Direct exchange of vitamin B12 is demonstrated by modelling the growth dynamics of algal-bacterial cocultures - PubMed (original) (raw)

Direct exchange of vitamin B12 is demonstrated by modelling the growth dynamics of algal-bacterial cocultures

Matthew A A Grant et al. ISME J. 2014 Jul.

Abstract

The growth dynamics of populations of interacting species in the aquatic environment is of great importance, both for understanding natural ecosystems and in efforts to cultivate these organisms for industrial purposes. Here we consider a simple two-species system wherein the bacterium Mesorhizobium loti supplies vitamin B12 (cobalamin) to the freshwater green alga Lobomonas rostrata, which requires this organic micronutrient for growth. In return, the bacterium receives photosynthate from the alga. Mathematical models are developed that describe minimally the interdependence between the two organisms, and that fit the experimental observations of the consortium. These models enable us to distinguish between different mechanisms of nutrient exchange between the organisms, and provide strong evidence that, rather than undergoing simple lysis and release of nutrients into the medium, M. loti regulates the levels of cobalamin it produces, resulting in a true mutualism with L. rostrata. Over half of all microalgae are dependent on an exogenous source of cobalamin for growth, and this vitamin is synthesised only by bacteria; it is very likely that similar symbiotic interactions underpin algal productivity more generally.

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Figures

Figure 1

Figure 1

Vitamin B12-requiring algae are only able to grow in the presence of cobalamin, which can only be synthesised by some bacteria. (a) The structure of cobalamin (vitamin B12). (b) The model laboratory system developed by Kazamia et al. (2012) to study this interaction. L. rostrata, a vitamin B12 auxotroph, grows in the presence of M. loti in autotrophic medium that does not contain cobalamin. Controls indicate that this is because of the exchange of a carbon source for vitamin B12.

Figure 2

Figure 2

The models describe the growth of L. rostrata and M. loti seen in experiments. In all graphs, the symbols represent the experimental data from Kazamia et al. (2012), collected with three biological replicates per time point per treatment, whereas the lines are output of the models. (a) The lines show the trends given by Model 2 at different initial conditions (different starting values of a and b), without add-back of nutrients. The algae (top panel) and bacteria (bottom panel) grow until they reach the carrying capacity. Model 1 gives a similar fit. (b) The equilibrium is broken by adding back either a carbon source in the form of glycerol (magenta lines and symbols) or vitamin B12 (green lines and symbols). The control (i.e. no addition) is shown in blue. The unregulated model (Model 1) recreates the increase in the number of algae when B12 is added to the medium (top panel), and the increase in the number of bacteria when carbon is added to the medium (bottom panel). The regulated model (Model 2) similarly recreates the increase in the number of algae and bacteria after the addition of B12 or carbon, respectively. It is also able to capture the corresponding decrease in algal numbers seen upon the addition of carbon (top panel, magenta line), and bacteria after addition of B12 (bottom panel, green line). Note that the scale of the y axis is logarithmic.

Figure 3

Figure 3

Levels of B12 in monoculture of M. loti versus coculture with L. rostrata. B12 was measured separately in the cell pellet, the medium and the total (i.e. before centrifugation to pellet the cells). In the monoculture, the medium has been supplemented with carbon. In the coculture, the cell fraction includes the L. rostrata cells. The values, normalised to the numbers of bacterial cells, are the average of three biological replicates. Note that the scale is logarithmic.

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