A new species, Mallomonas loricata sp. nov., is described from Vietnam based on silica-scale morphology studied using transmission and scanning electron microscopy. The species belongs to section Planae and has similar morphology to species from the Mallomonas matvienkoae complex. The new species is characterized by obovoid scales with a thick secondary layer with internal reticulation that covers almost whole surface of the scale. Scales of Mallomonas loricata were found at twenty-four localities of eight provinces in northern, central and southern areas of Vietnam. Scales of this new species have also been previously reported from Singapore and Malaysia. The distribution and habitat conditions of the new species are summarized.

This paper presents an annotated list of the Mallomonas and Synura taxa (order Synurales, class Chrysophyceae) found in Viet Nam. This list is based on materials from long-term studies (from 2008 to 2020) of 371 freshwater localities in 18 provinces in the country. A total of 67 taxa (morphotypes) of the genus Mallomonas and 7 of the genus Synura were reported. For each species, the range of abiotic environmental parameters under which they were found is given, as well as comments on the distribution of these taxa within Viet Nam and the tropical region.

Mallomonas fragariformis sp. nov. is described from Viet Nam based on silica-scale morphology studied by means of transmission and scanning electron microscopy. This species belongs to the Mallomonas mangofera species complex and is characterized by small scales ornamented with large papillae placed in shallow pits and stout four-edged serrated spines on the rear scales. Scales of Mallomonas fragariformis were found in eight localities from six provinces of Central and Southern Viet Nam. Also scales of this species have previously been reported from Java Island in Indonesia. The distribution and habitat conditions of the new species are summarized.

A new species, Cryptomonas indica sp. nov., is described from Western Ghats, India, based on morphological and molecular data. Phylogenetic relationships inferred from nuclear small and large subunit ribosomal DNA, internal transcribed spacer 2, and plastid psbA sequences show that the new species forms a separate lineage on the phylogenetic tree of the genus Cryptomonas. This new species is included in the basal clade of Cryptomonas tree, which consists of undescribed taxa from Europe and South Korea. Cells of this species are up to 19 µm in length, elliptical to slightly asymmetrical in broad view, and have a plastid with four pyrenoids. Cryptomonas indica has been observed in one locality, the Northern region of the Western Ghats. Notably, this is the first Cryptomonas taxon described from the Indian subcontinent with both morphology and molecular information.

Some diatoms are able to synthesize and accumulate HBIs – highly branched isoprenoids containing 25 carbon atoms, which are used in paleo-reconstruction of sea ice dynamics. The work assessed the ability of eight strains of diatoms to synthesize and accumulate IP25 (Ice Proxy C25). It was shown that among the studied strains only Navicula salinicola Hustedt and Pleurosigma intermedium W. Smith contain HBIs.