Horizontal gene transfer from gammaproteobacteria to cyanobacteria and others

It seems to me that Gammaproteobacteria can transfer a lot of their photosynthesis related genes to other bacteria.

Recent phylogenetic studies (Bryant and Liu, 2013; Sousa et al., 2013) showed that an ancestor of marine Synechococcus and Prochlorococcus strains obtained a set of bchLNB genes from gammaproteobacteria. Such an event of lateral gene transfer from a gammaproteobacterium into an ancestor of the Synechococcus/Prochlorococcus group also included other genes, encoding proteins such as CmpA involved in circadian output (Dvornyk, 2006), carboxisome proteins, rubisco (Marin et al., 2007), threonyl tRNA synthetase, and quite possibly many more (Zhaxybayeva et al., 2006; Zhaxybayeva et al., 2009).

Gemmatimonas photoautotrophica acquired photosynthesis via horizontal gene transfer from a gammaproteobacterium (Zeng et al., 2014) and the same seems true for a Firmicutes of the genus Alkalibacterium. I wonder why...

Microbial mat

References
Bryant, D.A., and Liu, Z.F. (2013). Green bacteria: Insights into green bacterial evolution through genomic analyses. Advances in Botanical Research 66, 99-150.

Dvornyk, V. (2006). Subfamilies of cpmA, a gene involved in circadian output, have different evolutionary histories in cyanobacteria. Microbioliology 152, 75-84.

Marin, B., Nowack, E.C.M., Glockner, G., and Melkonian, M. (2007). The ancestor of the Paulinella chromatophore obtained a carboxysomal operon by horizontal gene transfer from a Nitrococcus-like gamma-proteobacterium. BMC evolutionary biology 7.

Perreault, N.N., Greer, C.W., Andersen, D.T., Tille, S., Lacrampe-Couloume, G., Lollar, B.S., and Whyte, L.G. (2008). Heterotrophic and autotrophic microbial populations in cold perennial springs of the high arctic. Appl. Environ. Microb. 74, 6898-6907.

Sousa, F.L., Shavit-Grievink, L., Allen, J.F., and Martin, W.F. (2013). Chlorophyll biosynthesis gene evolution indicates photosystem gene duplication, not photosystem merger, at the origin of oxygenic photosynthesis. Genome biology and evolution 5, 200-216.

Zeng, Y.H., Feng, F.Y., Medova, H., Dean, J., and Koblizek, M. (2014). Functional Type 2 photosynthetic reaction centers found in the rare bacterial phylum Gemmatimonadetes. P. Natl. Acad. Sci. U. S. A. 111, 7795-7800.

Zhaxybayeva, O., Doolittle, W.F., Papke, R.T., and Gogarten, J.P. (2009). Intertwined evolutionary histories of marine Synechococcus and Prochlorococcus marinus. Genome biology and evolution 1, 325-339.

Zhaxybayeva, O., Gogarten, J.P., Charlebois, R.L., Doolittle, W.F., and Papke, R.T. (2006). Phylogenetic analyses of cyanobacterial genomes: Quantification of horizontal gene transfer events. Genome Research 16, 1099-1108.