Unraveling of Chlorella-Associated Bacterial Load, Diversity, and their Imputed Functions at High- and Low-Yield Conditions through Metagenome Sequencing.

Chlorella-associated bacteria can have a significant
influence on facilitating higher Chlorella biomass
yield due to their symbiotic relationship. In this
study, non-axenic Chlorella was cultivated in an
airlift photobioreactor at high and low-yield
conditions. The associated bacterial diversity was
analyzed using 16S rRNA metagenome sequencing.
At high-yield conditions, the bacterial load was
observed in the range of 108-1010CFU � mL−1,
whereas at low-yield conditions, bacteria were more
dominant and observed in the range of 1014–
1015 CFU � mL−1. The majority of the bacterial
species associated with Chlorella at high-yield
conditions belongs to Proteobacteria and
Bacteroidetes. Further, Bacteroidetes levels were
decreased at low-yield conditions and were highly
diversified with Planctomycetes, Firmicutes, and 18
others. Predicted functional genes indicated that
Chlorella-associated bacteria have the enzymes
involved in the metabolism and biosynthesis of Bcomplex
vitamins (i.e., vitamin B12, thiamin, biotin,
pyridoxine, and riboflavin). A critical evaluation
revealed that vitamin biosynthesis genes were more
abundant at low-yield conditions; however, vitamin
B12 transport genes (B12 transport ATP-binding
protein, B12 substrate-binding transportation, and
B12 permease protein) were less abundant,
indicating even though vitamins production occurs,
but their availability to Chlorella was limited due to
the lack of vitamin transport genes. Further, at high
yield, Chlorella-associated bacteria enabled higher
growth by supplementing the vitamins. In contrast,
at low-yield condition—an increased bacterial load,
diversity, and limited vitamin transport functional
genes affected the Chlorella yield. It can be inferred
that Chlorella yield was significantly affected by
three factors: associated bacterial load, diversity,
and transport functional genes of vitamins.