The emergence of a red tide resulting in yellow-brownish discoloration of waters in Porto Cesareo bay (Italy) during July–August 2018 is reported. The species responsible for the bloom was the dinoflagellate Margalefidinium cf. polykrikoides. Cell densities reached 9.1 × 10⁶ cells L⁻¹ during the initial outbreak. A second peak was observed about three weeks later reaching 6.7 × 10⁵ cells L⁻¹. Study of live specimens showed great variation in cell size and shape. Different cyst morphotypes were found in the water samples and in the sediment. For the first time, we followed several stages of the life cycle of M. cf. polykrikoides in natural samples. Fish die-offs in the bay were not observed, however this high-density bloom may have caused consequences on the ecosystem (amount of mucilage on the beach) and in turn, on tourism that is the main activity in the area during the summer season.

The diversity of free-living dinoflagellates in the coastal areas of the central Red Sea, Saudi Arabia, was studied from April 2016 to March 2017. A total of 106 dinoflagellates belonging to 36 genera, 20 families and 7 orders were identified and characterized using light microscopy. Of these, 47 taxa were potentially harmful, and 60 taxa were recorded for the first time from the Red Sea. The unexpectedly high species diversity, including new records, was due to the benthic species. The monthly variability of planktonic species records exhibited negative correlations with temperature and salinity, although in most cases, the links between them were insignificant. Subsequently, the dinoflagellates checklist for the entire Red Sea was updated and showed that there were currently 395 taxa and 66 genera. The results of this study provide a solid foundation for future studies of dinoflagellate biodiversity in the Red Sea, particularly for benthic and harmful species.

As part of a continuing toxic microalgae monitoring program, phytoplankton samples were collected on 16–17 August 2012 at several sampling sites in the southern part of the Bahía de La Paz. A bloom of the dinoflagellate Cochlodinium polykrikoides was detected. Abundance of C. polykrikoides ranged from 73 to 276×103cellsL−1 on the first day to 980–1425×103cellsL−1 on the second day. Study of live specimens showed great variation in cell size and form, mainly as single cells or chains of two cells. Live cells were 30–47μm long and 20–35μm wide (n=30). Seawater temperature during the bloom was 29–30°C. Low densities of Cochlodinium convolutum, Cochlodinium helicoides, and Cochlodinium shuettii were also found in the samples of the bloom event. These high densities of C. polykrikoides did not lead to fish die-offs in the bay.

The effects of reduced forms of nitrogen (NH₄⁺ and dissolved organic nitrogen (DON)) on the spatial distribution of diatoms and dinoflagellates in an estuarine-coastal water continuum were investigated from 2015 to 2019. The proportion of non-DIN in total nitrogen was utilized as an indicator of DON along with direct measurements of DON. While NO₃⁻ originated from Seomjin River, the abundant NH₄⁺ and DON occurred from Gwangyang Bay through Namhae. Diatoms were mostly confined to the upper estuarine system and dinoflagellates dominated in the regions with high levels of NH₄⁺ and DON. Generalized additive models also presented the different responses of diatoms and dinoflagellates to increases in NH₄⁺ and DON. Thus, our results highlight that diatoms dominate in NO₃⁻-replete water with full access to the source and dinoflagellates take over the ecologically open niche in an anthropogenically polluted estuary with full access to reduced forms of nitrogen.

Enrichment of reef environments with dissolved inorganic nutrients is considered a major threat to the survival of corals living in symbiosis with dinoflagellates (Symbiodinium sp.). We argue, however, that the direct negative effects on the symbiosis are not necessarily caused by the nutrient enrichment itself but by the phosphorus starvation of the algal symbionts that can be caused by skewed nitrogen (N) to phosphorus (P) ratios. We exposed corals to imbalanced N:P ratios in long-term experiments and found that the undersupply of phosphate severely disturbed the symbiosis, indicated by the loss of coral biomass, malfunctioning of algal photosynthesis and bleaching of the corals. In contrast, the corals tolerated an undersupply with nitrogen at high phosphate concentrations without negative effects on symbiont photosynthesis, suggesting a better adaptation to nitrogen limitation. Transmission electron microscopy analysis revealed that the signatures of ultrastructural biomarkers represent versatile tools for the classification of nutrient stress in symbiotic algae. Notably, high N:P ratios in the water were clearly identified by the accumulation of uric acid crystals.

