Phycology - Kostikov I.Yu. - 2009-2013

Chapter 3. General Characteristics of Algae

Species richness of Algae. Algae represent a phylogenetically heterogeneous and rather diverse ecological and biological group of photoautotrophic organisms. The species richness of currently known algae is estimated at 55,000–60,000 species. However, these data do not reflect the real picture, since, according to various authors, the actual number of algal species ranges from 150,000 to 10 million. An analysis of data on the discovery and description of new algal taxa confirms this forecast. For example, over the last decade alone, the number of known algal species has increased by almost a third (Table 3.1), primarily due to monograph revisions of certain groups of diatoms and The Development of the Taxonomy of a new Class of green algae, Trebouxiophyceae.

Table 3.1. Observed (based on 1996 and 2008 data) and expected species richness of various algal phyla

The flora of Ukraine currently records about 5,000 species of algae, represented by more than 6,000 infraspecific taxa, which accounts for about 10% of the species richness of the global algoflora. One of the phyla — Chlorarachniophyta — has not yet been discovered in our country.

Table 3.2. Diversity of various algal phyla in the flora of Ukraine⁴

One of the consequences of the phylogenetic heterogeneity of algae is the extraordinary diversity of their somatic Morphology, which visually manifests itself primarily in the variety of Sizes and Shapes of Cells and vegetative thalli, as well as in a wide spectrum of color variations.

Upon more detailed comparison, algal diversity is evident at all levels, ranging from Genome Organization features, biochemical Specificity, and ultrastructural Cell organization to the supra-organismal level, in particular the Composition and Structure of communities formed by different algae.

Sizes. Although the size characteristics of algal cells span a wide range, prokaryotic algal cells are generally smaller than eukaryotic ones.

The minimum cell sizes in prokaryotic algae are close to the limit of Light Microscope resolution (about 0.2 µm). In particular, the smallest algal cells have been noted in the unicellular thermophilic blue-green alga Cyanobium gracile: their minimum length is 0.4 µm and their width is 0.2 µm. The Cell diameter of the subaerophytic colonial alga Merismopedia minima, which grows on damp rocks, is 0.4–0.8 µm, while that of the colonial freshwater planktonic alga Aphanocapsa delicatissima is 0.5–0.8 µm. The colonial freshwater benthic alga Aphanothece nebulosa has cells 0.5–1.0 µm long and 0.3–0.4 µm wide. However, in the vast majority of prokaryotic algae, cell sizes range from 1 to 5–7 µm. The largest cells among prokaryotic algae are observed in the freshwater multicellular blue-green alga Oscillatoria princeps — their width reaches 80 µm, which is 400 (!) times greater than the minimum cell width recorded for Cyanobium gracile.

The size characteristics of eukaryotic algal cells vary across a much wider range than those of prokaryotes. Cell size is primarily influenced by the number of nuclei — uninucleate cells are typically smaller than multinucleate ones. The increase in cell size with an increase in the number of nuclei is a direct manifestation of the effect known as the relative constancy of the nucleoplasmic ratio, whereby a specific volume of cellular Cytoplasm is controlled by a specific volume of The Nucleus. Therefore, an increase in the number of nuclei leads to an increase in cytoplasmic volume and, consequently, to an increase in linear cell dimensions.

Among uninucleate eukaryotic algae, the smallest cells are recorded in the green algae Nannochloris coccoides (from 0.7 µm long and 0.5 µm wide) and Stichococcus atomus (from 1 µm long and from 0.5 µm wide). The largest unicellular uninucleate alga is the marine green alga Acetabularia. In its pre-reproductive state, its cell is differentiated into a basal system consisting of rhizoidal outgrowths and containing a single giant primary nucleus (up to 0.2 mm in diameter), an upright "stalk" portion up to 18 cm high and 1–2 mm thick, and a system of apical branched siphonous outgrowths forming the so-called cap — an apical structure up to 6 cm in diameter. Due to its giant cell size, Acetabularia has proved to be an extremely convenient object for various studies in experimental Cytology. It was on Acetabularia that classical experiments establishing the function of the nucleus were performed. However, in the absolute majority of cases, the sizes of uninucleate cells in eukaryotic algae range from 3 to 30 µm.

The maximum sizes of multinucleate cells far exceed the cell size of Acetabularia. For instance, the marine siphoneous (i.e., unicellular multinucleate) green alga Codium produces macroscopic thalli up to 40 cm in height. At the same time, the thallus, which represents a single giant cell, contains several tens of thousands of nuclei.

