Low diversity of a key phytoplankton group along the West Antarctic Peninsula

Abstract

The West Antarctic Peninsula (henceforth “Peninsula”) is experiencing rapid warming and melting that is impacting the regional marine food web. The primary phytoplankton groups along the Peninsula are diatoms and cryptophytes. Relative to diatoms, there has been little focus on regional cryptophytes, and thus our understanding of their diversity and ecology is limited, especially at the species level. This gap is important, as diatoms and cryptophytes play distinct roles in the regional marine food web and biogeochemistry. Here, we use a phylogenetic placement approach with 18S rRNA gene amplicon sequence variants to assess surface ocean cryptophyte diversity and its drivers at a high taxonomic resolution along the Peninsula. Data were collected over 5 years (2012–2016) during the regional research cruises of the Palmer Long‐Term Ecological Research program. Our results indicate that there are two major cryptophyte taxa along the Peninsula, consisting of distinct Geminigera spp., which in aggregate always comprise nearly 100% of the cryptophyte community (indicating low taxa evenness). The primary taxon dominates the cryptophyte community across all samples/years, which span a broad range of oceanographic conditions. A shift in cryptophyte community composition between a lower (higher) primary (secondary) taxon percentage is associated with distinct oceanographic conditions, including lower (higher) temperature, salinity, nutrients, and cryptophyte relative abundance (phytoplankton biomass and diatom relative abundance). These results emphasize the need for a full characterization of the ecology of these two taxa, as it is predicted that cryptophytes will increase along the Peninsula given projections of continued regional environmental change.

Spatiotemporal Variations in Antarctic Protistan Communities Highlight Phytoplankton Diversity and Seasonal Dominance by a Novel Cryptophyte Lineage

“…At the Tara Oceans WAP sites, clade IV had much lower contributions than those of clade VII to TPG relative abundance. Finally, we found that, like our V9 18S ASV1, the dominant cryptophyte V4 18S ASV identified in a recent WAP study ( 19 ) had 100% nucleotide identity (to the V4 region) of our clade VII full-length sequence.…”

“…We were able to compare the identified ASV and the V4 region of our full-length clade VII gene sequence and found they had 100% nucleotide identity. Hence, taking into account our integrative results and indications from prior studies of Ace Lake ( 40 ) and those of the nonfjord WAP transects ( 19 ), the novel TPG clade VII appears to be an important cryptophyte in multiple Antarctic regions, particularly in the WAP.…”

“…We observe that the clade VII cryptophyte and G. cryophila are consistently found in our samples at ratios greater than 4:1. Two recent studies, one involving microscopy ( 12 ) and the other V4 18S rRNA amplicon sequencing ( 19 ), also found that G. cryophila may not be the primary WAP cryptophyte species. The V4 18S ASV study did not sample fjords but rather focused on repeat transect sampling in WAP sites during summer and reported that a cryptophyte ASV that differed in sequence from that of G. cryophila had higher relative abundance than G. cryophila .…”

“…Phytoplankton communities have been addressed using the 18S rRNA gene to examine the entire microbial eukaryote community at a general level ( 6 , 14 – 18 ). Most recently, WAP transect data indicated that a single amplicon sequence variant (ASV) dominated the cryptophyte community at the majority of stations, spanning multiple years, with occasional offsets in peak relative abundances with the known Antarctic cryptophyte Geminigera cryophila ( 19 ). Overall, several primarily nonmolecular studies have suggested that general phytoplankton community dynamics in the WAP are shaped by the timing of sea ice retreat and extent of surface water stratification ( 8 , 9 , 13 ).…”

In contrast to diatoms, cryptophytes are susceptible to iron limitation, but not to ocean acidification

“…While diatoms form blooms in both open ocean and coastal regions of the SO (Smetacek et al, 2012), high abundances of cryptophytes are mainly observed in coastal SO waters (Brown et al, 2021; Moline et al, 2004; Montes‐Hugo et al, 2009). In this study, we provide evidence that cryptophytes such as G. cryophila cannot cope well with low Fe concentrations, which are typical for open SO waters.…”