Muharrem Balci

The diatom Pseudo-nitzschia delicatissima (Cleve) Heiden, 1928 is known a potential producer of domoic acid (DA), a neurotoxin which can cause Amnesic Shellfish Poisoning (ASP). The specific objectives of this study are to reveal the identification, based on morphological characterization via Scanning Electron Microscopy (SEM) and molecular phylogeny, and toxicity of a P. delicatissima strain (DEB132304) isolated from the Golden Horn Estuary (Sea of Marmara, Türkiye). SEM analysis revealed characteristic frustule features consistent with the general morphological description of P. delicatissima. Molecular and phylogenetic analyses based on ribosomal gene sequences from the LSU and ITS regions further supported the identification of the strain as P. delicatissima, with congruent phylogenetic placements obtained from both markers. The strain was harvested during the stationary growth phase and analysed for DA and its isomers; however, none were detected above the limit of detection (LOD) of 0.39 pg cell-1. Considering that the Pseudo-nitzschia species have previously formed blooms in the study area, regular monitoring of these species is essential. Further studies investigating the ecophysiological characteristics of different strains of P. delicatissima isolated from the same study area and the influence of environmental factors on DA production are needed to better understand its toxinological potential.

The growth response of a diatom Cylindrotheca closterium strain (DEB132303) isolated from the Golden Horn Estuary (Sea of Marmara) was investigated under different environmental conditions. C. closterium cells were grown in a silica-enriched K-medium under various temperatures (11, 18, 25 and 32°C), salinities (6, 12, 18, 24, 30 and 36 psu) and irradiances (10, 20, 50 and 65 μmol photons m⁻² s⁻¹). By the end of the acclimatization period, no viable C. closterium cells were present at temperatures of 4°C and 32°C and salinities of 6 and 36 psu, which marked the upper and lower boundaries of its environmental tolerance. The maximum specific growth rates (µmax) were 1.45 d⁻¹ at temperature of 18°C (18 psu, 50 μmol photons m⁻² s⁻¹), 1.08 d⁻¹ at salinity of 24 psu ( 18°C, 50 μmol photons m⁻² s⁻¹) and 1.80 d⁻¹ at irradiance of 65 μmol photons m⁻² s⁻¹ (18°C, 18 psu), in temperature, salinity and light experiments, respectively. The maximum cell density (Dmax) was 5×10⁵ cells mL⁻¹ at temperature of 25°C, salinity of 18 psu and irradiance of 50 μmol photons m⁻² s⁻¹. Growth rate varied significantly among different temperature, salinity and irradiance treatments (ANOVA, p<0.001) and there was a positive correlation between growth rate and irradiance (r= 0.98, p<0.01). These results demonstrate that these environmental parameters affect significantly the growth of this C. closterium strain.

The endosymbiotic association between the diatom Hemiaulus and the cyanobacterium Richelia was first observed in the Sea of Marmara in July 2021. The spatial distribution of the host diatom Hemiaulus spp. and the endosymbiont cyanobacterium Richelia intracellularis was investigated along with available physicochemical parameters. Three species of the Hemiaulus genus (H. hauckii, H. membranaceus, and H. sinensis) were morphologically identified in the study area. Hemiaulus hauckii and H. sinensis reached up to 128 × 10³ cells L⁻¹ and 38 × 10³ cells L⁻¹, respectively, while H. membranaceus was rarely observed. Each Hemiaulus cell contained one Richelia trichome, which had heterocysts at both ends. The surface water temperatures and salinity varied between 23.2°C and 28.5°C, 21.4 and 23.5, respectively. Dissolved oxygen levels ranged from 6.2 to 7.6 mg L⁻¹, while chlorophyll-a concentrations were between 0.3 and 6.8 µg L⁻¹. Nutrient concentrations varied between 0.05 and 0.18 μM for NO₃ + NO₂–N, 0.04–0.24 μM for NH₄–N, 0.02–0.39 μM for PO₄–P, and 0.18–1.42 μM for SiO₂–Si. This study reveals that the Hemiaulus–Richelia symbiosis may promote the proliferation of diatom populations and may play an important role in nutrient dynamics in nitrogen-limiting environments and in the overall functioning of the marine ecosystem.

