A Light and Electron Microscope Survey of Algal Cell Walls. II, Chlorophyceae

Under the light and electron microscopes, the structure of the cell walls of members of the 11 orders in the Class Chlorophyceae, Division Chlorophyta, were examined. With regard to the microfibrillar component of the cell walls, five types of wall structure were distinguished: (A) an apparent lack of a micro fibrillar component (Volvocales, Dasycladales, and some members of the Siphonales), (B) the microfibrils are arranged in a reticulate pattern (Tetrasporales, Schizogoniales), (C) the microfibrils are oriented in an axial direction (Ulotrichales, Oedogoniales, Zygnematales, and some members of the Siphonales), (D) the microfibrils are parallel to one another and arranged in lamellae (Ulvales), and (E) the microfibrils are parallel to one another, arranged in lamellae, and at right angles to the microfibrils in the lamellae above and below forming the crossfibrillar pattern (Cladophorales, Siphonocladales). Members of the Ulvales were found to have a cell wall similar to that of the brown algae while a member of the Schizogoniales, Prasiola, was found to have a cell wall similar to that of the red algae. A discussion of the taxonomic implications of cell wall structure is included. The Division Chlorophyta, as delimited by Smith (1955), includes two classes, the Chlorophyceae, with the majority of green algae from unicellular to filamentous and complex parenchymatous thalli, and the Charophyceae, with only one order. The evolution within this Division is of especial interest because of its presumed close relationship with extant land plants (Fritsch, 1935). Similarities between the Chlorophyta and land plants include (1) chromatophores with similar chlorophyll and carotinoid pigments; (2) starch as a storage product; (3) the structure and insertion of flagella; and (4) the presumed possession of walls containing cellulose. A considerable amount is known of the structure and chemical nature of certain algal cell walls, but in the class Chlorophyceae most of the previous work has been concentrated on members of two orders, the Cladophorales and Siphonocladales. In both orders, the cell wall consists of highly crystalline cellulose microfibrils arranged parallel to one another in lamellae. These lamellae are oriented at right angles to each other with a third, less common direction, sometimes bisecting the first two (Valonia, Cronshaw and Preston, 1958; Dictyosphaeria, Steward and Muhlethaler, 1953; Cladophora and Chaetomorpha, Frei and Preston, 1961a). Two members of the Cladophorales appear to lack a microfibrillar component in the cell wall. Instead, the wall is amorphous in nature (Spongomorpha and Acrosiphonia, Nicolai and Preston, 1959). Manuscript received October 26, 1964. THE OHIO JOURNAL OF SCIENCE 66(3): 317, May, 1966. 318 CLINTON J. DAWES Vol. 66 In other orders of the Chlorophyceae, evidence for a microfibrillar component in the cell wall has been found in Chlamydomonas (Sager and Palade, 1957); Spirogyra (a multinet growth pattern, Limaye et al., 1963); Viva and Enteromorpha (a reticulum of microfibrils, Cronshaw et al., 1958); Chlorella (a reticulum of microfibrils, Northcote et al., 1958); Hydrodictyon (lamellae of microfibrils, Northcote et al., 1960); Oedogonium (lamellae of microfibrils, Parker, 1965); Halicystis (a polylamellar reticulum of xyloglucan microfibrils, Roelofsen et al., 1953); Bryopsis, Caulerpa, Penicillus, Udotea, and Halimedia (an axial orientation of 1-3 xylan microfibrils, Frei and Preston, 1961b); and Dichotomosiphon (1-3 xylan component, Parker et al., 1963). Cell walls lacking a microfibrillar component have been described for Chlamydomonas (Lewin et al., 1951); members of the Volvocaceae and Astrephemonaceae (Lang, 1963; Parker, 1965); Pediastrum (the cell wall consists of hexagonal units of an unknown substance, Moner and Chapman, 1963; which is not cellulose but consists of chains of rings, Parker, 1965); Codium, Dasycladus, Acetabularia, and Batophora (an amorphous wall structure composed of mannan polymers, Frei and Preston, 1961b). This preliminary survey is a lightand electron-microscope study of the Chlorophycean cell wall structure, with special regard to the microfibrillar component. In order to cover as many representatives as possible in determining the usefulness of the green algal cell wall as a taxonomic character, a review of previous work is included. Although the microfibrillar component may be but a small portion of the total cell wall, microfibrils can be set off from other wall substances as the primary framework because of the unusual size and high degree of molecular order. Consistent patterns within a taxa might then indicate relationships. The Charophyceae are not included since they have been treated comprehensively by Green (1960). Detailed studies of the chemical composition of the apparently amorphous walls and the interfibrillar substances which do not mask the fundamental pattern of the wall of specific forms will be considered in future work. MATERIALS AND METHODS Specimens of the green algae were collected in tide pools and by skin diving on the coasts of southern California, southern Florida, Puerto Rico, and southern Victoria, Australia. Fresh-water specimens were