Interactions between algae and bacteria are widespread in aquatic and terrestrial ecosystems and play fundamental roles in nutrient cycling and biomass production. However, the chemical basis for these interactions remains poorly characterized and understood. Numerous studies have shown that the mutualism of plant growth-promoting bacteria (PGPB) with algae improves physiological activity, and increases the accumulation of lipids, proteins, pigments, starch, and cell density of these microalgae.

   Chlamydomonas cells can scavenge N from amino acids in three ways: direct uptake by specific amino acids transporters, amino acid deamination mediated by LAO1, and mutualistic interaction with bacteria. By the LAO1 activity, Chlamydomonas can grow on most amino acids, but there are some of them as the L-proline that this alga cannot use. In our lab, we have discovered a species-specific mutualistic interaction between Chlamydomonas and Methylobacterium aquaticum that allows growth in the presence of L-proline as the only N source. This algal-bacterial interaction resulted in a metabolic complementation based on a C-N exchange. M. aquaticum performs the mineralization of L-proline to ammonium, which is used by Chlamydomonas as an N source, and Chlamydomonas performs the photosynthetic fixation of CO2, producing glycerol that is used by M. aquaticum as C source.