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The strange Pelagophyceae: Now also defined ultrastructurally?

A problem for the classification of plants and animals, including algae, is determining whether taxa at different taxonomic levels such as species, genera, and families can be distinguished using morphological, biochemical, and/or ecological characters, or whether molecular data are required for identification. An interesting example of how morphological and molecular data can—perhaps—be combined is presented by Wetherbee et al. (2021) in the present issue. Wetherbee and co-workers describe three new marine benthic species of heterokont algae from Australia belonging to two new genera: Gazia and Glomerochrysis. They belong to a class of heterokont algae, the Pelagophyceae, a little-known group of tiny golden-brown algae. This class is one of several that includes species formerly placed in the Chrysophyceae. The Pelagophyceae was erected by Andersen et al. (1993) to include a single, tiny flagellate, Pelagomonas calceolata, collected in the North Pacific in 1973 and kept in culture at the CCMP. Pelagomonas calceolata was found to be a highly unusual flagellate; it was truly uniflagellate, even a second basal body was missing in the cell, and Andersen et al. (1993) found no flagellar roots. Subsequent studies added to the unusual features; prior to cell division, the single (hairy) flagellum shortened but instead of transforming into a new smooth flagellum, it simply disappeared. Two new hairy flagella subsequently formed de novo. There was no eyespot. Pelagomonas gave the impression of being a modified (reduced) golden-brown flagellate, but neither ultrastructure nor molecular studies gave information about closely related species. The authors, therefore, established the new class Pelagophyceae for this species only, but they wisely suggested that several other very small unicells might also belong there. Only four years later, two minute (picoplanktonic) coccoid species, Pelagococcus subviridis from the North Pacific (Lewin et al. 1977, Vesk and Jeffrey 1987) and the mussel-death-causing Aureococcus anophagefferens from Narragansett Bay (Sieburth et al. 1988), were added to the Pelagophyceae, as was the Texas brown tide species Aureoumbra lagunensis (DeYoe et al. 1997). At the same time, Saunders et al. (1977) found the enigmatic order Sarcinochrysidales (Gayral and Billard 1977) to form a sister group to Pelagophyceae and tentatively included it in this class, to form the second order of the class. This added the three genera Sarcinochrysis, Ankylochrysis, and Chrysoreinhardia (Pulvinaria) to the class, which has continued to grow rapidly in the number of described genera and species. Thus, no less than 5 new genera of (mainly) benthic, colonyforming genera were described by Han et al. (2018): Arachnochrysis, Chrysocystis, Pelagospilus, Sargassochrysis, and Sungminboa. Even a filamentous species, Andersenia (apparently a uniseriate filamentous colony, with two species) was described by Wetherbee et al. (2015), and a monostromatic species, Aureoscheda bahamensis, was described by Wynne et al. (2014) from a dried herbarium sheet (!); the material collected on dead corals in the Bahamas. The group presently comprises 16 genera, and individual cells are generally tiny, smaller than 10 lm in size. They all possess plastids but lack eyespots. Phagotrophy has apparently not been reported. The classification of the 16 genera is presently in two orders and three families, the Pelagomonadales with the single-family Pelagomonadaceae, and the Sarcinochrysidales with two families: Sarcinochrysidaceae and Chrysocystaceae. The diagnostic features used to characterize the orders and the families are restricted to Pelagomonadales when this was first described: “flagellate cells,” and Pelagomonadaceae: “thecate cells.” The Sarcinochrysidales and its families were defined solely using molecular characters. In fact, all families and orders within the class are now based on gene sequence comparisons. Does this bewildering assemblage of minute marine species share any morphological characters or is the class characterized only by gene sequences? At first sight, the morphological diversity needs to be examined for phylogenetic importance, including “gross” morphological features like unicell (flagellate or coccoid) versus colony-forming (up to 2,500 cells in a colony), and the colonies of the different genera being sarcinoid, sheet like, or forming a uniseriate filament. Unfortunately, this “gross” J. Phycol. 57, 393–395 (2021) © 2021 Phycological Society of America DOI: 10.1111/jpy.13130