THE AVAILABILTY OF UREA AND ITS MICROBIAL UPTAKE IN THE SOUTHEASTERN MEDITERRANEAN SEA

Urea is an important bioavailable organic nitrogen (N) compound directly utilized by heterotrophic bacteria as well as some photoautotrophs. In this study, the distribution of urea was determined in the upper water column of the southeastern Mediterranean Sea during three consecutive periods: 1) a thermally stratified oligotrophic period depleted in dissolved inorganic N and phosphorus; 2) temporary mesotrophy after a winter storm that injected a major pulse of nutrients into the photic zone; 3) decline in dissolved inorganic nutrients during the subsequent seasonal thermal stratification. Urea and total dissolved organic nitrogen (DON) concentrations peaked during the winter period after Storm Carmel (Dec 2021) and decreased during the following summer. We show evidence of urea removal above the nitracline, a zone of high nitrification rates, with increased urea below that depth. Our study showed that bioavailable N was predominantly taken up by the microbial populations in the following order ammonia>urea>nitrate, excluding immediately after Storm Carmel when the preferred utilization was ammonia> nitrate>urea. The in-situ uptake ratio of inorganic C:bioavailable N ranged from 1.5 to 4 while the measured C:N ratio was 8-10. We hypothesize that this discrepancy results from microbial utilization of dissolved organic carbon, not labelled by the spiked sodium bicarbonate, and thus not accounted for. The highest rates of in-situ N uptake were for urea, which emphasises the importance of DON uptake as a source of N in the extreme inorganic N-depleted southeastern Mediterranean.

Presentation Preference: Oral

Primary Presenter: Ilana Berman-Frank, University of Haifa (iberman2@univ.haifa.ac.il)

Authors:

Alon Blachinsky, Leon H. Charney School of Marine Sciences, University of Haifa  (blachinski111@gmail.com)

Tal Ben-Ezra, Leon H. Charney School of Marine Sciences, University of Haifa  (tal.benezra13@gmail.com)

Anat Tsemel, Leon Charney School of Marine Sciences, University of Haifa  (anattsemel@gmail.com)

Shiran Gozali, Leon H. Charney School of Marine Sciences, University of Haifa  (shirkans07@gmail.com)

Dan Tchernov, Leon H. Charney School of Marine Sciences, University of Haifa  (dtchernov@univ.haifa.ac.il)

Mark Altabet, School for Marine Science & Technology, UMASS Dartmouth  (maltabet@umassd.edu)

Ilana Berman-Frank, Leon H. Charney School of Marine Sciences, University of Haifa  (iberman2@univ.haifa.ac.il)

Michael Krom, Leon H. Charney School of Marine Sciences, University of Haifa and University of Leeds  (M.D.Krom@leeds.ac.uk)