Dinoflagellate community structure from two semi-enclosed areas along the South Andaman region, India, was investigated to assess the anthropogenic impact on coastal water quality. At the densely inhabited Port Blair Bay, the dominance of mixotrophs in water and Protoperidinoids in sediments was attributed to anthropogenic nutrient enrichment and prey availability. A significant decrease in dinoflagellate abundance from inner to outer bay emphasize the variation in nutrient availability. The dominance of autotrophs and Gonyaulacoid cysts at the North Bay highlight low nutrient conditions with less anthropogenic pressure. The occurrence of oceanic Ornithocercus steinii and Diplopsalis sp. could evince the oceanic water intrusion into the North Bay. Nine potentially harmful and red-tide-forming species including Alexandrium tamarense complex, A. minutum were identified in this study. Although there are no harmful algal bloom (HABs) incidences in this region so far, increasing coastal pollution could support their candidature towards the future HABs initiation and development.

Testing phytoplankton viability within ballast tanks and receiving waters of ballast water discharge remain understudied. Potentially harmful dinoflagellates and diatoms are transported via ballast water to Galveston Bay, Texas (USA), home to three major ports: Houston, Texas City and Galveston. Ballast water from vessels transiting the North Atlantic Ocean was inoculated into treatments representing low and high salinity conditions similar to the Ports of Houston and Galveston respectively. Phytoplankton in ballast water growout experiments were deemed viable and showed growth in low and mid salinities with nutrient enrichment. Molecular methods identified several genera: Dinophysis, Gymnodinium, Gyrodinium, Heterocapsa, Peridinium, Scrippsiella, Chaetoceros and Nitzschia. These phytoplankton genera were previously identified in Galveston Bay except Scrippsiella. Phytoplankton, including those capable of forming harmful algal blooms leading to fish and shellfish kills, are transported to Galveston Bay via ballast water, and are viable when introduced to similar salinity conditions found in Galveston Bay ports.

In this study, we conducted two cruises in the Bohai Bay (China) focusing on phytoplankton community and relation to water quality. The evaluation revealed that most of the open area was non-eutrophic, whereas the river inlet had severe eutrophication. Phytoplankton populations respond differently to different aquatic environments and are controlled by more than two factors, as revealed by aggregated boosted tree analysis. Notably, a shift in the phytoplankton community structure was observed during the seasonal transition, from the dominance of diatoms to the co-dominance of diatoms-dinoflagellates. However, the relative abundance of dinoflagellates increased by 14 % in autumn, when the harmful algae species Akashiwo sanguinea exclusively predominated; this was primarily linked to the nutrient ratios, temperature, and dissolved oxygen. The eutrophication and organic pollution had direct effects on phytoplankton abundance. Overall, our findings may provide further insights into the impacts of eutrophic environments on phytoplankton community structure in coastal systems.

A circadian rhythm of the dinoflagellate Peridinium quadridentatum was studied at a time-series station in the southwestern Gulf of Mexico, in May 2007. Different substrates (water column, the seagrass Thalassia testudinum, macroalgae, coral rubble and sandy sediment surface) were sampled at the site at 1.5–3.5m depth. In the samples of coral rubble, P. quadridentatum was scarce. In the water column, the species showed an abundance peak at 15:00. The cell abundance of P. quadridentatum in Thalassia samples increased from 15:00 until 18:00 (1.81×104cells/gsubstratewet weight), and then continuously decreased until 06:00. Changes in P. quadridentatum cell abundance on macroalgae followed the same trend as on Thalassia, with the maximal value at 18:00. The higher abundance of P. quadridentatum (up to 1.40×104cells/gSWW) in macroalgae samples showed the preference for seaweeds. P. quadridentatum has a neritic tropical–boreal distribution. A new combination is proposed: Peridinium quadridentatum var. trispiniferum.