collected near the Mountain Lake Biological Station, Pembroke, Virginia, and in various mountain streams in southern California. The material was carried alive in polyethylene bags to the laboratory for study or was fixed immediately in 2 per cent formalin. The validity of the microchemical stains used, the methods of removal of incrusting substances, and the details of maceration including ultrasonic fragmentation have been previously discussed (Dawes et al., 1961). The cell wall of members of the Cladophorales and Siphonocladales can be separated into individual lamellae by removal of pectic material if the filament is bathed for 20 min in 2 per cent NH4OH at 60°C. Transverse, ultrathin sections (fig. 6a, 6b) were obtained using a Si-Ro-Flex EXPLANATION OF FIGURES All unit marks equal one micron. FIGURE 1. Acetabularia crenulata. The chemically cleared, ultrasonically macerated filament wall consists of an amorphous matrix, which is finely striated possibly due to folding. X 9,600. FIGURE 2. Tetraspora sp. A single cell isolated from the colonial matrix by ultrasonic maceration has a cell wall composed of a microfibrillar reticulum. The opaque structure is due to folding of the wall after drying. X 10,000. The insert is a portion of Tetraspora wall showing detail of microfibrillar "weft". X30,000. No. 3 ALGAL CELL WALLS OF CHLOROPHYCEAE 319 320 CLINTON J. DAWES Vol. 66 ultramicrotome after fixation in unbuffered 2 per cent KMnC>4 for 15 min at 0°C, in sea water (pH 6.7), dehydration in 10 per cent steps of butyl alcohol for 1 hr at each step, and embedding in methyl methacrylate. The sections were then washed twice in chloroform to remove the methacrylate and shadowed with platinum. The carbon replica of the interior wall of Apjohnia (fig. 7) was prepared by the wet replica technique (Wardrop, 1964). OBSERVATIONS AND DISCUSSIONS The following species were examined under the light and electron microscopes in the present study. Chlamydomonas reinhardi Urospora penicilliformis Haematococcus australis Anadyomene stellata Volvox sp. Spongomorpha coalita Tetraspora sp. Ulva taeniata Stigeoclonium sp. Entermorpha sp. Ulothrix sp. Prasiola meridionales Spirogyra sp. Chlorella sp. Chaetomorpha aerea Codium fragile C. torta Bryopsis pennata C. Darwinii Cladophorophsis membranacea Cladophora trichotoma Apjohnia laetevirens C. gramineae Struvea plumosa C. micro cladioides Dasycladus vermicular is C. sp. (fresh water) Acetabularia crenulata Rhizoclonium sp. Batophora oerstedi On the basis of their known cell-wall structure, each of these green algae can be placed into one of five categories according to the presence of a microfibrillar component where none was previously reported. Group A.—-The cell walls appear to lack a microfibrillar component. The representatives examined include unicellular, colonial, and siphonaceous forms from the orders Volvocales (Chlamydomonas, Haematococcus, Volvox), Siphonales (Codium), (Cladophorales (Spongomorpha), and Dasycladales (Acetabularia, Batophora, Dasycladus). Under the light microscope, with microchemical stains, the cell wall gives a positive reaction for pectin substances (ruthenium red) and a positive reaction for cellulose (I2KI, H2SO4) with the exception of Spongomorpha, Codium, Dasycladus, Batophora, and Acetabularia. Under the electron microscope, the cell wall of the members examined is granular in appearance and striations or folds are present (Acetabularia fig. 1). Regardless of chemical treatment used to remove possible incrusting substances, EXPLANATION OF FIGURES FIGURE 3A. Prasiola meridionalis. The chemically cleared and ultrasonically macerated cell wall of three cells consists of a reticulum of microfibrils with an impregnating amorphous matrix. X 10,800. The insert is a portion of Prasiola wall, showing microfibrils (arrow) and the covering amorphous matrix. X32,400. FIGURE 3B. Stigeoclonium sp. The chemically cleared cell wall of the filament consists of microfibrils arranged in an axial direction in the older, elongated cells. Plasmodesmata are visible transversing the end walls and connecting adjacent protoplasts. X 11,200. The insert is a portion of Stigeoclonium wall showing axially directed microfibrils. X33.600. FIGURE 4. Enteromorpha sp. A cell wall with parallel microfibrils in lamellae, of a portion of three cuboidal cells of the chemically cleared and ultrasonically macerated thallus is visible. A pit composed of a group of pores with abundant amorphous material covering the microfibrils is present on one side of the cell wall (arrow) X 12,000. No. 3 ALGAL CELL WALLS OF CHLOROPHYCEAE 321 322 CLINTON J. DAWES Vol. 66 no fibrillar material is found. The results of Frei and Preston (1961b) on the cellular wall structure of Dasycladus, Batophora, and Acetabularia (Dasycladales) is in agreement with the present study. Studies on Pediastrum (Moner and Chapman, 1963; Parker, 1964) indicate that it also lacks a microfibrillar component and should be placed in this group. However, microfibrils are present in other members of the Siphonales (Frei and Preston, 1961b; Parker et al., 1963) and Chloroccales (Northcote et al., 1960), indicating probable taxonomic divergences within the orders. Group B.