Species of the diatom genus Thalassiosira are an important component of the marine phytoplankton all over the world, and the Mexican Pacific Ocean planktonic flora is also rich in species of this genus. In this study 37 net plankton samples from the Gulf of Tehuantepec, Mexico were analyzed, yielding 23 taxa of the genus. Most of the species were studied using combined light and electron microscopy. New records for the Mexican Pacific Ocean are Thalassiosira hendeyi, T. intrannula, T. pseudonana, T. tenera and T. visurgis. The species of Thalassiosira recorded here seem to indicate that the Gulf of Tehuantepec represents a subtropical zone at least in one period of the year.

During a phytoplankton survey, non-motile (cyst-like) cells were isolated and cultured from the tropical Mexican Pacific, which belong to the thecate dinoflagellate Vulcanodinium rugosum Nézan et Chomérat, recently described from the Mediterranean and recently identified as a source of pinnatoxins in Australia, New Zealand, and Japan. Motile cells and putative cysts were studied by light microscopy and scanning electron microscopy. The most characteristic features of the species agree with a previous description: (i) large and conspicuous apical pore, which extrudes mucilaginous material; (ii) polygonal and striated theca; (iii) ventral, sigmoid canal running from the apical pore to the anterior margin of the cingulum; (iv) three connected anterior intercalary plates; and (v) production of spherical cyst-like cells. The species is photosynthetic, containing many small greenish-brown chloroplasts. The tabulation of the species is Po, X, 4′, 3a, 7″, 6c, 5s?, 5′″, 2″″. Organic cyst-like cells were nearly spherical to subspherical, solitary, or aggregations linked by mucilaginous material; they are considered to be a stage in the life cycle of the species. Vulcanodinium rugosum is a new record in the study area. The ecology and the geographical distribution of the species are also discussed.

Hemidiscus cuneiformis, type of the family Hemidiscaceae, is a diatom with a distinctive semi-circular shape in valve view. A strain of H. cuneiformis was established from the coastal region of the tropical southwestern Atlantic Ocean off Brazil. Cells of this strain showed semi-circular to asymmetrically elliptical valves, indicating morphological plasticity. In the small subunit (SSU) rDNA phylogeny, the type species of Hemidiscus branched between two clades of Actinocyclus, one of which was a sister group, the other in a basal position. Beyond cell shape, there are no significant morphological differences between Hemidiscus and Actinocyclus. We propose the transfer of H. cuneiformis and H. kanayanus into Actinocyclus. The family Hemidiscaceae remains for the genus Actinocyclus.

Two coastal zones (influenced by the Coatzacoalcos and Grijalva-Usumacinta rivers) and an oceanic one (Campeche Canyon), from the southern Gulf of Mexico, were sampled during the ‘nortes’ or windy (February, 2004) season, to study the concentration and distribution of the phytoplanktonic pigments and the picoeukaryote fraction. Vertical profiles of accessory pigments indicated the presence and potential importance of autotrophic picoplankton in the waters of the Southern Gulf of Mexico. These pigments showed different distributions, associated with different hydrographic conditions between the oceanic zone and the coastal regions (e.g. presence and depth of a thermocline), indicating contrasting phytoplankton communities. The highest concentrations of accessory pigments (fucoxanthin, zeaxanthin, divinyl-chlorophyll a) were associated with shallow coastal zones (above 1% PAR_I), except for prasinoxanthin that also showed high concentrations in the Campeche Canyon. Pigments associated with prokaryotic picoplankton (zeaxanthin, divinyl-chlorophyll a) were more distinguishable in the Campeche Canyon than in the coastal zones. Chlorophyll a was not directly related to accessory pigments in most of the stations studied. Relatively high cell abundances of Prasinophyte Micromonas pusilla occurred at one coastal zone station, using molecular probes (FISH-TSA), and revealed the potential importance of this Prasinophyte in the picoeukaryote fraction from the study area, providing a new insight into their ecological role, relative to diatoms and coccolithophorids.