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The oligotrophy of the southern Adriatic Sea is characterized by seasonal stratification which enables nutrient supply to the euphotic layer. A set of interdisciplinary methods was used to elucidate the diversity and co-dependency of bacterio- and phytoplankton of the water column during the stratification period of July 2021. A total of 95 taxa were determined by microscopy: 58 diatoms, 27 dinoflagellates, 6 coccolithophores, and 4 other autotrophs, which included Chlorophyceae, Chrysophyceae, and Cryptophytes. Nanophytoplankton abundances were higher in comparison to microphytoplankton. The prokaryotic plankton community as revealed by HTS was dominated by Proteobacteria (41–73%), Bacteroidota (9.5–27%), and cyanobacteria (1–10%), while the eukaryotic plankton community was composed of parasitic Syndiniales (45–80%), Ochrophyta (2–18%), Ciliophora (2–21%), Chlorophytes (2–4%), Haptophytes (1–4%), Bacillariophyta (1–13%), Pelagophyta (0.5–12%) and Chrysophyta (0.5–3%). Flow cytometry analysis has recorded Prochlorococcus and photosynthetic picoeukaryotes as more abundant in deep chlorophyll maximum (DCM), and Synechococcus and heterotrophic bacteria as most abundant in surface and thermocline layers. Surface, thermocline, and DCM layers were distinct considering community diversity, temperature, and nutrient correlations, while extreme nutrient values at the beginning of the investigating period indicated a possible nutrient flux. Nutrient and temperature were recognized as the main environmental drivers of phytoplankton and bacterioplankton community abundance.

The paraphyletic diatom genus Nitzschia comprises over 1000 morphologically distinct pennate taxa, known from the benthos and plankton of freshwater, brackish and marine environments. The principal diagnostic characters for delimitation of Nitzschia species include valve shape, the position and structure of the raphe, presence/absence and shape of the proximal raphe endings the shape of the terminal raphe fissures, areola structure, and specific morphometric features such as cell size, and stria and fibula density. In this study, we isolated 12 diatom strains into culture from samples collected at the surface or greater depths of the southeastern Adriatic Sea. Morphological analyses included LM, SEM and TEM observations, which, along with specific morphometric features, allowed us to distinguish three new Nitzschia species. These findings were congruent with the results of phylogenetic analyses performed on nuclear-encoded SSU (18S) rDNA and chloroplast-encoded rbcL and psbC genes. One of the new species (Nitzschia dalmatica sp. nov.) formed a lineage within a clade of Bacillariaceae containing members of the Nitzschia sect. Dubiae, which was sister to Psammodictyon. A second lineage was part of a novel clade that is significantly distinct from other Nitzschia species sequenced so far and includes Nitzschia adhaerens sp. nov. and N. cf. adhaerens. A further new species was found, Nitzschia inordinata sp. nov., which appeared as the sister group to the N. adhaerens clade and the conopeoid Nitzschia species in our phylogenetic trees. Our findings contribute to the overall diversity of genus Nitzschia, especially in identifying some deep branches within the Bacillariaceae, and highlight under-scoring of this genus in marine plankton.

A new marine strain of picoplanktonic algae, PMPFPPE4, was isolated from a mixed net-phytoplankton sample taken from the upper euphotic layer of the southeastern Adriatic Sea. Evaluation of the new strain included morphological investigation (by light and electron microscopy), phylogenetic analysis (utilizing plastid 16S rRNA and nuclear 18S rRNA genes), and physiological characterization (screening of pigment/lipid composition and capturing photosynthesis measurements). The new strain was proven to belong to the genusPicochlorumand the lipid composition revealed an unexpected accumulation of triacylglycerols, indicating an evolutionary adaptation for growth under unfavourable conditions. In addition, lipid remodelling in the exponential to stationary growth phase was characterized by an increased share of membrane-forming digalactosyldiacylglycerols and phosphatidylcholines. Maximum photosynthetic activity measured was at 30 degrees C, but the most rapid increase of photosynthetic activity was at lower temperatures (15-20 degrees C). Moreover, the thermotolerant strain did not exhibit photoinhibition below 40 degrees C and survived a one-month cultivation period in complete darkness. The strain's survival in low light and dark conditions suggests a potential shift from autotrophy to mixotrophy under unfavourable growth conditions. Thus, the unique physiological attributes represented by a high growth rate, thermotolerance, phototolerance and high triacylglycerol synthesis may render the strain highly attractive for biofuel production and growth in large outdoor systems.

