Maja Mejdandžić

Colonization of diatoms and bacteria on artificial substrates in the northeastern coastal Adriatic Sea

Abstract Every surface that is immersed in seawater becomes rapidly covered with an unavoidable biofilm. Such biofilm formation, also known as fouling, is a complex multistage process and not yet thoroughly investigated. In this study, the succession of diatoms and bacteria was investigated during a one month exposure on an artificial substrate of plexiglass (polymer of methyl methacrylate) mounted above the seafloor at a depth of 5 m. For biofilm analyses, the fouling was investigated using selective agar plates, epifluorescence, light and electronic microscopy, as well as high performance liquid chromatography (HPLC) pigment analysis. During biofilm development, the abundance of all biofilm components increased and reached maximum values after a one month exposure. In the bacterial community, heterotrophic marine bacteria were dominant and reached 1.96 ± 0.79 × 104 colony forming units (CFU) cm-2. Despite the fact that faecal coliforms and intestinal enterococci were detected in the water column, faecal coliforms were not detected in the biofilm and intestinal enterococci appeared after one month of exposure but in the negligible number of 60 ± 10 CFU cm-2. The phototrophic component of the biofilm was dominated by diatoms and reached a concentration of 6.10 × 105 cells cm-2, which was supported by pigment analysis with fucoxanthin as dominant pigment in a concentration up to 110 ng cm-2. The diatom community was dominated by Cylindrotheca closterium and other pennate benthic diatoms. A detailed taxonomic analysis by electronic microscopy revealed 30 different taxa of diatoms. The study confirmed that a plexiglass surface in a marine environment is susceptible to biofouling within 30 days of contact. Furthermore, the co lonization process sequence firstly involved bacteria and cyanobacteria, and secondly diatoms, which together formed a primary biofilm in the sea.

Multigene phylogeny and morphology of newly isolated strain of Pseudo-nitzschia mannii Amato & Montresor (Adriatic Sea)

An increasing number of cryptic and pseudo-cryptic species have been found within many newly described diatom species. To resolve the phylogenetic relationships of the genus Pseudo-nitzschia, molecular markers are being widely used in combination (or separately) with different morphological characters. Sequence analysis of ribosomal DNA markers (18S, ITS and 28S) and morphological analyses of Pseudo-nitzschia mannii strain (CIM_D-4), isolated from the Telašćica Bay (Adriatic Sea), differentiate it from all other currently reported strains of this species.

Morphological diversity and phylogeny of the diatom genus Entomoneis (Bacillariophyta) in marine plankton: six new species from the Adriatic Sea

The diatom genus Entomoneis is known from the benthos and plankton of marine, brackish, and freshwaters. Entomoneis includes diatoms with a bilobate keel elevated above the valve surface, a sigmoid canal raphe, and numerous girdle bands. Owing mostly to the scarcity of molecular data for a diverse set of species, the phylogeny of Entomoneis has not been investigated in depth. The few previous studies that included Entomoneis were focused on broader questions and the available data were from a small number of either unidentified Entomoneis or well‐known species (e.g., E. paludosa). Since the first description of new species combining both molecular and morphological characters (E. tenera), we have continued to cultivate and investigate Entomoneis in the plankton of the Adriatic Sea. Combined multigene phylogeny (SSU rDNA sequences, rbcL, and psbC genes) and morphological observations (LM, SEM and TEM) revealed six new Entomoneis species supported by phylogenetic and morphological data: E. pusilla, E. gracilis, E. vilicicii, E. infula, E. adriatica, and E. umbratica. The most important morphological features for species delineation were cell shape, the degree and mode of torsion, valve apices, the appearance and structure of the transition between keel and valve body, the ultrastructure and the shape of the girdle bands, and the arrangement and density of perforations along the valve and valvocopulae. Our results highlight the underappreciated diversity of Entomoneis and call for a more in‐depth morphological and molecular investigation of this genus especially in planktonic habitats.

Blue Diatoms: Global Phenomenon of ˝Greening˝ in Shellfish and Record of Planktonic Haslea Species in the South Adriatic Sea

Diatoms are unicellular, photoautotrophic eukaryotic microorganisms, often forming colonies and can be found in most aquatic and moist terrestrial habitats. All known diatoms today have specific golden-brown pigment fucoxanthin that masks chlorophylls in diatom plastid, but one genus represents an exception, having the additional specific pigment marennine due to whom the cells appear blue. Blue diatoms from genus Haslea cause a global phenomenon of ˝greening˝ in shellfish (mostly oysters) affecting them in both positive and negative ways. In this study, historical and recent review regarding blue diatoms and physiological and behavioural effect of marennine as well as challenges in shellfish farming from diatom perspective is addressed. This study is also a first record of blue Haslea diatom in the South Adriatic Sea during BIOTA (Bio-tracing Adriatic Water Masses) cruise in March 2016. Investigated blue diatom was cultured in laboratory and morphologically analysed with light microscopy. Diatom investigations are very important for better understanding of the ecology of specific marine area, but also for the economy, aquaculture and tourism. The emergence of green coloured flesh of shellfish in the Adriatic Sea has not been recorded yet, but this finding of the blue diatom from genus Haslea does not rule out this possibility in the future.