Turgay Durmus

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.

In this study, the distribution and composition of epipelic diatoms in Büyükçekmece Reservoir, which is one of the most important and main freshwater resources for İstanbul, Türkiye were investigated seasonally. Epipelic diatom samples were collected from five different stations each season (February, May, August, and November) in 2019. A total number of eighty-seven taxa belonging to thirty-seven genera were identified, six of which were centric and eighty-one were pennate; five of the epipelic species were new record diatoms for the Büyükçekmece Reservoir. The highest number of epipelic diatom species were encountered in February while the lowest was in November. Considering the frustule counts, it was observed that the species that increased their relative abundance changed in each sampling period. Consequently, it was observed that Gomphonella olivacea was the most abundant in February, Navicula reichardtiana in May, Achnanthidium sp. in August, and Cocconeis placentula in November. Shannon-Weaver Diversity Index showed that the highest diversity of diatom species was determined in February (station 4), and the lowest was observed in August (station 3). According to the results of redundancy analysis, it was observed that silicate-Si, conductivity, and temperature were the most efficient variables in the distribution of the epipelic diatom species.

Zooplanktonic organisms are vital components of marine ecosystems, playing significant roles in the carbon cycle. Zooplankton are primary consumers that transfer energy from phytoplankton to higher trophic levels, serving as a primary food source for economically important fish species. Their diversity, shaped by environmental factors, is crucial for maintaining ecosystem balance and productivity. Climate change significantly affects zooplankton distribution, abundance, and ecological roles, prompting adaptive responses or changes in their prevalence. Adaptive and evolutionary changes in zooplankton populations reshape food web dynamics and biochemical cycles. Understanding the adaptive capacity of zooplanktonic organisms and their effects on ecosystems is necessary for monitoring resilience and adaptability in the face of environmental changes. Ensuring the sustainability of marine ecosystems requires comprehending these changes and implementing effective conservation strategies. Conservation efforts should focus on reducing greenhouse gas emissions, protecting marine habitats, and enhancing ecosystem resilience. Collective efforts to combat climate change are essential for preserving the health and resilience of marine ecosystems.

Craspedostauros E.J. Cox is a diatom genus comprising 17 taxa reported from various regions of the world. While many species of Craspedostauros are epibiontic, the taxa have variable ecological preferences. In this study we formally describe Craspedostauros nazmiisp. nov., an epilithic species discovered along the Turkish Black Sea Coast, based on light and scanning electron microscopy. Craspedostauros nazmiisp. nov. is characterized by valves that are lanceolate to narrowly lanceolate, slightly constricted near the apices with uniseriate, parallel throughout the whole valve, transapical striae and and the presence of an apical silica flap. The areolae are distributed over the valve face and the mantle. The differences and similarities between C. nazmiisp. nov. and established species of Craspedostauros are discussed. Based on shape and morphometrics, the most similar species is Craspedostauros capensis, but it is easily distinguished from C. nazmiisp. nov. by its lack of an apical silica flap.