Title: Origin and evolution of the genera Pleodorina and Volvox (Volvocales)

Author(s): Nozaki H

Source: BIOLOGIA 58 (4): 425-431 JUL 2003

Abstract: The previous molecular phylogenetic study using 6021 base pairs from five chloroplast genes suggested that two species of Pleodorina (P. californica, P. japonica) might have evolved from a Volvox-like alga by the decrease in colony cell number and size. However, number of species of the genus Volvox was very limited especially in the section Merrillosphaera.

In the present study, 6021 base pairs of the concatenated five chloroplast genes from 10 strains representing seven taxa of the genus Volvox were added to the previous data matrix. The sequence data resolved two anisogamous/oogamous clades within a large monophyletic group comprising five advanced genera of the Volvocaceae (Yamagishiella, Platydorina, Eudorina, Pleodorina and Volvox), one containing Volvox sect. Volvox and the anisogamous genus Platydorina (32-celled flattened colony), and the other (Eudorina group) composed of three other sections of Volvox, Pleodorina and Eudorina. The isogamous genus Yamagishiella (32-celled colony) was positioned basally to the Eudorina group. Therefore, evolution of anisogamy with sperm packets from isogamy might have occurred twice within the Volvocaceae. Based on the present molecular phylogenetic analysis, species of Volvox and Pleodorina within the Eudorina group represented three and two, respectively, separate lineages. One the three Volvox lineages [composed of V (sect. Merrillosphaera) carteri, V (sect. Merrillosphaera) obversus, V. (sect. Merrillosphaera) tertius, V. (sect. Merrillosphaera) africanus and V (sect. Copelandosphaera) dissipatrix] was sister to the monophyletic group consisting of one of the two Pleodorina lineages (P. californica and P. japonica) and V (sect. Janetosphaera) aureus. In addition, species of Eudorina were basal to the two lineages of Pleodorina and three Volvox lineages within the Eudorina group, representing the ancestral situation of Pleodorina/Volvox (excluding sect. Volvox). Thus, reverse evolution from a Volvox-like alga to Pleodorina suggested previously appears unlikely.

Strojsova A, Vrba J, Nedoma N, et al.

Seasonal study of extracellular phosphatase expression in the phytoplankton of a eutrophic reservoir
EUR J PHYCOL 38 (4): 295-306 NOV 2003

Abstract:
Many phosphorus-deficient algae and cyanobacteria produce extracellular, mostly cell-attached, phosphatases, presumably to make ambient organically bound phosphorus available. The distribution of phosphatase activity among natural phytoplankton populations and its ecological significance is largely unknown. Bulk extracellular phosphatase activity of plankton (using a standard fluorometric assay) and expression of phosphatases at the level of single phytoplankton cells were studied in the eutrophic Rimov reservoir during three consecutive seasons. The new enzyme labelled fluorescence (ELF) technique was modified by introducing (i) fixation with HgCl2 to preserve fragile species and (ii) use of polycarbonate filters to concentrate the phytoplankton. After enzymatic hydrolysis of artificial substrate (ELF(R)97 phosphate), the fluorescent product (ELF(R)97 alcohol, ELFA) formed insoluble precipitates at the site of phosphatase activity. Inhibition experiments suggested that both the standard assay and the ELF technique detected the same group of phosphatases. Weak ELFA formation and/or stability at pH > 8 might prevent sufficient detection of alkaline phosphatases using the ELF technique. ELFA labelling was detected in most algal classes, except for Euglenophyceae and the majority of Cryptophyceae and Chrysophyceae. Surprisingly, phosphatase activity was almost absent in the dominant populations. No ELFA labelling occurred in the phytoplankton in early spring 2000 and during the clear-water phases in all sampling years. Species of Cyanobacteria, Chlorophyceae and Conjugatophyceae showed phosphatase activity mainly in summer and at the beginning of autumn, while one species of Chrysophyceae (Synura petersenii) and three diatoms (Aulacoseira italica, Cyclotella spp., and Stephanodiscus hantzschii) produced phosphatases in spring. Several green algae (Ankyra ancora, Ankyra judayi, Coelastrum pseudomicroporum, Eudorina elegans, and Pediastrum spp.) were ELFA-labelled whenever present in the phytoplankton. Some species produced the ectoenzyme only in one season, such as Aphanizomenon flos-aquae (Cyanobacteria), Elakatothrix genevensis (Chlorophyceae), or Cryptomonas spp. On the other hand, one third of the 56 species studied never expressed any ELFA labelling. Large variability of phosphatase production was found not only among different algal species, but also within the population of one species. Not all cells of the population were equally ELFA-labelled and also the level of ELFA fluorescence was different. In particular cases, ELFA labelling on algal cells could be produced by bacterial rather than algal ectoenzymes. In comparison to standard methods, the ELF method provided more specific information about the variability of phosphatase activity (i.e. phosphorus stress) within the whole phytoplankton community as well as within one species populations.

Rengefors K, Ruttenberg KC, Haupert CL, et al.

Experimental investigation of taxon-specific response of alkaline phosphatase activity in natural freshwater phytoplankton
LIMNOL OCEANOGR 48 (3): 1167-1175 MAY 2003

Abstract:
It is widely accepted that alkaline phosphatase activity (APA) is an efficient indicator of phosphate limitation in freshwater phytoplankton communities. In this study, we investigated whether the response in APA to phosphate limitation differs among the taxa in a mixed phytoplankton assemblage. We used the new enzyme-labeled fluorescence (ELF) technique, which allows microscopic detection of phosphate limitation in individual cells of multiple species. The most prominent findings of this study were that alkaline phosphatase (AP) was induced in many, but not all taxa and that different taxa, as well as different cells within a single taxon, experienced different degrees of phosphate stress under the same environmental conditions. Our approach was to manipulate the limiting nutrient in a natural freshwater phytoplankton community by incubating lake water in the laboratory. We induced nitrogen (N) or phosphate limitation through additions of inorganic nutrients. Both the ELF assay and bulk APA indicated that the lake phytoplankton were not phosphate limited at the start of the experiment. During the experiment, several chlorophyte taxa (e.g., Eudorina and an unidentified solitary spiny coccoid) were driven to phosphate limitation when inorganic N was added, as evidenced by a higher percentage of ELF-labeled cells relative to controls, whereas other chlorophyte taxa such as Actinastrum and Dicryosphaerium were not phosphate stressed under these conditions. In the phosphate-limited treatments, little or no ELF labeling was observed in any cyanobacterial taxa. Furthermore, all taxa observed after the ELF labeling procedure (>10-mum fraction) were labeled with ELF at least on one occasion, demonstrating the wide applicability of the ELF method. By using ELF labeling in tandem with bulk APA, the resolution and analysis of phosphate limitation was increased, allowing the identification of specific phosphate-stressed taxa.