Stimulated growth promotion via signal exchange in microalgal-bacterial symbiotic relationships

Microalgae interact with bacteria on many different levels and form symbiotic relationships with specific bacterial species. Synergistic mutualism between the organisms can be used to benefit the aquaculture industry by promoting growth and improving overall biomass production. There are two main kinds of mutualism occurring between microalgae and bacteria, the exchange of materials and resources and the communication of signals between the species.

Early studies in the literature have focussed on the exchange of material and resources in order to benefit their survival. Some species rely on symbiotic relationships to aid metabolic mechanisms such as bacteria providing vitamin-B12, to many species of microalgae that don’t possess the gene that codes for vitamin-B12. The bacteria will provide this vitamin in exchange for dissolved organic matter that the bacteria can use for as energy for growth.

Less information is available on the importance of signal exchange in the synergistic mutualistic relationship between microalgae and bacteria. The exchange of substances are used for communication, not as nutrients, and are used to activate or inhibit gene expression or biological activity, resulting in the change in growth and metabolism of cells. Recently, these interactions have been receiving more attention and new important discoveries are being made. It has recently been reported that Chlorella vulgaris secretes a certain signal substance that inactivates the signal substance of the bacterial acyl-homoserine lactones (AHLs), thereby inhibiting the production of bacterial toxins, while Azospirrillum could promote the growth of the microalgae by secreting some hormones such as IAA – Indole-3- Acetic Acid.

The aim of this study was to investigate the relationship between microalgae and symbiotic bacteria and how the hormone IAA can promote the microalgae culture process.This Journal appears to be one of the first experiments to use high-throughput screening for research in microalgae-promoting bacteria. This provides a new method into research on microalgae-bacteria symbiosis. Using this form of screening allowed for the characteristics of the isolated bacteria and the small molecules that they excreted to be studied.

High throughput multiple-well plate method was developed for efficient isolation of bacteria from microalgae (Scenedesmus sp. LX1) cultivation. Scenedesmus sp. LX1 was isolated from tap water and cultivated to be used as the microalgae inoculation and bacterial community associated were obtained using the gradient dilution plate method. For detecting IAA secretion ability, bacteria were cultured under 6 different conditions (including axenic condition) and each was inoculated. The IAA concentration was then measured using an HPLC tandem (High Performance Liquid Chromatography) with a photodiode detector.

The results of this experiment presented that in comparison to the axenic control, most bacteria not only increased microalgal biomass yield but also the growth rate. 16S ribosomal DNA phylogenetic analysis presented that that some of the main isolates present were Pseudomonas and Bacillus. These bacteria strains proved to significantly enhance the growth of microalgae by secretion of IAA, in addition to carbon supply, in Scenedesmus sp. LX1. However, it also showed that simultaneously, microalgae were also secreting other unknown small molecules that seemed to be stimulating the bacteria to secrete IAA, confirming a mutualistic approach.

In summary, IAA is considered to be an important signal substance in plants and bacteria, and the knowledge of this could be potentially used in the aquaculture industries as an economic strategy in large-scale microalgae cultivation to promote blooms that can be used as feed in farm fisheries.

References 
Dao, G.H., Wu, G.X., Wang, X.X., Zhang, T.Y., Zhan, X.M., & Hu, H.Y. (2018). Enhanced microalgae growth through stimulated secretion of indole acetic acid by symbiotic bacteria. Algal Research , 33, 345-351.
https://www.sciencedirect.com/science/article/pii/S2211926418301334

Cho, D.H., Ramanan, R., Heo, J., Lee, J., Kim, B.H., Oh, H.M., & Kim, H.S (2015). Enhancing microalgal biomass productivity by engineering a microalgal-bacterial community. Bioresour. Technol. 175, 578-585.