Heterologous expression of genes from a heterocystous cyanobacterial endosymbiont highlights organic carbon exchange with its diatom host

Abstract

ABSTRACTA few genera of diatoms are widespread and thrive in low nutrient waters of the open ocean due to their close association with N2-fixing, filamentous heterocyst-forming cyanobacteria. In one of these symbioses, the symbiont,Richelia euintracellularis, has penetrated the cell envelope of the host,Hemiaulus hauckii, and lives inside the host cytoplasm. How the partners interact, including how the symbiont sustains high rates of N2fixation is unstudied. SinceR. euintracellularishas evaded isolation, heterologous expression of genes in model laboratory organisms was performed to identify the function of proteins from the endosymbiont. Gene complementation of a cyanobacterial invertase mutant and expression of the protein inEscherichia colishowed thatR. euintracellularisHH01 possesses a neutral invertase that splits sucrose producing glucose and fructose. Several solute binding proteins (SBPs) of ABC transporters encoded in the genome ofR. euintracellularisHH01 were expressed inE. coliand their substrates were characterized. The selected SBPs directly linked the host as the source of several substrates, e.g., sugars (sucrose, galactose), amino acids (glutamate, phenylalanine) and a polyamine (spermidine), to support the cyanobacterial symbiont. Finally, transcripts of genes encoding the invertase and SBPs were consistently detected in wild populations ofH. hauckiicollected from multiple stations and depths in the western tropical North Atlantic. Our results support the idea that the diatom host provides the endosymbiotic cyanobacterium with organic carbon to fuel N2fixation. This knowledge is key to understand the physiology of the globally significantH. hauckii-R. euintracellularissymbiosis.SIGNIFICANCEDiatom diazotroph associations (DDAs) between diatoms and N2-fixing bacteria (diazotrophs) have a relevant impact on N2fixation-based production, but the mechanisms underlying their integrated N2and CO2fixation remain unstudied. In the association between the diatomHemiaulus hauckii(host) and the N2-fixing, heterocyst-forming cyanobacteriumRichelia euintracellularis(endosymbiont), the cyanobacterium is uncultivable. Here we used heterologous expression of genes from the endosymbiont to identify the function of proteins involved in the utilization of organic carbon from the host. The importance of these proteins was also confirmed by estimating gene expression in environmental samples. Our results show that the metabolisms of the symbiotic partners are integrated allowing the host to sustain the physiology of the endosymbiont for an important ecological role.