Achoma brachiatum Novis & Visnovsky, 2012

Novis, Phil M. & Visnovsky, Gabriel, 2012, Novel alpine algae from New Zealand: Chlorophyta, Phytotaxa 39, pp. 1-30 : 20

Achoma brachiatum Novis & Visnovsky , gen. et sp. nov. ( Figs 4A–C )

Cellulae sphaericae vel ellipsoideae, 4.6–7.7 µm longae, 3.5–7.3 µm latae. Paries cellularis nullus. Chloroplastus parietalis, cupulatus, lobis incisurisque instructus, pyrenoide unica in forma reticulata e intrusionibus thylacoidium composita in vagina amylacea segmentata includenti continens. Autosporis 2–4–8 per sporangium regenerans. Zoosporae nudae biflagellatae visae, sed sporangia non visa.

Type:— NEW ZEALAND: Westland : Mt Philistine, 1400 m, preserved cultured specimen from sample collected 30 November 2007, CHR610487 .

Cells single, spherical to ellipsoidal, 4.6–7.7 µm long, 3.5–7.3 µm wide. Cell wall absent ( Fig. 4B ). Chloroplast parietal, cup-shaped, with lobes and incisions, containing single pyrenoid ( Fig. 4A ) with reticulate pattern of thylakoid intrusions, surrounded by segmented starch sheath ( Figs 4B, C ). Reproduction by autospores, 2–4–8 per sporangium. Naked biflagellate zoospores observed, but sporangia not seen.

Habitat:— Alpine herbfield soil.

Distribution:— New Zealand.

Etymology:— “Without a bulwark” (wall), “branched” (referring to shape of intruding thylakoids in pyrenoid matrix).

Observations:— Vegetative cells often resemble chlamydomonad cells, with cup-shaped chloroplast and obvious polarity, but lack flagella ( Fig. 4A ). In this feature the cells resemble Chloronomala cuprecola ( Groover & Bold 1969) in the Gloeodendrales (sensu Ettl & Gärtner 1995), but the latter forms palmelloid colonies. Cystomonas (Ettl & Gärtner 1987) also resembles chlamydomonad cells, but retains a papilla, and other chlorococcaleans have different chloroplast and/or cell shapes, The rbc L phylogeny ( Fig. 9 ) rules out its placement in any of the chlamydomonad clades known to date. Pyrenoid structure, with branched/reticulate thylakoids inside the matrix, is reminiscent of the structure found in pyrenoids of Hamakko caudatus Nakada & Nozaki 2009 . Although no robust splits separate these two species in our rbc L phylogeny, there is also no indication that the two species are close relatives, and relationships in this part of the tree were not resolved. Attempts to amplify the 18S rDNA gene from Achoma were unsuccessful; this could provide more information on the phylogenetic position of the genus.

Cultures:— LCR-CG11.

Figures

FIGURE 4. A–C. Achoma brachiatum (photographed from culture LCR-CG11). A. Light micrograph of vegetative cells showing variable shape, prominent pyrenoid (pyr), and incised parietal chloroplast (cp). B, C, Transmission electron micrographs of zoospore showing mitochondria (mito), chloroplast containing starch (st) and prominent pyrenoid (pyr), components of flagellar basal apparatus (fba), absence of cell wall, and branching pattern of thylakoids (tm) in the pyrenoid matrix. D, E Schizochlamydella orbicularis (photographed from culture LCR-CG9). D. Light micrograph of mature cells evenly spaced in mucilage, and newly released autospores (arrow). E. Transmission electron micrograph of cell showing mitochondrion (mito), nucleus (nuc) cell wall (cw), and parietal chloroplast containing starch (st) but no pyrenoid. Scales: 10 µm in D (use for A and D), 1 µm in B, C, and E.

FIGURE 9. Phylogenetic analysis of a combined dataset representing the Chlorophyceae and Trebouxiophyceae inferred from rbcL gene sequences, with the third codon position omitted. Sequences contributed in this study are in bold. The tree typology is that inferred using MrBayes v3.0B4 (5 million generations). Numbers associated with nodes are Bayesian PP/MPB percentages (500 replicates). Scale bar represents 0.1 changes/site.