Simple Summary Diatoms are microalgae known for their ecological importance. Among them, just a few species are able to produce a blue pigment. We describe Haslea silbo sp. nov., a cosmopolitan species of blue diatoms, found on both sides of the Atlantic Ocean. The description includes the use of both microscopy and next generation sequencing. It has been possible to observe its reproduction in the laboratory, and the blue pigment it produces has also been studied. Abstract Specimens of a new species of blue diatoms from the genus Haslea Simonsen were discovered in geographically distant sampling sites, first in the Canary Archipelago, then North Carolina, Gulf of Naples, the Croatian South Adriatic Sea, and Turkish coast of the Eastern Mediterranean Sea. An exhaustive characterization of these specimens, using a combined morphological and genomic approach led to the conclusion that they belong to a single new to science cosmopolitan species, Haslea silbo sp. nov. A preliminary characterization of its blue pigment shows similarities to marennine produced by Haslea ostrearia, as evidenced by UV–visible spectrophotometry and Raman spectrometry. Life cycle stages including auxosporulation were also observed, providing data on the cardinal points of this species. For the two most geographically distant populations (North Carolina and East Mediterranean), complete mitochondrial and plastid genomes were sequenced. The mitogenomes of both strains share a rare atp6 pseudogene, but the number, nature, and positions of the group II introns inside its cox1 gene differ between the two populations. There are also two pairs of genes fused in single ORFs. The plastid genomes are characterized by large regions of recombination with plasmid DNA, which are in both cases located between the ycf35 and psbA genes, but whose content differs between the strains. The two sequenced strains hosts three plasmids coding for putative serine recombinase protein whose sequences are compared, and four out of six of these plasmids were highly conserved.

The genus Entomoneis includes diatoms with an elevated bilobate keel, a sigmoid raphe canal and numerous girdle bands. It is known to inhabit various environments, from freshwater to marine, both plankton and benthos. During a phytoplankton investigation in the Kungälv estuary on the west coast of Sweden, we observed numerous cells belonging to Entomoneis. Further morphological investigations performed with light and electron microscopy allowed us to describe Entomoneis annagodhei, sp. nov., with a unique set of morphological characters. Most importantly, a still undocumented character proved to be an oblique transapical fascia across the centre of the valve. Other distinctive features of this species are a pronounced raphe canal with dense raphe fibulae, very fine striation, resolvable only with electron microscopy, external lanceolate slit-like opening of the central nodule, and fine, linear to undulate external ridges running more or less parallel and adjacent to the raphe and valve margin. The morphological characters and morphometry are discussed in comparison with similar taxa. Our results contribute to the under-appreciated diversity of Entomoneis, especially inhabiting the marine plankton.

When the nameEntomoneis annagodheiAl-Handal &Mucko was published (Al Handal et al.2020), an errorwas made in giving the name of the species. The species isnamed after the late Professor Dr Anna Godhe and hencethe name should have the feminine ending-ae.Asthisisa correctable error according to art. 60.8 of the Interna-tional Code of Nomenclature for algae, fungi, and plants(Shenzhen Code), we therefore wish to correct the name toEntomoneis annagodheaeAl- Handal & Mucko (Turlandet al.2018).The PhycoBank registration for this species name, http://phycobank.org/102503 shows the correction.

Bacteria are the most important microorganisms in the world oceans, accounting for up to 75% of the total biomass. They are responsible for fundamental biogeochemical processes and therefore often used as ecological indicators. In this study, bacteria were quantified by flow cytometry and their diversity assessed by High Throughput Sequencing (HTS) in the southern Adriatic Sea. The most abundant bacterial groups were also quantified by qPCR. The samples were collected from the surface to the seabed over a total of 16 different depths at four stations during the late winter BIOTA (BIO-Tracing Adriatic water masses) cruise conducted in March 2016. The investigated area showed unusual water mass properties and was characterized by a shallow mixed layer, which differed from the usual winter convection conditions, typical of middle-altitude ecosystems and important for the seasonal picoplankton dynamics of this area. Heterotrophic bacteria were separated into HNA (relative High Nucleic Acid content) and LNA (Low Nucleic Acid content) subpopulations with abundances up to 1.8 × 105 and 8.8 × 105 cells mL−1, respectively. HNA dominated at offshore stations reaching their maximum at depths below the euphotic zone. The bacterial community was dominated by Alphaproteobacteria, accounting for >40% of the total sequence reads and were mainly represented by the SAR11 clade (90.84%), followed by Marinimicrobia (18% of the total sequence reads), mainly represented by clade SAR406 (8.44%). Distinctive bacterial groups were found in the euphotic layer (Bacteroidetes and Actinobacteria) and aphotic layer samples (Deltaproteobacteria, Marinimicrobia, Chloroflexi, Acidobacteria and Planctomycetes). Results of the qPCR analyses further confirmed HTS results with highest abundances obtained for Alphaproteobacteria, followed by Gammaproteobacteria and Bacteroidetes. The adopted multiple approach, combining different molecular tools, critically supported by optics and flow cytometry, reveal changes in the bacterial assemblages during the unusual thermohaline conditions observed in 2016 in the southern Adriatic Sea.

