A microalga and its microbiome: Diversity, variability and not just a question of B12?

Microalgae are a diverse group of phototrophic organisms, that have the potential to produce a wide range of products important in creating a sustainable future. Microalgae have been seen to have associated bacterial populations and often grow poorly without this interaction. There is much debate about the underlying mechanisms contributing to these relationships which are believed to be highly complex and specific, based on current findings. This may be due to many factors including the transfer of products such as cobalamin (B12), carbon sources and nitrogen sources. The proprietary microalgae ALG01, as held by Algaecytes, is in the class of Eustigmatophyceae, which produces Eicosapentaenoic acid (EPA) an important omega-3 oil, needed by many living things, for growth and repair. ALG01 is grown phototrophically and is normally grown with associated microflora. The aim of this study was to investigate whether ALG01 could be grown axenically and to gain an insight into the role of the associated microflora.

In the present study, axenic ALG01 (devoid of microbiome) was shown not to grow well, with reduced chlorophyll content (qualitative observations only), like many other documented axenic strains. The addition of B12, tryptone and glucose improved the growth of axenic cultures (approx. 50% increase) but did not return growth to normal levels when compared to cultures with the total microbiome. 87 morphologically different bacterial isolates were taken from an ALG01 culture through agar isolation, 83 were able to be subcultured, a further 3 were previously isolated from AlgaeCytes and all were tested against the axenic ALG01 on agar cultures. 65 showed a positive interaction, 18 showed no effect and none showed a detrimental effect. The addition of the bacteria only had a visible impact on algal growth on agar cultures where direct contact occurred. Bacterial additions into liquid algal cultures had no statistically significant effect on growth or lipid production when cultivated for 8 days, until the cultures entered stationary phase of growth within a small-scale system. Of the 90 strains tested the 10 with the most significant effect on agar were sequenced and were identified as Pseudomonas, Pimelobacter, Brevundimonas, Microbacterium, Comamonas, and Sphingopyxis species. Genomic analysis of these strains showed them to have a plethora of possible products that may underpin any interaction with ALG01 i.e. B12 production and denitrification capabilities. The growth of ALG01 axenically on a large scale is unrealistic and its microbiome has been shown to be more significant in the overall health and stability of the culture than initially thought. More investigation is needed into the dynamics of all the species within ALG01 cultures and our study reflects what others have found with other microbiome studies, namely that they are closely associated with the microalgae and are beneficial to growth, but their precise function is unknown. This has a large implication in the industrial uses of microalgae by being able to use the information for culture management, potential increase in product production, and as an indicator of culture health and performance.