The siphoneous yellow-green alga Vaucheria sessilis, whose thalli appear as branched multinucleate tubes, is capable of forming macroscopic growths resembling a felt-like mat, where the total length of all tubes in a single thallus can reach thousands of kilometers. For example, given an average tube diameter of about 25 µm in a mat covering an area of about 1 m² and only about 5 cm thick, left on the soil after a puddle dried up in the vicinity of the Kaniv Nature Reserve, the calculated linear length of the thallus was over 40,000 km — that is, approximately equal to the Earth's equatorial circumference. Furthermore, such a thallus represented a single cell, albeit a multinucleate one.

The thalli of multicellular algae can also be either microscopic or clearly visible to the naked eye, i.e., macroscopic. Algae with multicellular microscopic thalli are found in the phyla Cyanophyta, Chrysophyta, Xanthophyta, Phaeophyta, Haptophyta, Rhodophyta, and Chlorophyta. Macroscopic multicellular algae are represented in only three phyla — Phaeophyta, Rhodophyta, and Chlorophyta.

Among multicellular algae, the largest thalli are observed in the marine brown alga Macrocystis pyrifera. Its vegetative body consists of a holdfast with rhizoids and a stipe from which leaf-like blades (sporophylls) arise. The stem length in adult specimens is about 30–50 m. The largest reliably recorded specimen of Macrocystis had a stem 107 m long. However, some sources cite data on stems of this alga reaching even greater lengths — from 300 to 400 m.

Diversity of body shape. Algae exhibit an extremely wide range of vegetative body shapes. In a significant number of algae, the entire body consists of a single cell. However, the variety of shapes of such cells is extraordinary — from simple spherical, ellipsoidal, and cylindrical cells to cells with complex geometry: polyhedral, with sharp and obtuse angles, with various spines, bristles, outgrowths, and folds. Cells can be symmetric and asymmetric, polar and non-polar, dorsiventral or not. Sometimes cells possess specialized attachment structures — stalks, holdfasts, rhizoidal outgrowths, etc.

Compared to unicellular algae, the cells of multicellular algae are more uniform in shape, with cylindrical cells being the most widespread variant. However, such cells of geometrically simple shape can form not only simple, mostly filamentous, but also complexly branched thalli, for the description of which the terminology developed by vascular plant morphologists is applied.

Coloration. The absolute majority of algae, as photoautotrophic organisms, possess vivid coloration. Typically, the coloration is determined by a specific set of Photosynthetic Pigments (chlorophylls, phycobilins, carotenoids) that are obligatory Components of the elementary structural units of the photosynthetic apparatus — thylakoids. Since the pigment composition varies across different algal phyla, the color spectrum of algae, unlike that of higher plants, is much broader. For example, most blue-green and all glaucocystophyte algae are colored purple, blue, or blue-green; diatoms, brown, golden, dictyochophyte, and haptophyte algae range in various shades of yellow, from golden to dark brown. Euglenoids, chlorarachniophytes, yellow-green, eustigmatophytes, and most green algae, similarly to higher plants, are green. Among red algae, species with red coloration are overwhelmingly predominant.

In some algal phyla, coloration is not constant and universal within the phylum, but rather a species-specific trait. For example, cryptophyte algae include species with red, blue, green, and yellow coloration, as well as colorless forms. A similar picture is observed in the phylum dinoflagellates (Dinophyta).

In many species of green algae, coloration changes depending on cell age or environmental conditions. For instance, the green alga Haematococcus pluvialis has green-colored cells in its youth, which turn red as they age. A similar color change is demonstrated by the salt lake inhabitant Dunaliella salina, though triggered by a sudden change in salt concentration in the brine.

Some species of blue-green algae change color depending on the spectral characteristics of light: when illuminated with short-wavelength rays (ultraviolet, violet, and blue rays), the cells turn red, and under long-wavelength rays, they turn blue. The phenomenon of changing color depending on light quality is known as chromatic adaptation.

Distribution and main ecological groups of algae. Most algae are organisms whose existence is tied to the aquatic environment (which is reflected in their very name in Ukrainian — "alga-growth" [vodo-rist])⁵. In aquatic biotopes, they are a component of the hydrosphere's plant population and, together with higher aquatic plants, form the hydrophyton. The number of algal species recorded within the hydrophyton is estimated at approximately 50,000 (i.e., over 90% of the total species richness of all known species).

