| Outdoor cultivation of algae is practiced on an industrial scale for high value commodities, such as nutritional supplements, but yields are often hampered by contamination by bacteria, zooplankton, or other algae resulting in production systems having to cease periodically for decontamination. If algae are to be grown on a large scale for the production of a cheap commodity such as biodiesel, yields have to be more reliable and the algal culture more resilient to bacterial attack. We suggest that algal monocultures are inherently unstable systems that are likely to increase in complexity naturally through the invasion of competitors, predators and bacterial colonisers in an open outdoor system. We hypothesise that carefully constructed communities of algae and bacteria grown together are more robust systems, less likely to collapse and are therefore a better model for cultivation. Previously, we have shown that a beneficial algal-bacterial interaction can result, in which the bacteria supply vitamin B12 to the algae in return for a carbon source (Croft et al. 2005 Nature 438:90-93). We investigate the dynamics of equilibrium formation between algae and bacteria that grow in this symbiotic relationship using the B12-dependent Lobomonas rotrata, a close relative of Chlamydomonas reinhardtii, grown with the soil bacteria Mesorhizobium loti or Rhizobium leguminosarum. We show stable equilibria are formed irrespective of starting inoculation ratios, and are investigating the stability of these equilibria under non-selective conditions. Vitamin B12 is an expensive nutrient; if the necessity to add it to culture medium can be by-passed by inclusion of symbiotic bacteria, significant costs savings may be made. We investigate this in a Life Cycle Analysis. |