ORIGINAL ARTICLE Dynamics of Heterocapsa sp. and the associated attached and free-living bacteria under the influence of dispersed and undispersed crude oil T. Severin, H.P. Bacosa, A. Sato and D.L. Erdner Marine Science Institute, The University of Texas at Austin, Port Aransas, TX, USA Significance and Impact of the Study: In the environment, oil spills have the capacity to modify phyto- plankton communities, with important consequences on the food web and the carbon cycle. We are just beginning to understand the oil resistance of phytoplankton species, making it difficult to predict community response. In this study we highlighted the strong resistance of Heterocapsa sp. to oil, which could be associated with its resilient attached bacteria and oil degradation by the free-living bacteria. This finding suggests new directions to explore in the understanding of oil impacts and interactions between eukaryotic and prokaryotic microbes. Keywords attached bacteria, dinoflagellate, dispersant, free-living bacteria, oil, phytoplankton- bacteria interaction. Correspondence Tatiana Severin, Marine Science Institute, The University of Texas at Austin, 750 Channel View Drive, Port Aransas, TX 78373-5015, USA. E-mail: tatiana.severin@utexas.austin.edu 2016/0827: received 18 April 2016, revised 11 July 2016 and accepted 20 August 2016 doi:10.1111/lam.12661 Abstract While many studies have examined the impact of oil on phytoplankton or bacteria, very few considered the effects on the biological complex formed by phytoplankton and their associated phytoplankton-attached (PA) and free- living (FL) bacteria. However, associated bacteria can affect the physiology of phytoplankton and influence their stress responses. In this study, we monitored the growth of Heterocapsa sp., an armoured dinoflagellate, exposed to crude oil, Corexit dispersant, or both. Growth of Heterocapsa sp. is unaffected by crude oil up to 25 ppm, a concentration similar to the lower range measured on Florida beaches after the Deepwater Horizon oil spill. The PA bacteria community was resistant to exposure, whereas the FL community shifted towards oil degraders; both responses could contribute to Heterocapsa sp. oil resistance. The growth rate of Heterocapsa sp. decreased significantly only when exposed to dispersed oil at 25 ppm, indicating a synergistic effect of dispersant on oil toxicity in this organism. For the first time, we demonstrated the decoupling of the responses of the PA and FL bacteria communities after exposure to an environmental stress, in this case oil and dispersant. Our findings suggest new directions to explore in the understanding of interactions between unicellular eukaryotes and prokaryotes. Introduction Because they are responsible for about 50% of annual glo- bal carbon dioxide fixation (Field 1998), marine phyto- plankton are a crucial part of the global carbon cycle and form the base of the food web in aquatic systems. These important functions can be altered by both acute and chronic disturbances such as oil spills. The Deepwater Horizon (DWH) oil spill in April 2010 is the largest acci- dental oil spill in the United States history and affected the Gulf of Mexico coast from Louisiana to Florida (Sam- marco et al. 2013). A remote sensing study suggests that this massive oil spill stimulated an intense phytoplankton bloom observed 4 months afterwards in the north-eastern Gulf of Mexico (Hu et al. 2011). However, most labora- tory experiments observe a decrease in total chlorophyll a after oil addition (Gonz alez et al. 2009; Jung et al. 2012). They also show that oil responses are species-dependent with a greater tolerance in diatoms and dinoflagellates (Gilde and Pinckney 2012; € Ozhan et al. 2014). Letters in Applied Microbiology 63, 419--425 © 2016 The Society for Applied Microbiology 419 Letters in Applied Microbiology ISSN 0266-8254