In aquatic biotopes (both marine and continental), algae are integral components of the biocenoses of the Water Column (plankton), the bottom (benthos), solid substrates raised above the bottom (periphyton), and the surface microlayer of water (neuston), forming phytoplankton, phytobenthos, phytoperiphyton, and phytoneuston, respectively. Algae inhabiting different biotopes possess specific suites of adaptations tailored precisely to those environments. In many cases, particularly among aquatic organisms, these adaptations are vividly reflected in their morphology.

Specifically, planktonic algae exhibit adaptations that keep them suspended within the water column. For instance, some species bear various outgrowths, spines, or bristles (e.g., Golenkiniopsis), or form specific colonies, such as flat ones (Pediastrum, Merismopedia). Through these mechanisms, planktonic algae increase their body surface area and, consequently, water frictional resistance, thereby reducing their sinking rate to the bottom. Others accumulate substances that lower the specific gravity of their cells and multicellular thalli—such as oil droplets in the cytoplasm (diatoms), gas mixtures in gas vacuoles (cyanobacteria), or atmospheric air in air bladders (certain species of the genus Sargassum). A quite diverse group of phytoplankton comprises so-called active plankters—algae that counteract sedimentation by actively swimming using flagella (Chlamydomonas, Peridinium). In many cases, a single Organism possesses multiple different adaptations for a planktonic lifestyle. For example, in several Scenedesmus species, cells forming flat colonies also bear spines and bristles; in species of the genus Gonium, flat colonies are composed of flagellated cells; while individual Mallomonas cells simultaneously feature flagella, silica scales with very long spines, and oil droplets. Furthermore, the very small size of cells helps many microscopic planktonic algae remain suspended in the water column.

The morphological features of benthic algae are driven by different factors. In particular, clay and sand particles—brought in by currents and runoff from land, alongside dead remains of planktonic and neustonic hydrobionts—constantly settle onto the bottom of water bodies from the water column. To avoid being buried under this solid "rain," some unicellular benthic algae produce various mucous stalks that continuously grow and elevate the cells above the bottom surface. Others possess mechanisms enabling efficient locomotion precisely among solid particles, such as raphes in diatoms, contractile fibrils in the cell walls of cyanobacteria, cytoskeletal elements for metabolic body shape changes and vermiform movement (in many euglenoids), or, more rarely, flagella. In multicellular benthic algae, the primary adaptation against sediment "rain" is most often the growth of the thallus, frequently reaching macroscopic sizes (which is especially pronounced in marine brown and red algae).

The habitus features of periphytic algae are most strongly linked to the necessity of remaining on the substrate surface while withstanding The impact of currents, wind, and wave action.

About 5,000–7,000 algal species (roughly 10% of the known total) inhabit non-aquatic biotopes. Specifically, around 4,000 species permanently reside in soils, forming the edaphon. Over 1,000 species live on substrates elevated above the soil—such as tree bark, rock surfaces, and various terrestrial structures—constituting the aerophytion. In addition, algae develop On the surface of snow and ice (cryophytion) and in grottoes and caves (so-called hypogean algae).

A considerable number of algae form symbioses with various organisms. Among them, the most diverse group (exceeding 1,000 species) enters into Symbiosis with Fungi to form Lichens; such algae are termed lichenophilous. A relatively small number of algae (several dozen species) are intracellular and intratissue symbionts of animals—primarily certain Ciliates, rotifers, crustaceans, Coelenterates, and Sponges.

¹ Based on: Norton T.A., Melkonian M., Andersen R.A. Algal biodiversity. - Phycologia, 1996, vol. 35, № 4. - 308-326.

² ibid.

³ in parentheses – including fossil representatives

⁴ Based on data from: Raznoobrazie vodoroslej Ukrainy / ed. by Wasser S.P., Tsarenko P.M. - Algologia, 2000, vol. 10, № 4: 3-309; Tsarenko P.M., Petlevanny O.A. Supplement to "Algae Diversity of Ukraine". - Kyiv: M.G. Kholodny Institute of Botany, NASU, 2001. - 130 p.; Kostikov I.Yu. et al. Soil Algae of Ukraine. - Kyiv: Phytosociological Center, 2001. - 300 p.; Algae of Ukraine / eds. Tsarenko P.M., Wasser S.P., Nevo E. - Ruggell: A.R.A. Gantner Verlag, 2006. - 714 p.

⁵ The author of the term "водорості" in Ukrainian and "Водоросли" in Russian is Professor M.O. Maksymovych, the first rector of St. Volodymyr University of Kyiv (now Taras Shevchenko National University of Kyiv).