water Article Picoplankton Distribution and Activity in the Deep Waters of the Southern Adriatic Sea Danijela Šanti´c 1,* , Vedrana Kovaˇcevi´c 2, Manuel Bensi 2, Michele Giani 2 , Ana Vrdoljak Tomaš 1 , Marin Ordulj 3 , Chiara Santinelli 2, Stefanija Šestanovi´c 1, Mladen Šoli´c 1 and Branka Grbec 1 1 Institute of Oceanography and Fisheries, Šetalište Ivana Meštrovi´ca63, POB 500, 21000 Split, Croatia 2 National Institute of Oceanography and Applied Geophysics, Borgo Grotta Gigante 42/c, 34010 Sgonico (Ts), Italy 3 University of Split, University Department of Marine Studies, Ruđera Boškovi´ca37, 21000 Split, Croatia * Correspondence: [email protected]; Tel.: +385-21-408-006; Fax: +385-21-358-650 Received: 19 July 2019; Accepted: 8 August 2019; Published: 10 August 2019 Abstract: Southern Adriatic (Eastern Mediterranean Sea) is a region strongly dominated by large-scale oceanographic processes and local open-ocean dense water formation. In this study, picoplankton biomass, distribution, and activity were examined during two oceanographic cruises and analyzed in relation to environmental parameters and hydrographic conditions comparing pre and post-winter phases (December 2015, April 2016). Picoplankton density with the domination of autotrophic biomasses was higher in the pre-winter phase when significant amounts of picoaoutotrophs were also found in the meso-and bathy-pelagic layers, while Synechococcus dominated the picoautotrophic group. Higher values of bacterial production and domination of High Nucleic Acid content bacteria (HNA bacteria) were found in deep waters, especially during the post-winter phase, suggesting that bacteria can have an active role in the deep-sea environment. Aerobic anoxygenic phototrophic bacteria accounted for a small proportion of total heterotrophic bacteria but contributed up to 4% of bacterial carbon content. Changes in the picoplankton community were mainly driven by nutrient availability, heterotrophic nanoflagellates abundance, and water mass movements and mixing. Our results suggest that autotrophic and heterotrophic members of the picoplankton community are an important carbon source in the food web in the deep-sea, as well as in the epipelagic layer. Besides, viral lysis may affect the activity of the picoplankton community and enrich the water column with dissolved organic carbon. Keywords: picoplankton community; deep-sea; Southern Adriatic; Mediterranean Sea 1. Introduction Autotrophic members of the picoplankton community are important primary producers, while bacteria are consumers of the dissolved organic matter that can originate from primary production, thus transferring carbon towards higher trophic levels. Bacteria represent the main component of plankton, which is involved in the degradation of organic matter and in transforming inorganic compounds into the shapes adequate for primary producers. The role of the picoplankton community has become more important in oligotrophic and phosphorus-limited (P-limited) areas, such as the open sea area of the Adriatic Sea (Mediterranean Sea), where bacteria, Synechococcus, Prochlorococcus, and picoeukaryotes play an important role in the production and flow of biomass and energy [1–3] than in eutrophic areas. However, previous studies on picoplankton communities were mostly focused on investigating the epipelagic layer (i.e., depths less than 200 m). The deep-sea is characterized by the absence of light, i.e., conditions that are unfavorable for the primary production. Tanaka and Water 2019, 11, 1655; doi:10.3390/w11081655 www.mdpi.com/journal/water Water 2019, 11, 1655 2 of 21 Rassoulzadegan [4] pointed out the importance of bacteria and their biomass in carbon flux in the deep-sea. Moreover, Arístegui et al. [5] have highlighted that the deep ocean represents a key site for remineralization of organic matter and long-term carbon storage. The discovery of cyanobacteria Synechococcus in the deep part of the Adriatic Sea revealed that they could be used to gain a better understanding of the effects of deep-ocean convection, such as ventilation and renewal of deep waters [6]. Hence, the vertical distribution of the picoplankton in the open southern Adriatic Sea, below the euphotic zone, has recently started to be investigated more intensively [7–10]. Aerobic anoxygenic phototrophs (AAPs), a bacterial group recently discovered in the Adriatic Sea [11,12] and primarily unknown in the investigated area, are up to 3 bigger than other bacterial cells [13,14], and hence they × could represent a remarkable source of carbon in the marine environment. These photoheterotrophic microorganisms can harvest light energy using pigment bacteriochlorophyll a to supplement their primarily organotrophic metabolism only in the presence of oxygen [15]. The Adriatic Sea is an elongated semi-enclosed basin of the Eastern Mediterranean Sea. According to its morphology and bathymetry, it can be divided into three sub-basins (northern, middle, and southern). The Southern Adriatic Pit (hereafter SAP) is the deepest area, with a depth reaching ~1250 m. Adriatic is characterized by a cyclonic basin-scale circulation. Through the Strait of Otranto at its southern end (~80 km wide, with a sill depth of ~800 m) [16,17], the Adriatic exchanges water and mass with the adjacent Ionian Sea. The Eastern Adriatic Current (EAC) brings northward warm and saline waters from the Ionian Sea and the Western Adriatic Current (WAC), transporting southward colder and fresher waters [18]. Waters from the Ionian Sea enrich the P-limited Adriatic Sea [19,20] with nutrients and organic substances, causing changes in the food web [21] and the distribution of organisms [6,8,10,22]. The prevalent heat and salt import from the Ionian involve the surface and intermediate layers of the Adriatic Sea, between the surface and 800 m depth. According to Gaˇci´cet al. [23], periodical changes in the sense of the rotation between cyclonic and anticyclonic phases of the North Ionian Gyre (NIG) deviate the branch of the Atlantic water, entering the Eastern Mediterranean through the Strait of Sicily, towards the Adriatic (during anticyclonic phase) or towards the Cretan and Levantine basins (during cyclonic phase). Under these two opposite conditions, changes in the heat and salt content of the SAP may occur out of phase compared to those observed in the Levantine basin [24]. Moreover, water masses flowing into the Adriatic along its eastern flank have important ecological implications. Indeed, the pool of available nutrients in the SAP changes according to the phases of the NIG [25–27]. The deep part of the SAP occasionally receives the North Adriatic Dense Water (NAdDW), that ventilates the deepest layers after winter cooling [28,29]. Locally, the open-sea winter convection triggers vertical mixing and produces dense water (Adriatic Deep Water, AdDW) that reaches depths between 300 and 1000 m [30–32]. Subsequently, AdDW outflows across the Strait of Otranto sinks into the Ionian Sea abyss and ventilates the deep layers of the eastern Mediterranean [33]. The picoplankton community is exposed to sudden physical-chemical changes in a dynamic environment. The ability of certain members to acclimate physiologically determine their presence and activity or absence in the water column. Hence, environmental conditions (vertical convection, lateral exchanges, long-term trends, and multiannual oscillatory signals) make the Southern Adriatic an ideal laboratory to study vertical distribution and activity of picoplankton members in the deep zones (below epipelagic layer), where they can represent a significant source of carbon through their biomass. The primary goal of the ESAW (evolution and spreading of the Southern Adriatic Waters) cruise was to assess and compare the pre-and post-winter hydrographic and environmental (physical, biogeochemical) conditions in the middle and southern Adriatic basins. The objective of the present study was to evaluate for the first time, the abundance of aerobic anoxygenic phototrophs (AAPs) in the deep waters of the Southern Adriatic Pit (hereafter SAP). Beside the AAPs, we determined the distribution of Synechococcus, Prochlorococcus, picoeukaryotes, and heterotrophic bacteria and estimated their contribution to carbon budget from epipelagic to deep waters. Our study shows and Water 2019, 11, 1655 3 of 21 discusses results obtained from the analysis of picoplankton biomasses and their activity concerning environmental parameters and hydrographic conditions and the effect of grazing and viral lysis. 2. Material and Methods 2.1. Physical and Biochemical Sampling In this study, we used data collected during the two ESAW cruises (Evolution and spreading of the Southern Adriatic Waters), supported by the EUROFLEETS2 program. The first one was conducted in December 2015 (10–15 December). The second one was conducted in April 2016 (5–10 April), when the hydrographic and environmental conditions were supposed to be changed after the wintertime occurrence of vertical convection, and consequent dense water formation. The intention was to measure the signals of a possible spread of dense water from the middle to the southern Adriatic, because the former may be both an accumulation site and an occasional formation site for dense waters [34]. Here, we focused only on data collected in the SAP, at hydrological stations regularly spaced along a transect between Bari (Ba; Italy) and Dubrovnik (Du; Croatia, Figure1). A conductivity-temperature-depth (CTD) SBE 911plus probe (Sea-Bird Electronics, Bellevue,