Fig. 6: Hypothesized contributions to excess oxygen removal rates relative to bacterioplankton organic carbon removal rates.

Two hypothesized mechanisms involving the breakdown of DOM could have contributed to the observation of higher oxygen removal rates than were predicted from the rate of carbon removal and commonly used respiratory quotients (see O₂ vs. TOC departures in Fig. 1b). a In mechanism (a), methyl sugars are partially oxidized, and methanol or formaldehyde is released. b Mechanism (b) involves partial oxidation of nitrogen-containing organoheterocyclic compounds, an example organoheterocyclic compounds of those that were significantly correlated with the oxidation state of removed DOM in Fig. 3a. Detected library matches within the nitrogen-containing organoheterocyclic class, and that were consistently decreasing with time in all bioassays, included tryptophan, quinoline, and caffeine. In this second mechanism, we hypothesize oxygen will be used and incorporated into the compounds with comparatively low bacterioplankton respiration of TOC. Also noted are both the enzyme commission (EC) numbers reported in BioCyc for the degradation of those compounds and the responders in Fig. 5b, whose publicly available genomes (>99.0% similarity to our sequences) also contained the same EC numbers.