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The spatial patterns of aerobic anoxygenic phototrophs abundances were investigated, for the first time, in the Adriatic Sea. Also, the spatial patterns of the whole picoplankton community as well as the environmental factors that potentially influence these patterns were highlighted. AAP abundances was in average 66.9 ± 66.8 × 10 3 cell mL −1 , and their proportion in total bacteria was 7.3 ± 4.3%. These values are in the upper range of AAP abundances observed in marine environments. Multivariate analyses proved that environmental factors influenced the picoplankton community interdependently. Chl a was the main driving factor for the picoplankton community, accounting for 33.3% of picoplankton community variance, followed by NO 2 − (17.9% of variance explained) and temperature (14.2% of variance explained). Chl a showed stronger correlation with AAPs, non-pigmented bacteria and Picoeucaryotes than with cyanobacteria. Abundance of cyanobacteria was stronger correlated to salinity and the N:P ratio than to nutrient concentrations. [ABSTRACT FROM AUTHOR]

Bacteria are an active and diverse component of pelagic communities. The identification of main factors governing microbial diversity and spatial distribution requires advanced mathematical analyses. Here, the bacterial community composition was analysed, along with a depth profile, in the open Adriatic Sea using amplicon sequencing of bacterial 16S rRNA and the Neural gas algorithm. The performed analysis classified the sample into four best matching units representing heterogenic patterns of the bacterial community composition. The observed parameters were more differentiated by depth than by area, with temperature and identified salinity as important environmental variables. The highest diversity was observed at the deep chlorophyll maximum, while bacterial abundance and production peaked in the upper layers. The most of the identified genera belonged to Proteobacteria, with uncultured AEGEAN-169 and SAR116 lineages being dominant Alphaproteobacteria, and OM60 (NOR5) and SAR86 being dominant Gammaproteobacteria. Marine Synechococcus and Cyanobium-related species were predominant in the shallow layer, while Prochlorococcus MIT 9313 formed a higher portion below 50 m depth. Bacteroidota were represented mostly by uncultured lineages (NS4, NS5 and NS9 marine lineages). In contrast, Actinobacteriota were dominated by a candidatus genus Ca. Actinomarina. A large contribution of Nitrospinae was evident at the deepest investigated layer. Our results document that neural network analysis of environmental data may provide a novel insight into factors affecting picoplankton in the open sea environment. [ABSTRACT FROM AUTHOR]

Distribution and abundance of two Synechococcus ecotypes, phycocyanin-rich cells (PC-SYN) and phycoerythrin-rich cells (PE-SYN) were studied in the surface layer of the central Adriatic Sea during the 2015-2016 period. The studied area included several estuarine areas, and coastal to open sea trophic gradients, covering a wide range of seawater temperatures (11.82 - 20.75oC), salinity (4.47 - 38.84) and nutrient concentration. The abundance of PC-SYN ranged from 0 to 79.79 x 103 cell mL-1 and that of PE-SYN from 5.01 x 103 to 76.74 x 103 cell mL-1. Both ecotypes coexisted in the studied waters with PC-SYN cells dominating during spring and PE-SYN during winter and autumn. PC-SYN showed a significant positive relationship with temperature and strong positive responses to nitrogen nutrients, whereas PE-SYN positively responded to phosphate availability. The relative ratio of phosphorus availability and total inorganic nitrogen nutrients (N/P ratio) affects the spatial distribution of the two Synechococcus ecotypes. [ABSTRACT FROM AUTHOR]

Global and atmospheric climate change is altering the thermal conditions in the Adriatic Sea and, consequently, the marine ecosystem. Along the eastern Adriatic coast sea surface temperature (SST) increased by an average of 1.03 °C during the period from 1979 to 2015, while in the recent period, starting from 2008, a strong upward almost linear trend of 0.013 °C/month was noted. Being mainly oligotrophic, the middle Adriatic Sea is characterized by the important role played by the microbial food web in the production and transfer of biomass and energy towards higher trophic levels. It is very important to understand the effect of warming on microbial communities, since small temperature increases in surface seawater can greatly modify the microbial role in the global carbon cycle. In this study, the Self-Organizing Map (SOM) procedure was used to analyse the time series of a number of microbial parameters at two stations with different trophic status in the central Adriatic Sea. The results show that responses of the microbial food web (MFW) structure to temperature changes are reproducible in time. Furthermore, qualitatively similar changes in the structure of the MFW occurred regardless of the trophic status. The rise in temperature was associated with: (1) the increasing importance of microbial heterotrophic activities (increase bacterial growth and bacterial predator abundance, particularly heterotrophic nanoflagellates) and (2) the increasing importance of autotrophic picoplankton (APP) in the MFW. [ABSTRACT FROM AUTHOR]

The Mediterranean Sea (including the Adriatic Sea) has been identified as a ‘hotspot’ for climate change, with the prediction of the increase in water temperature of 2–4 °C over the next few decades. Being mainly oligotrophic, and strongly phosphorus limited, the Adriatic Sea is characterized by the important role of the microbial food web in production and transfer of biomass and energy towards higher trophic levels. We hypothesized that predicted 3 °C temperature rise in the near future might cause an increase of bacterial production and bacterial losses to grazers, which could significantly enlarge the trophic base for metazoans. This empirical study is based on a combined ‘space-for-time substitution’ analysis (which is performed on 3583 data sets) and on an experimental approach (36 in situ grazing experiments performed at different temperatures). It showed that the predicted 3 °C temperature increase (which is a result of global warming) in the near future could cause a significant increase in bacterial growth at temperatures lower than 16 °C (during the colder winter-spring period, as well as in the deeper layers). The effect of temperature on bacterial growth could be additionally doubled in conditions without phosphorus limitation. Furthermore, a 3 °C increase in temperature could double the grazing on bacteria by heterotrophic nanoflagellate (HNF) and ciliate predators and it could increase the proportion of bacterial production transferred to the metazoan food web by 42%. Therefore, it is expected that global warming may further strengthen the role of the microbial food web in a carbon cycle in the Adriatic Sea. [ABSTRACT FROM AUTHOR]