Artificial tripartite symbiosis involving a green alga (Chlamydomonas), a bacterium (Azotobacter) and a fungus (Alternaria): Morphological and physiological characterization

Abstract

A long-living artificial tripartite symbiosis involving a green alga (Chlamydomonas), a bacterium (Azotobacter) and a fungus (Alternaria) was established on carbon- and nitrogen-free medium. The basis of the interdependence is the complementation of photosynthetic CO2 assimilation and atmospheric nitrogen fixation. Green color of the colonies indicated that the algal cells had enough nitrogen to synthesize chlorophylls. The chlorophyll content was nearly 40% of the control cells. The relatively high rate of photosynthetic oxygen evolution proved that nitrogen was effectively used for building up a well functioning photosynthetic apparatus. This was supported by the analysis of photosystems and ultrastructural investigations. In comparison with degreened algae cultured on nitrogen-free medium, the chloroplasts in the symbiont algal cells contained a well-developed, stacked thylakoid membrane system without extreme starch or lipid accumulation. The occurrence of the fungus in the association greatly increased the chlorophyll content. Far fewer types of amino acids were excreted by the tripartite cultures than by pure cultures. Cystathionine, which is a common intermediate in the sulfur-containing amino acid metabolism, was produced in high quantities by the tripartite symbiosis. This can mostly be attributed to the activity of the fungus.

Association with an Ammonium-Excreting Bacterium Allows Diazotrophic Culture of Oil-Rich Eukaryotic Microalgae

“…As a proof of concept, this work demonstrates the feasibility of the approach by the assembly of an artificial symbiosis between an efficient nitrogen fixer (even at supra-ambient O 2 concentrations) genetically manipulated for high-level ammonium excretion and a fast-growing oleaginous microalga. Results presented in this work together with data from previous reports (29,33) suggest that Azotobacter spp. might be a robust platform for this strategy using a variety of microalgae.…”

Co-culture systems and technologies: taking synthetic biology to the next level

“…Some of the examples include very diverse species grown in close proximity. For example, Lõ rincz et al [51] studied an artificial symbiosis between a photosynthetic green alga, a nitrogen-fixing bacterium and a fungus grown on carbon-and nitrogen-free medium. Although this synthetic community was discovered serendipitously, it shows that co-cultures require similar conditions to be stable over generations as natural symbioses.…”

From molecular manipulation of domesticated Chlamydomonas reinhardtii to survival in nature

“…Several beneficial interactions of C. reinhardtii have been described, including interactions with growth-promoting bacteria and even mutualism (e. g. Nikolaev et al, 2008 ; Lörincz et al, 2010 ; Kim et al, 2014 ). These findings provide the basis for future studies that address regulatory mechanisms and identify specific compounds that impact the biology of C. reinhardtii in nature.…”