Cryptophytes: An emerging algal group in the rapidly changing Antarctic Peninsula marine environments.

The western Antarctic Peninsula (WAP) is a climatically sensitive region where foundational changes at the basis of the food web have been recorded; cryptophytes are gradually outgrowing diatoms together with a decreased size spectrum of the phytoplankton community. Based on a 11-year (2008-2018) in-situ dataset, we demonstrate a strong coupling between biomass accumulation of cryptophytes, summer upper ocean stability, and the mixed layer depth. Our results shed light on the environmental conditions favoring the cryptophyte success in coastal regions of the WAP, especially during situations of shallower mixed layers associated with lower diatom biomass, which evidences a clear competition or niche segregation between diatoms and cryptophytes. We also unravel the cryptophyte photo-physiological niche by exploring its capacity to thrive under high light stress normally found in confined stratified upper layers. Such conditions are becoming more frequent in the Antarctic coastal waters and will likely have significant future implications at various levels of the marine food web. The competitive advantage of cryptophytes in environments with significant light level fluctuations was supported by laboratory experiments that revealed a high flexibility of cryptophytes to grow in different light conditions driven by a fast photo-regulating response. All tested physiological parameters support the hypothesis that cryptophytes are highly flexible regarding their growing light conditions and extremely efficient in rapidly photo-regulating changes to environmental light levels. This plasticity would give them a competitive advantage in exploiting an ecological niche where light levels fluctuate quickly. These findings provide new insights on niche separation between diatoms and cryptophytes, which is vital for a thorough understanding of the WAP marine ecosystem.

Asterionellopsis tropicalis (Bacillariophyceae): a new tropical species found in diatom accumulations.

The diatom Asterionellopsis glacialis sensu lato forms high-density patches in the surf zone of some sandy beaches worldwide and was until recently considered a cosmopolitan species. With the recent description of four cryptic species, the identity of specimens found in these accumulations remains uncertain. In this study, diatom patches were sampled from two sandy beaches of the Brazilian coast: one tropical (Futuro Beach, 3° S; 38° W) and one subtropical (Cassino Beach, 32° S; 52° W). Fine structure of frustules and the sequencing of three phylogenetic markers revealed the subtropical strains to be A. guyunusae and the tropical strains to be a new species, here described as Asterionellopsis tropicalis sp. nov. A. tropicalis was differentiated morphologically by the number of striae in 10 µm at the foot pole and head (39-44; 38-45, respectively), from A. lenisilicea (46-55; 46-64), A. maritima (46-51; 46-60), and A. thurstonii (42-58; 55-70). The number of striae at the head region of the valvocopula (10 µm) helped to distinguish A. tropicalis (56-62) from A. guyunusae (61-64), but A. tropicalis was morphologically undistinguishable from A. glacialis. The sequence divergence from other identified Asterionellopsis species was 13%-16% (Cox1), 11%-12% (5.8S + ITS2) and 2%-6% (RbcL), and A. tropicalis formed a distinct monophyletic clade with high support in all analyzed phylogenetic trees (single or multi-locus). This work will aid in the understanding of the ecological and physiological diversity of diatom patches that are key to the trophic webs of sandy beaches.