Phytoplankton, a group of single-celled organisms capable of photosynthesis, are vital to marine ecosystems due to their primary production and role in biogeochemical cycles. They adapt to diverse environments and utilize complex nutritional strategies. Phytoplankton contribute to the global carbon cycle through carbon fixation and sequestration, particularly by diatoms and coccolithophores. However, climate change and anthropogenic impacts are altering their dynamics, with rising temperatures, acidification, and pollution affecting growth, productivity, and community structure. Harmful algal blooms (HABs), driven by eutrophication and increased nutrient loads, pose significant threats to marine ecosystems, causing toxic effects and disrupting food webs. Effective management of these issues requires extensive research, international cooperation, and informed policy measures. Understanding phytoplankton’s responses to environmental changes is crucial for predicting marine ecosystem futures and developing strategies to mitigate the adverse effects of climate change and pollution.

In the intense mucilage formation observed in the Sea of Marmara in 2021, Phaeocystis pouchetii (Prymnesiophyceae) together with Skeletonema costatum, Cylindrotheca closterium, Thalassiosira rotula (Bacillariophyceae), and Gonyaulax fragilis (Dinophyceae) were detected in the foamy mucilage in the surface layer, and Chrysoreinhardia giraudii and Nematochrysopsis marina (Chrysophyceae), which are known to produce filamentous mucilage in benthic habitat. In addition, with the contribution of these groups, a higher cell abundance (2.1×107 cells/L) and chlorophyll-a value (15.9 µg/L) was reached than the mucilage event experienced in previous years. In the microscopic observations, typical dominant genera of the Sea of Marmara such as Protoperidinium and Tripos were observed very little in terms of species composition, and the fact that three previously unobserved species became dominant and the Cyanophyceae group was represented by different species indicated that phytoplankton composition changed in the mucilage formation in this period. The changing species composition with these 3 species that are known to make mucilage and which are new records for the Sea of Marmara point to the transportation by ship ballast waters or the inflow of brackish water with heavy rains. It is recommended to take the necessary measures to control domestic and industrial wastes and terrestrial inputs, which cause these species to reach numerical abundance and form mucilage, to carry out fisheries in a controlled manner, and to prevent the discharge of ship ballast waters and bilge waters.

Domoic acid (DA) is produced by almost half of the species belonging to the diatom genus Pseudo-nitzschia and causes amnesic shellfish poisoning (ASP). It is, therefore, important to investigate the diversity and toxin production of Pseudo-nitzschia species for ASP risk assessments. Between 2018 and 2020, seawater samples were collected from various sites around Aotearoa New Zealand, and 130 clonal isolates of Pseudo-nitzschia were established. Molecular phylogenetic analysis of partial large subunit ribosomal DNA and/or internal transcribed spacer regions revealed that the isolates were divided into 14 species (Pseudo-nitzschia americana, Pseudo-nitzschia arenysensis, Pseudo-nitzschia australis, Pseudo-nitzschia calliantha, Pseudo-nitzschia cuspidata, Pseudo-nitzschia delicatissima, Pseudo-nitzschia fraudulenta, Pseudo-nitzschia galaxiae, Pseudo-nitzschia hasleana, Pseudo-nitzschia multiseries, Pseudo-nitzschia multistriata, Pseudo-nitzschia plurisecta, Pseudo-nitzschia pungens, and Pseudo-nitzschia cf. subpacifica). The P. delicatissima and P. hasleana strains were further divided into two clades/subclades (I and II). Liquid chromatography-tandem mass spectrometry was used to assess the production of DA and DA isomers by 73 representative strains. The analyses revealed that two (P. australis and P. multiseries) of the 14 species produced DA as a primary analogue, along with several DA isomers. This study is the first geographical distribution record of P. arenysensis, P.cuspidata, P. galaxiae, and P. hasleana in New Zealand coastal waters.