbes do so, they say that they "harvest light energy to facilitate the turnover and transformation of organic materials in the environment through microbial food webs (Eiler, 2006)." In addition, they indicate that most studies of PPB have been conducted on those found in aquatic ecosystems (Karr et al., 2003; Waidner and Kirchman, 2005; Fourcans et al., 2006; Hu et al., 2006; Cho et al., 2007), and that very little is known about those that live in terrestrial soil environments (Feng et al., 2009)," which is where -- and why -- Feng et al. (2011) studied them.

What was done
Feng et al. (2011) conducted a free-air CO2-enrichment or FACE experiment with a rice-wheat rotation system at the Nianyu Experimental Station, Jiangsu Province, China, on a field typical of the majority of agricultural lands used for rice production in that country. It began in 2002 and continued for a full six years, over which period community structures and abundances of PPB were determined by denaturing gradient gel electrophoresis (DGGE) and real-time quantitative polymerase chain reaction (PCR), respectively, targeting the pufM gene, which encodes a protein in the light reaction centre of PPB.

What was learned
The four researchers with the State Key Laboratory of Soil and Sustainable Agriculture of the Chinese Academy of Sciences discovered that the extra 200 ppm of CO2 they supplied to the rice and wheat crops increased the abundance and biodiversity of PPB in the soil in which the crops grew, although the soil PPB communities were more diverse and larger under rice than wheat, which they attributed to flooding during the time rice was grown. Nevertheless, there was also a phylogenetically diverse mixture and large abundance of PPB when wheat was grown, the two dominant species of which were related to Bradyrhizobium and Rhodopseudomonas palustris. And this finding was most important, for they say that "Bradyrhizobium and Rhodopseudomonas palustris have been reported to promote the growth of rice (Biswas et al., 2000; Harada et al., 2005)."

What it means
The Chinese scientists write that "the positive responses of the PPB community to elevated CO2 concentration suggest that the ecological role of PPB in a soil environment has been neglected in the past," while noting that PPB "may also play an important role in soils, as well as in seas and lakes." And they say that that role may be to "enhance the microbial food chain and promote the growth and yield of crops," which the results of the studies of Elbadry et al. (1999) and Harada et al. (2005) suggest that PPB do indeed do.

References
Biswas, J.C., Ladha, J.K. and Dazzo, F.B. 2000. Rhizobia inoculation improves nutrient uptake and growth of lowland rice. Soil Science Society of America Journal 64: 1644-1650.

Bryant, D.A. and Frigaard, N.-U. 2006. Prokaryotic photosynthesis and phototrophy illuminated. Trends in Microbiology 14: 488-496.

Cho, J.-C., Stapels, M.D., Morris, R.M., Vergin, K.L., Schwalbach, M.S., Givan, S.A., Barofsky, D.F. and Giovannoni, S.J. 2007. Polyphyletic photosynthetic reaction centre genes in oligotrophic marine Gammaproteobacteria. Environmental Microbiology 9: 1456-1463.

Eiler, A. 2006. Evidence for the ubiquity of mixotrophic bacteria in the upper ocean: implications and consequences. Applied and Environmental Microbiology 72: 7431-7437.

Elbadry, M., Gamal-Eldin, H. and Elbanna, K. 1999. Effects of Rhodobacter capsulatus inoculation in combination with graded levels of nitrogen fertilizer on growth and yield of rice in pots and lysimeter experiments. World Journal of Microbiology and Biotechnology 15: 393-395.

Feng, Y., Lin, X., Wang, Y., Zhang, J., Mao, T., Yin, R. and Zhu, J. 2009. Free-air CO2 enrichment (FACE) enhances the biodiversity of purple phototrophic bacteria in flooded paddy soil. Plant and Soil 324: 317-328.

Fourcans, A., Sole, A., Diestra, E., Ranchou-Peyruse, A., Esteve, I., Caumette, P. and Duran, R. 2006. Vertical migration of phototrophic bacterial populations in a hypersaline microbial mat from Salins-de-Giraud (Camargue, France). FEMS Microbiology Ecology 57: 367-377.

Harada, N., Nishiyama, M., Otsuka, S. and Matsumoto, S. 2005. Effects of inoculation of phototrophic bacteria on grain yield of rice and nitrogenase activity of paddy soil in a pot experiment. Soil Science and Plant Nutrition 51: 361-367.

Hu, Y., Du, H., Jiao, N. and Zeng, Y. 2006. Abundant presence of the ?-like Proteobacterial pufM gene in oxic seawater. FEMS Microbiology Letters 263: 200-206.

Karr, E.A., Sattley, W.M., Jung, D.O., Madigan, M.T. and Achenbach, L.A. 2003. Remarkable diversity of phototrophic purple bacteria in a permanently frozen Antarctic lake. Applied and Environmental Microbiology 69: 4910-4914.

Waidner, L.A. and Kirchman , D.L. 2005. Aerobic anoxygenic photosynthesis genes and operons in uncultured bacteria in the Delaware River. Environmental Microbiology 7: 1896-1908.