Enhancing species discovery and description in algal turfs: A case study in the green alga <i>Pseudoderbesia</i> (Bryopsidales)

Algal turfs are assemblages consisting of small marine green, brown, and red algae on the scale of millimeters to a few centimeters. Due to their small size, they have been less intensively studied by macroalgal taxonomists, and they also fall outside the scope of microalgal taxonomists, who tend to focus on smaller, often unicellular, taxa. They often have a rather simple structure and a tendency to converge onto similar morphologies with creeping and upright axes. Because of all of this, there is a substantial amount of undocumented algal biodiversity in turfs, as has been shown in several molecular surveys. Our aim in this paper was to explore some integrative taxonomic methods that could help accelerate the discovery and description of very small turf species. We focused on Pseudoderbesia, a genus of extremely small green algae from the family Bryopsidaceae. We used a combination of multifocal imaging of field-collected samples, microsample genomics, and culturing to document the Pseudoderbesia biodiversity from Heron Island on the Great Barrier Reef. Algorithmic species delimitation based on rbcL and tufA marker genes indicated that likely six (possibly five) species exist in Pseudoderbesia, but only two have been described. We have formally described the two species discovered at Heron Island as P. luxurians and P. epilithica. The latter was described using a multifocal image as the holotype, following an exception to the nomenclatural code for microscopic algae. We have justified this choice extensively, both based on an interpretation of the code and on the broader conceptual need to name newly discovered species, facilitating their use in science, conservation, and policy.