Microbial community structure in the sea surface microlayer at two contrasting coastal sites in thenorthwestern Mediterranean Sea

International audienceIn an attempt to compare the microbial community structure between the sea surfacemicrolayer (SML) and subsurface waters (SSW), we determined the enrichment factors (EF: the ratioof abundance or activity in the SML to abundance or activity in SSW) of 13 biological parameters.Samples were taken at 2 contrasting coastal sites in the Mediterranean Sea, corresponding to a high(Barcelona, Spain) and low (Banyuls-sur-Mer, France) urbanized area. Principal component analysisshowed that temporal variability was much higher at Barcelona than at Banyuls, and that thecharacteristics of the SML and SSW samples were more closely related at Barcelona. At both sites, theSML was weakly enriched in heterotrophic bacteria (on average 1.1-fold), Synechococcus spp. (onaverage 1.1-fold), and photosynthetic picoeukaryotes (on average 1.5-fold) relative to SSW. Incontrast, the SML was considerably enriched in chlorophyll a (chl a) (on average 1.9-fold), phaeophytina (on average 7.4-fold), autotrophic (on average 6.1-fold) and heterotrophic nanoflagellates(on average 5.1-fold) relative to SSW. Enrichments in bacterial production and culturable bacteriawere highly variable. EF were significantly different between the 2 sites only for concentrations of chla, b, and c and the abundance of autotrophic nanoflagellates, with higher EF at the Barcelona site.Except for autotrophic flagellates, the abundance or activity of the parameters determined in the SMLwas highly correlated with that determined in SSW, suggesting that enrichment of the SML resultsmainly from upward transport of microorganisms attached to buoyant particles or bubble scavenging.In contrast, the high contribution of autotrophic nanoflagellates to overall phytoneuston biomass(mean = 26%) is likely due to their rapid colonization of the SML. The high abundances of auto- andheterotrophic nanoflagellates in the SML indicated that these organisms play a key role in thefunctioning of the microbial food webs at the air–sea interface