Marine picoplankton, unicellular organisms with cell sizes up to 3 μm in diameter, numerically dominate marine ecosystems, encompassing Archaea, Bacteria, Eukarya (protists and fungi) as well as viruses. Autotrophic and heterotrophic picoplankton abundance and community composition with a focus on picoeukaryotes (PEs) were investigated in the winter of 2016 at three stations along a coast-to-offshore transect in the southern Adriatic Sea. Abundances were estimated by flow cytometry, while community composition by Illumina High-Throughput Sequencing (HTS) of 16S and 18S rRNA genes. The photosynthetic picoplankton diversity was also investigated by High-Performance Liquid Chromatography (HPLC) of liposoluble pigments. Heterotrophic bacteria and cyanobacteria (Prochlorococcus and Synechococcus) accounted for up to 7 × 105; 2.3 × 104 and 2.5 × 104 cells mL−1, respectively, while photosynthetic picoeukaryotes peaked with 3 × 103 cells mL−1. Prokaryotes, as revealed by HTS were dominated by Alphaproteobacteria (mainly SAR11, 44.91% of total 16S sequence reads), followed by Gammaproteobacteria (Oceanospirillales and Pseudomonadales, 14.96%), Bacteroidetes (mainly Flavobacteriales, 13%), Cyanobacteria (Prochlorococcus and Synechococcus, 9.52%), Marinimicrobia (SAR406, 7.97%), Deltaproteobacteria (SAR324, 3.83%), Actinobacteria (2.24%) and Chloroflexi (SAR202, 1.90%). Photosynthetic pigment concentrations were very low (12.12 μgL−1 at the most) and taxonomic pigments could be attributed to Prochlorococcus, Synechococcus, Prymnesiophyceae, Bacillariophyceae, Chrysophyceae, and Prasinophyceae. HTS data revealed that PEs were dominated by heterotrophs, such as Syndiniophyceae, parasitic dinoflagellates (79.67% of total 18S sequence reads), Dinophyceae (8.7%) and the radiolarians Collodaria belonging to Sphaerozoidae (22.1%) and Spumellaria (5.0%). On the other hand, photoautotrophs, including Chlorophyta (Mamiellophyceae, Prasinophyceae, Trebouxiophyceae, and Ulvophyceae), Stramenopiles (Bacillariophyta, Chrysophyceae, Dictyochophyceae, Pelagophyceae), photoautotrophic Cryptophyta and some Haptophyta (Prymnesiophyceae), did not exceed 5% of total sequence reads. This study provides the first snapshot of the PEs diversity in oligotrophic euphotic waters of the southern Adriatic Sea, hence setting the stage for large-scale surveying and characterization of the eukaryotic diversity in the entire basin.

Rod Entomoneis Ehrenberg obuhvaća vrste penatnih dijatomeja koje su uglavnom opisane iz bentičkih brakičnih i morskih staništa te ih vrlo rijetko nalazimo u planktonu i slatkim vodama. Ovaj rod dijatomeja karakterizira složena struktura silicijevih ljušturica, sinusoidni uzdignuti kanal rafe, morfološki promjenjive fibule, brojni pojasevi između valvi te česta torzija samih stanica s obzirom na uzdužnu os. U Jadranskom moru nalazimo samo šest vrsta roda Entomoneis, od kojih smo prvu planktonsku vrstu, Entomoneis tenera, otkrile 2015. prilikom terenskog istraživanja južnog Jadrana te objavile opis vrste 2017. godine (Mejdandžić i i sur. 2017). Istraživanja u sklopu projekta Hrvatske zaklade za znanost - „Bio-tracing Adriatic water masses“ (BIOTA) su nastavljena i u ožujku 2016. sa nizom uzorkovanja i mjerenja na području obalnog i otvorenog mora od Dubrovnika do južnojadranske kotline. Za potrebe detaljnijih taksonomskih istraživanja prikupljali su se živi uzorci fitoplanktona s različitih dubina (površina, 30 m, 100 m, 250 m) u kojima smo primijetile nove, do sada nepoznate vrste roda Entomoneis. Pojedinačne stanice iz uzoraka smo izolirale u dvanaest monoklonalnih laboratorijskih kultura na kojima su se provela iscrpna morfološka (svjetlosna i elektronska mikroskopija) i molekularna (tri genska markera i filogenetske analize) istraživanja. Rezultati morfometrije, morfoloških karakteristika i filogenije potvrdili su da se radi o šest novih vrsta za znanost (Sl. 1) koje su nazvane redom: Entomoneis pusilla Bosak & Mejdandžić što znači „vrlo mala“ jer je do sada to najmanja vrsta ovog roda, E. gracilis Mejdandžić & Bosak prema „vitkom” izgledu stanica, E. infula Mejdandžić & Bosak opisujući sedlasti izgled stanica uslijed torzije, E. adriatica Mejdandžić & Bosak koja je ime dobila prema lokalitetu pronalaska, Jadranskom moru, te E. umbratica Mejdandžić & Bosak čije ime znači „živi u sjeni“ jer je pronađena u uzorku uzetom s 250 m dubine. Posljednja novootkrivena vrsta je dobila ime E. vilicicii Bosak & Mejdandžić u čast profesoru emeritusu Damiru Viličiću koji je svojim radom značajno pridonio straživanjima fitoplanktona Jadrana. Svi opisi novih vrsta iz Jadrana napravljeni su u suradnji s kolegama s Biološkog odsjeka PMF-a (Zagreb), Instituta Ruđer Bošković, Centra za forenzična ispitivanja, istraživanja i vještačenja Ivan Vučetić te Sveučilišta u Arkansasu (SAD) te je rad objavljen u prestižnom znanstvenom časopisu Journal of Phycology.