Microbial transformation of jellyfish organic matter affects the nitrogen cycle in the marine water column — A Black Sea case study Tinkara Tinta a, ⁎ ,1 , Tjaša Kogovšek a,c,1 , Valentina Turk a , Tamara A. Shiganova b , Alexander S. Mikaelyan b , Alenka Malej a a National Institute of Biology, Marine Biology Station, Fornače 41, 6330 Piran, Slovenia b P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Nakhimovsky Prospekt 36, 117997 Moscow, Russia c Graduate School of Biosphere Science, Hiroshima University, 4-4 Kagamiyama 1 Chome, Higashi-Hiroshima 739-8528, Japan abstract article info Article history: Received 1 May 2015 Received in revised form 7 September 2015 Accepted 30 October 2015 Available online xxxx Keywords: Jellyfish POM Bacterial community Stable isotopes Nitrogen cycle The Black Sea The increasing trend in jellyfish blooms that have been observed in some coastal areas around the world can have serious ecological consequences. In particular, the fate of jellyfish organic matter (jelly-OM), after the decay of jellyfish blooms, and their implications for marine biogeochemical cycles and ecosystem functioning, are still un- clear. In order to study bacteria–jelly-OM interactions and the associated fate of the jelly-OM, we conducted two sets of short-term jelly-OM enrichment experiments using coastal and offshore ambient pelagic bacterial assem- blages from the Black Sea, where the scyphozoan medusa Aurelia aurita blooms seasonally. The microbial trans- formation of the jelly-OM was followed using a stable δ 13 C and δ 15 N analyses of particulate jelly-OM together with standard organic and inorganic matter chemical analyses. The effect of the jelly-OM on the ambient bacterial community was investigated by following changes in bacterial abundance, growth rates, and community struc- ture. The Black Sea's surface bacterial assemblages from both systems, coastal and offshore, responded rapidly to the jelly-OM enrichment, preferentially utilizing nitrogen-rich constituents of the jelly-OM, leaving carbon- enriched particulate OM (hypothetically recalcitrant) in the system. The end products of the bacteria-mediated jelly-OM degradation process, i.e. total dissolved nitrogen and ammonium, accumulated in the system, indicating possible implications for the nitrogen cycle. Despite the differences in the Black Sea's coastal and offshore seawa- ter background nutrient concentrations and particulate OM quality, the nitrogen budget was very much the same in both studied systems, however there were differences in the bacterial community function/performance from these two environments. The addition of jelly-OM triggered different structural changes in the coastal and offshore ambient bacterial communities, suggesting that different bacterial groups were capable of utilizing jelly-OM. A comparison of the response of natural bacterial community to the jelly-OM and the bacterial trans- formation of the jelly-OM in different marine ecosystems indicates that the degree of bacterial growth rate and the rate of ammonium accumulation depend on the incidence of jellyfish occurrence, physiochemical environ- mental conditions, and possibly also on ambient bacterial community composition. Our study provides insights into the nature of bacteria–jelly-OM interactions, the processes and mechanisms of bacterial jelly-OM transfor- mation, and the consequences for marine nitrogen (and carbon) cycle, as well as for the functioning of different coastal marine ecosystems. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Over the past few decades, longer and more frequent jellyfish out- breaks have been reported in some coastal areas, as well as in some large marine ecosystems around the world (Kogovšek et al., 2010; Brotz et al., 2012; Condon et al., 2013). There is a debate over whether jellyfish populations are indeed increasing globally or that the recorded increase in their blooms is just a part of long-term natural oscillations in jellyfish populations (Condon et al., 2012, 2013 and the reference there- in). In spite of that controversy, jellyfish can have serious ecological, as well as socio-economic consequences (Purcell et al., 2007; Richardson et al., 2009). Although the exact drivers of jellyfish fluctuations are un- known, they are most likely a consequence of the combined effect of multiple anthropogenic stressors and/or climate changes (Molinero et al., 2008; Richardson et al., 2009; Purcell et al., 2007; Duarte et al., 2013; Purcell, 2005, 2012). When blooming, jellyfish populations can attain an enormous bio- mass in a relatively short period of time (Condon and Steinberg, 2008; Condon et al., 2011). During their life span, jellyfish play an important role in providing carbon, nitrogen, and other nutrients to the microbial Journal of Experimental Marine Biology and Ecology 475 (2016) 19–30 ⁎ Corresponding author. E-mail address: tinta@mbss.org (T. Tinta). 1 Authors contributed equally to the manuscript. http://dx.doi.org/10.1016/j.jembe.2015.10.018 0022-0981/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe