TRANSCRIPTIONAL RESPONSE OF PROCHLOROCOCCUS TO CO-CULTURE: DIFFERENCES BETWEEN STRAINS AND THE INVOLVEMENT OF PUTATIVE INFOCHEMICALS

Interactions between marine microorganisms may determine the structure and function of microbial communities. Here we show that two strains of the globally abundant marine cyanobacterium Prochlorococcus, MED4 and MIT9313, which belong to two different ecotypes, differ markedly in their response to co-culture with a marine Alteromonas. Alteromonas macleodii strain HOT1A3 enhanced the growth of MIT9313 at low cell densities, yet inhibited it at a higher concentration, whereas it had no clear effect on MED4 growth. The early transcriptomic responses of Prochlorococcus cells after 20 hours in co-culture showed no evidence of nutrient starvation, whereas the expression of genes involved in photosynthesis, protein synthesis and stress responses was affected. Differential expression of genes involved in outer membrane modification, efflux transporters and, in MIT9313, lanthipeptides (prochlorosins), suggests that Prochlorococcus mount a specific response to the presence of other cells in the cultures. Intriguingly, many of the differentially expressed genes encoded short proteins, including two new families of co-culture responsive genes, CCRG-1 which is found across the Prochlorococcus lineage and CCRG-2, which are released from the cells into the media and may be related to prochlorosins.