Marine microbial community response to molecular classes of DOM

Here, we examine if and how natural microbial communities from the ultra-oligotrophic Eastern Mediterranean Sea (EMS) respond to addition of different molecular classes: peptides, amino acids, amino sugars, disaccharides, monosaccharides, and organic acids.Surface seawater (from 10m depth) were collected at the continental slope of the EMS, where the bottom depth was 800m (Latitude - 32 30.26 N; Longitude - 034 37.52 E) during November 11, 2019. Seawater were incubated in quadruplicate in 4.5L bottles in running seawater pools to maintain ambient temperature of 25C in the dark for 24 hours. Seawater were amended in the lab, approximately 15 hours after collected onboard. Seawater were amended with six defined molecular classes of DOM, each at a concentration of 25 uM. We also included a negative control without any modification, and a control amended only with NH4 and PO4. Added concentration of NH4 was set to 130 uM and PO4 to 5 uM. This resulted in N:P ratio of 27:1, which is in accordance with the seep water masses of the EMS (Ben Ezra et al., 2021).