The golden paradox – a new heterokont lineage with chloroplasts surrounded by two membranes

Abstract

A marine, sand‐dwelling, golden‐brown alga is described from clonal cultures established from a high intertidal pool in southeastern Australia. This tiny, unicellular species, which we call the “golden paradox” (Chrysoparadoxa australica gen. et sp. nov.), is benthic, surrounded by a multilayered cell wall and attached to the substratum by a complex adhesive plug. Each vegetative cell gives rise to a single, naked zoospore with heterokont flagella that settles and may become briefly amoeboid prior to dividing. Daughter cells are initially amoeboid, then either permanently attach and return to the benthic stage or become motile again prior to final settlement. Two deeply lobed chloroplasts occupy opposite ends of the cell and are surrounded by only two membranes. The outer chloroplast membrane is continuous between the two chloroplasts via the outer membrane of the nuclear envelope. Only two membranes occupy the chloroplast–nucleus interface, the inner membrane of the nuclear envelope and the inner chloroplast membrane. A small pyrenoid is found in each chloroplast and closely abuts the nucleus or protrudes into it. It contains an unusual, membrane‐bound inclusion that stains with SYBR green but is unlikely to be a nucleomorph. Phylogenies inferred from a 10‐gene concatenated alignment show an early‐branching position within the PX clade. The unusual morphological features and phylogenetic position indicate C. australica should be classified as a new class, Chrysoparadoxophyceae. Despite an atypical plastid, exploration of the C. australica transcriptome revealed typical heterokont protein targeting to the plastid.

A new marine prasinophyte genus alternates between a flagellate and a dominant benthic stage with microrhizoids for adhesion

“…We expect that these relationships will be clarified when more complete data sets become available. The discovery of Microrhizoidea confirms that many divergent lineages can be found in benthic marine habitats (see also Kai et al 2008, Zechman et al 2010, Grant et al 2011, Wetherbee et al 2015, Wetherbee and Verbruggen 2016, Wetherbee et al 2019. Like many other sand-dwelling algae, M. pickettheapsiorum has adapted to maintain its place in a dynamic habitat with alternating benthic and short-lived planktonic stages (see below).…”

“…, Wetherbee and Verbruggen , Wetherbee et al. ). Like many other sand‐dwelling algae, M. pickettheapsiorum has adapted to maintain its place in a dynamic habitat with alternating benthic and short‐lived planktonic stages (see below).…”

“…2010, Wetherbee et al. ). It is worth noting that many of the lineages in our 18S rDNA phylogenies are only known from planktonic environmental sequences, but one should not dismiss the possibility that more of these lineages have alternating benthic and planktonic stages, and it would be highly informative to have more environmental 18S rDNA surveys of benthic habitats, as the great majority of the work done to date has been on plankton.…”

“…Like many other sand-dwelling algae, M. pickettheapsiorum has adapted to maintain its place in a dynamic habitat with alternating benthic and short-lived planktonic stages (see below). The phylogenetic position of Microrhizoidea in a largely planktonic lineage suggests that in this case the adaptation to benthic habitats was relatively recent, but there certainly are examples of much more ancient transitions of planktonic algae into this habitat, leading to the discovery of new order-and class-level lineages in benthic habitats (e.g., Kai et al 2008, Zechman et al 2010, Wetherbee et al 2019. It is worth noting that many of the lineages in our 18S rDNA phylogenies are only known from planktonic environmental sequences, but one should not dismiss the possibility that more of these lineages have alternating benthic and planktonic 1220 stages, and it would be highly informative to have more environmental 18S rDNA surveys of benthic habitats, as the great majority of the work done to date has been on plankton.…”

Metabolic quirks and the colourful history of the Euglena gracilis secondary plastid

“…Interestingly, proline residues in the TP were recently proposed to be important for correct import of plastid proteins with multiple transmembrane domains in plants (Lee et al, 2018), but it is unclear what the significance of general proline-richness in TPs, as observed here, could be. However, the recently described structure of the plastid of the stramenopile Chrysoparadoxa suggests that this membrane topology can indeed arise (Wetherbee et al, 2018). The absence of unambiguously identifiable additional components, such as Npl4 and Ubx, leaves it unclear if a translocon analogous to SELMA is functional in E. gracilis.…”