Uptake of <i>Prochlorococcus</i> ‐derived metabolites by <i>Alteromonas macleodii</i> <scp>MIT1002</scp> shows high levels of substrate specificity

Abstract Seawater contains small biomolecules, or metabolites, that are highly labile components of dissolved organic matter (DOM). Marine microbes interact by exchanging metabolites, thus shaping marine microbial ecology, DOM composition, and global carbon cycling. To better constrain one set of microbe‐metabolite interactions, we cultured the marine gammaproteobacterium Alteromonas macleodii MIT1002 on a range of compounds excreted by a sympatric cyanobacterium, Prochlorococcus . Alteromonas macleodii MIT1002 could metabolize the branched‐chain amino acids leucine, isoleucine, and valine, as well as 3‐methyl‐2‐oxobutanoic acid (a branched‐chain ketoacid intermediate of valine metabolism), but not thymidine, kynurenine, 4‐hydroxybenzoic acid, nor the other branched‐chain ketoacids. The utilization of branched‐chain amino acids indicates that A. macleodii MIT1002 can metabolize each corresponding ketoacid, suggesting that transporter specificity underlies the observed substrate specificity for 3‐methyl‐2‐oxobutanoic acid. These experiments show that even subtle changes in chemical structure can result in different microbial interactions and different fates for dissolved metabolites.