Mutualistic interactions between vitamin B₁₂‐dependent algae and heterotrophic bacteria exhibit regulation

Abstract

Summary Many algae are auxotrophs for vitamin B12 (cobalamin), which they need as a cofactor for B12‐dependent methionine synthase (METH). Because only prokaryotes can synthesize the cobalamin, they must be the ultimate source of the vitamin. In the laboratory, a direct interaction between algae and heterotrophic bacteria has been shown, with bacteria supplying cobalamin in exchange for fixed carbon. Here we establish a system to study this interaction at the molecular level. In a culture of a B12‐dependent green alga Chlamydomonas nivalis, we found a contaminating bacterium, identified by 16S rRNA analysis as Mesorhizobium sp. Using the sequenced strain of M. loti (MAFF303099), we found that it was able to support the growth of B12‐dependent Lobomonas rostrata, another green alga, in return for fixed carbon. The two organisms form a stable equilibrium in terms of population numbers, which is maintained over many generations in semi‐continuous culture, indicating a degree of regulation. However, addition of either vitamin B12 or a carbon source for the bacteria perturbs the equilibrium, demonstrating that the symbiosis is mutualistic and facultative. Chlamydomonas reinhardtii does not require B12 for growth because it encodes a B12‐independent methionine synthase, METE, the gene for which is suppressed by addition of exogenous B12. Co‐culturing C. reinhardtii with M. loti also results in reduction of METE expression, demonstrating that the bacterium can deliver the vitamin to this B12‐independent alga. We discuss the implications of this for the widespread distribution of cobalamin auxotrophy in the algal kingdom.

Tetraselmis suecica F&M‐M33 growth is influenced by its associated bacteria

“…Nevertheless, Muricauda is also reported to inhibit Nannochloropsis gaditana (Han et al ., 2016) and kill Skeletonema (Shi et al ., 2013). Strains of the genus Mesorhizobium have been shown to provide vitamin B 12 to cultured algae (Kazamia et al ., 2012; Grant et al ., 2014), but this is probably not the reason for increased growth of T. suecica F&M‐M33 co‐cultivated with Mesorhizobium sp. strain LB4, as vitamins (including B 12 ) were added to the culture medium.…”

Torn apart and reunited: impact of a heterotroph on the transcriptome of Prochlorococcus

“…Many marine microbes are known to be vitamin B 12 auxotrophs as they require this cofactor for essential functions such as methionine biosynthesis, yet lack the capability to make it themselves (Croft et al, 2005;Sañudo-Wilhelmy et al, 2013); this has led to the suggestion that exchange of vitamin B 12 may have an important ecological role in the marine environment (Kazamia et al, 2012;Bertrand et al, 2015). Owing to its massive population size, Prochlorococcus may be a globally important supplier of this cofactor for the ocean ecosystem (Bonnet et al, 2010).…”

Inference of interactions in cyanobacterial–heterotrophic co-cultures via transcriptome sequencing

“…The spectrum of behavioral responses varies greatly, as the origin of excreted material ranges from targeted secretion of photosynthetic intermediates (for example, glycolate, osmolytes and fatty acids) and extracellular polymeric substance (Seymour et al, 2010;Bruckner et al, 2011), to the products of cell lysis that can include sugars, proteins, lipids and nucleic acids (Grossart, 1999;Stevenson and Waterbury, 2006;Shen et al, 2011). In exchange, heterotrophic bacteria are thought to provide essential micronutrients, such as vitamins, amino acids and bioavailable trace metals (Amin et al, 2009;Hayashi et al, 2011;Kazamia et al, 2012;Xie et al, 2013), necessary to maintain high photosynthetic productivity.…”