Stichococcus marinus (Wille) Hazen (A)

This collection formed thick green carpets on rock in a flowing freshwater brook where it drained into Sam Orr’s Pond, NB (Featured Image above). The filaments are straight to slightly wavy, up to 4 cm tall, very fine and a dull to light green colour (Image A). Filaments are unbranched, the cells 5-6 µm wide by 12-26 µm tall, and contain a single parietal chloroplast that occupies only a portion of the cell wall and may lack pyrenoids (hyaline bodies are present, well developed in some cells, but inconsistent; Images B & C). Filaments are attached by a slightly modified basal cell (Image C).

This is a tentative identification awaiting molecular confirmation. There is considerable confusion regarding the morphology and taxonomy of this entity in the various published accounts. The single collection was in full freshwater outflow that likely receives exposure to seawater at the highest tides only. When I first looked at it under the microscope and failed to key it out to anything meaningful (local guides report Stichococcus marinus attaining only a few cells in length; Mathieson & Dawes 2017, Sears 2002) I almost discarded it as a freshwater Ulothrix(?) assuming I had ventured too far up the creek. Time will tell… If this identification is correct, I concur with Pröschold & Darienko (2020) that this species is incorrectly placed in Stichococcus. Preliminary phylogenetic analyses place this genetic group sister to the lineage including Chlorothrix and Ulothrix.

Image A. Filaments of a collection from a freshwater inflow to Sam Orr’s Pond, NB (GWS044454).

Image B. View of the thin vegetative filaments and their cells, each with a single parietal chloroplast that appear to lack (indistinct?) pyrenoids (GWS044454).

Image C. Attachment is by a slightly modified basal cell (GWS044454).