—The cell wall contains a microfibrillar component in a reticulate pattern. Under the light microscope, with microchemical stains, the cell walls of Tetraspora (Tetrasporales, colonies of spherical cells embedded in a matrix) and Prasiola (Schizogoniales, a parenchymatous thallus) give a positive reaction for cellulose and pectic substances. Under the electron microscope, the loose weft arrangement of microfibrils in the cells is present in Tetraspora (fig. 2) and Prasiola (fig. 3a). No change in orientation is visible in the older cell walls of the thallus of Prasiola. In the cell wall of Tetraspora, small white spots, possibly remaining protoplasm, are visible (fig. 2) when no chemical clearing is undertaken. The colonial matrix of Tetraspora is amorphous in nature. Chlorella pyrenoidosa (Chlorococcales) has been shown to possess a cell wall composed of microfibrils in a reticulate pattern (Northcote et ai., 1958) and therefore falls into this group. Group C.—The cell wall is constructed of microfibrils arranged in an axial direction. Under the light microscope, and with microchemical stains, the cell wall of the filamentous representatives of the Zygnematales (Spirogyra) and Ulothrichales (Stigeoclonium, Ulothrix) gives a positive reaction for cellulose and pectic substances. Plasmodesmata are visible between proplasts of the plasmolyzed cells of the filaments. The mucilaginous sheath in Spirogyra gives a strong reaction for pectic substances. Under the electron microscope, an axial orientation of microfibrils is visible either without chemical treatment in older, elongated cells of Stigeoclonium (fig. 3b) and Ulothrix, or after removal of a mucilaginous sheath from the filaments of Spirogyra (fig. 5b). A reticulate pattern of microfibrils is evident in the bulbous, tip-cells of filaments of Stigeoclonium and Ulothrix and in the mucilage and outermost layer of microfibrils in untreated filaments of Spirogyra (fig. 5a). In Spirogyra, this microfibrillar reticulum covers a layer of axial microfibrils (fig. 5a, 5b). In a third and innermost layer, microfibrillar bands occur in transverse orientation (fig. 5c). Such a difference in microfibrillar arrangement (see also Limaye et al., 1963) in the inner and outer surfaces of the cell wall is consistent with the multinet growth hypothesis (Houwink and Roelofsen, 1954), although it appears that the microfibrils are reoriented as groups and not individually (in Spirogyra). Some members of the Siphonales (Bryopsis, Caulerpa, Penicillus, Udotea, Halimeda, Frei and Preston, 1961b), and possibly one member of the Chlorococcales, (Hydrodictyon, Northcote et al., 1960) possess microfibrils in axial orientation; however, the microfibrils consist of a xylan component. Group D.—The cell walls consist of microfibrils parallel to one another and arranged in lamellae. Under the light microscope and with microchemical stains the parenchymatous thalli of Ulva and Enteromorpha, members of the Ulvales, give a positive reaction for pectic substances and cellulose. As seen under the electron microscope after chemical clearing, the cell walls of both genera consist of parallel microfibrils in lamellae {Enteromorpha, fig. 4), which are arranged in flat spirals around the cell. Pits are either simple, thin areas in the cell wall, a random dispersion of pores, or groups of pores defined by an amorphous matrix (arrow, fig. 4). Oedogonium (Oedogoniales) has also been shown to possess lamellae of parallel microfibrils (Parker, 1964) and is placed here. This group appears to be intermediate between Group C and E in microfibrillar organization. No. 3 ALGAL CELL WALLS OF CHLOROPHYCEAE 323 FIGURE 5. Spirogyra sp. The outer wall in untreated filaments consists of a reticulum of microfibrils and amorphous mucilage (fig. 5a). After chemical clearing, the outermost randomly arranged microfibrils are visible (fig. 5b). With maceration, an inner wall of transverse bands of microfibrils is visible (fig. 5c). Arrows indicate the filament's axis. Figures 5a and 5b, X 10,000; figure 5c, X 14,600. 324 CLINTON J. DA WES Vol. 66 Group E.—The cell walls consist of microfibrils oriented in a cross-fibrillar structure. Under the light microscope with microchemical stains, the thick stratified cell walls of members of the Cladophorales (Cladophora, Chaetomorpha, Rhizoclonium, Urospora) and members of the Siphonocladales {Apjohnia, Struvea, Cladophoropsis, and Anadyomene) all give positive reactions for pectic substances and cellulose. After chemical clearing, the lamellated cell wall of all the specimens, examined under the light microscope, shows two directions of striations, one transverse to the filament's axis in a flat spiral, the other parallel to the filament's axis in a steep spiral. As seen under the electron microscope, the cell walls of Apjohnia (fig. 6a, 7) and Chaetomorpha (fig. 6b) have a thin outermost reticulate layer of microfibrils {Chaetomorpha, fig. 6b), which covers the filament and frays out onto the formvar. The remainder of the wall consists of lamellae of parallel microfibrils which are at right angles to the microfibrils in the lamellae above or below {Apjohnia, fig. 7), so that in cross section, the microfibrils of every third lamella are similar in orientation {Apjohnia, fig. 6a and Chaetomorpha, fig. 6b). Also, the inner lamellae are thicker than the outer ones (fig. 6a, 6b).