The first list of epiphytic diatom species on the seagrass Thalassia testudinum from the Yalahau lagoon area in the Mexican Caribbean is given. One hundred and seven diatom taxa living on T. testudinum were identified including species and varieties. Nine taxa are new records for Mexico. The Bacillariophyceae with 87 taxa were the most diverse group. The most abundant taxa were: Cocconeis lineata, Mastogloia crucicula var. crucicula, Cocconeis thalassiana, Cocconeis scutellum var. scutellum and Mastogloia ovata, representing approximately 75% of the total valve count (N = 1527). Yalahau lagoon samples from site 1 yielded a high number of rare taxa. Species richness (S), diversity (H′, 1-λ) and equitability (J′) for the lagoon were lower than for Cabo Catoche (site 2). Based on presence/absence of species, the measured similarity (60%) suggests that the diatom assemblages growing on T. testudinum in the two localities are different.

Recent investigations at previously unexplored localities on the Mexican coast have confirmed the high taxonomic potential of benthic marine diatom assemblages (BMDA) in the region. An exploratory study of epiphytic diatoms of macroalgae in the Revillagigedo Archipelago (RA) suggested that further studies would yield many more taxa, prompting the hypothesis that diversity measurements, based on ecological indices, would be among the highest in pristine environments. Thus, the aim of this research was to enrich the record of epiphytic diatom floristics of the RA, and to estimate species diversity based on information theory (H′). Floristically, 167 identified taxa are added here to the BMDA species list of the RA, bringing the total to 397 taxa overall, including 52 taxa that are potentially new records for the Mexican Pacific coast. Among the most conspicuous genera are Mastogloia with five new taxa and it remains the most diverse genus with 55 taxa overall, followed by Cocconeis (27), Nitzschia (24), Amphora (23), Navicula (19), Diploneis (17) and Grammatophora (15). As expected for a pristine environment, the computed species diversity values for the BMDA were high, ranging from H′ = 3.92–5.2, depicting stability. Future surveys are expected to further increase the species richness of BMDA for the RA.

Halamphora siqueirosii sp. nov. is a marine benthic diatom isolated as a monoclonal culture from a hypersaline evaporation pond at the Guerrero Negro Saltworks, Baja California Sur, Mexico. Analysis of the valve ultrastructure of this taxon using light and electron microscopy indicated that its uniseriate striae and the density of its dorsal striae are useful characteristics for discriminating H. siqueirosii from other species with similar morphological patterns. The phylogenetic analyses performed with nuclear ribosomal genes, the small (18S) and large subunits (28S), and chloroplast gene ribulose-bisphosphate carboxylase (rbcL) fragments showed that H. siqueirosii is closely related to H. americana, H. calidilacuna and H. incelebrata. Morphological differences between this new species and similar ones are discussed.

A taxonomic analysis of diatoms found on Phyllodictyon pulcherrimum yielded a total of 244 diatom taxa (all illustrated) within 27 orders, 45 families, and 86 genera. The Taxa were briefly documented in a list including identification references and morphometric data. Thirty-eight of the taxa identified at the species and infraspecific levels represent new records for the coasts of Mexico. Seven were recorded for the first time on the American continent: Auricula flabelliformis, A. pulchra, Campylodiscus scalaris, Coscinodiscus mesoleius, Dimeregramma fulvum, Navicula palpebralis var. angulosa, and Seminavis barbarae, and one, Nitzschia fusiformis, for the Pacific Ocean. This is the second record of the chlorophyte P. pulcherrimum in the north Pacific and the third for Mexican waters. The results confirm that surveying rare macroalgae species as hosts for epiphytic diatoms provides opportunities to seek new records of diatom taxa, or even new taxa, in regions around the world.

The environmental contingency caused by the recent massive arrivals of pelagic sargasso (Sargassum natans and S. fluitans) on Mexico’s Caribbean coasts have given rise to several areas of scientific research. Our work proposed identifying the diatom flora adhered to the thalli of these two sargasso species collected on the coasts of Cancun, Isla Mujeres, and Puerto Morelos. We recorded 184 diatom taxa (all illustrated) from 68 genera. Taxa from the genera Mastogloia (37), Cocconeis (11), Nitzschia (10), Diploneis (8), and Amphora (9) represented 41% of the total, while 44 of these genera were represented by a single species. In the total floristic count, 41 taxa occurred exclusively on S. fluitans, 53 exclusively on S. natans (22 and 29%, respectively) and 90 (49%) were found on both. Species of Navicula were scarce, and Navicula barbara var. densestriata was here transferred to the genus Lyrella (Lyrella barbara var. densestriata (Foged) López-Fuerte & Siqueiros Beltrones comb. nov.). Overall, 17 (9%) of the identified taxa were new recordings for Mexico’s coasts. Supporting the hypothesis proposed, the 37 Mastogloia taxa suggested a tropical affinity, while the high species richness denoted that the surfaces of both sargasso species constituted favorable substrata for the growth of diatom assemblages.

The ecological significance of benthic diatom taxocoenoses in marine ecosystems renders them a suitable reference when making decisions on conservation, as well as ecological and biogeographical issues. This study focuses on floristics of epiphytic diatoms of macroalgae from the central region of the Gulf of California and their biogeographical affinities. Based on current floristics for benthic diatoms of the Mexican NW, it was hypothesized that species richness would be higher for the central gulf (transitional) than for the tropical Revillagigedo Archipelago. Additionally, it was hypothesized that it would be composed mainly of common taxa of the region, with tropical and temperate components. Likewise, it was hypothesized that Mastogloia (tropical) would include less taxa and Lyrella would include more taxa in the transitional central gulf. The diatom flora yielded 333 taxa and 101 genera, and out of which the best represented were Navicula with 29 taxa, Amphora (27), Cocconeis (25), Nitzschia (24), Diploneis (19), Achnanthes (13), Halamphora (14), Fallacia (10), Lyrella (9), and Mastogloia (8), adding up to 53% of the total. The inspected diatom taxocoenosis included mostly taxa commonly distributed in the Mexican NW, albeit with 16 new records. This, plus the high species richness, as well as the proportions of selected taxa depicting this diatom taxocoenosis as being of transitional biogeographical affinity, back the posed hypotheses.