Genome sequences of Chlorella sorokiniana UTEX 1602 and Micractinium conductrix SAG 241.80: implications to maltose excretion by a green alga

Abstract

Green algae represent a key segment of the global species capable of photoautotrophic-driven biological carbon fixation. Algae partition fixed-carbon into chemical compounds required for biomass, while diverting excess carbon into internal storage compounds such as starch and lipids or, in certain cases, into targeted extracellular compounds. Two green algae were selected to probe for critical components associated with sugar production and release in a model alga. Chlorella sorokiniana UTEX 1602 – which does not release significant quantities of sugars to the extracellular space – was selected as a control to compare with the maltose-releasing Micractinium conductrix SAG 241.80 – which was originally isolated from an endosymbiotic association with the ciliate Paramecium bursaria. Both strains were subjected to three sequencing approaches to assemble their genomes and annotate their genes. This analysis was further complemented with transcriptional studies during maltose release by M. conductrix SAG 241.80 versus conditions where sugar release is minimal. The annotation revealed that both strains contain homologs for the key components of a putative pathway leading to cytosolic maltose accumulation, while transcriptional studies found few changes in mRNA levels for the genes associated with these established intracellular sugar pathways. A further analysis of potential sugar transporters found multiple homologs for SWEETs and tonoplast sugar transporters. The analysis of transcriptional differences revealed a lesser and more measured global response for M. conductrix SAG 241.80 versus C. sorokiniana UTEX 1602 during conditions resulting in sugar release, providing a catalog of genes that might play a role in extracellular sugar transport.