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.

Studies of the genus Amphiroa (Corallinales, Rhodophyta) along the southern Baja California Peninsula, México, were undertaken to determine how many species were present and to assess the characters that have been or could be used in species delimitation. After analysis of 800 specimens from 60 localities, we conclude that five species are present: A. valonioides Yendo, A. misakiensis Yendo, A. vanbosseae Lemoine, A. rigida Lamouroux and A. beauvoisii Lamouroux. Amphiroa brevianceps Dawson and A. magdalensis Dawson are considered heterotypic synonyms of A. misakiensis. Characters used here for the first time to delimit species of Amphiroa include shape of gametangial conceptacle chambers, ontogeny of the pore of the gametangial conceptacle, the type of spermatangia present, the position of gonimoblast filaments and the form of the cortical cells. Characters used in species delimitation in Amphiroa are discussed in detail and a key to the species found in southern Baja California is provided.

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.

Diatoms of the genus Mastogloia are naviculoid forms that are generally easy to discriminate due to the presence of marginal chambers known as partecta. They exhibit a wide marine distribution, and are basically benthic forms usually conspicuous in epiphytic and epipelic diatom assemblages. Our objective was to identify species of Mastogloia epiphytic on Thalassia testudinum blades from the southern Gulf of Mexico region. Blades of T. testudinum were collected off the coast of Campeche State, Mexico; the epiphytic diatoms were scrapped off, cleaned and mounted on permanent slides for identification under light microscopy. A total of 76 Mastogloia taxa were recorded. This is the highest number of Mastogloia taxa recorded for Mexican coasts. Seven taxa had not been observed for the region: Mastogloia angusta, M. belaensis, M. depressa, M. ovalis, M. pseudolacrimata, M. regula, and M. urveae. Likewise, 20 Mastogloia species are new records for the Gulf of Mexico, while 15 are new records for Mexican coasts overall. This study confirms that Mastogloia species are mostly distributed in tropical and subtropical habitats, and shows thatmuch floristics and formal taxonomy are required on benthic diatoms from different types of substrata for Mexican littorals.