Phycology - Kostikov I.Yu. - 2009-2013

Chapter 6. Morphological Diversity of Algae

The Water/57.html">Diversity of Algae is extraordinarily apparent in their external appearance. Algae can be extremely small and visible only under a Microscope (some requiring Electron Cell/15.html">Microscopy), or they can reach large (and even giant) sizes measuring tens of meters. Algal thalli may have a simple Structure represented by single Cells, or complex differentiation into Tissues and Organs. Some algae are capable of active movement, thus resembling animals, while others lack motility and lead an attached, strictly plant-like lifestyle, or float in the water Column, passively migrating with currents. Algal thalli may consist of uninucleate cells, or their cells may contain numerous nuclei and grow to sizes easily discernible by the unaided eye.

The morphological diversity of algae reflects the major evolutionary trends of their body plan, with the general trajectory driven by the refinement of a true plant-like lifestyle strategy: increasing the photosynthetic surface area, transitioning to an attached mode of existence, growing larger for defense against grazing, and so forth. Under similar environmental conditions, morphological evolution along this primary trajectory resulted in the convergent body development of phylogenetically distant algal groups. For instance, the overarching trend associated with maximizing the photosynthetic surface and protecting against predators led to The Emergence of multicellularity in both prokaryotic and eukaryotic algae. This phenomenon of convergent evolution in algal body plans is known as morphological parallelism.

Specific structural blueprints of algae that reflect the principal stages of morphological evolution are referred to as types of morphological body structure, or simply structural types. Algae exhibit 8 types of morphological structure, three of which occur in unicellular forms (monadoid, hemimonadoid, coccoid), three in multicellular uninucleate algae (trichal, heterotrichal, tissue-grade), and two in multinucleate or so-called non-cellular algae (siphonous and siphonocladous). METABOLISM/2.html">THE CONCEPT OF morphological structural types applies exclusively to eukaryotic algae.

The body evolution of unicellular eukaryotic photoautotrophs proceeded toward the loss of motility and the transition to a stationary, plant-like existence.

The simplest structural type is considered to be the monadoid type. The primary feature of the monadoid structure is the presence of motile flagella. Monadoid algae are uninucleate and typically possess a eyespot (stigma) and contractile vacuoles.

Algae with a hemimonadoid type of structure are non-motile, yet they retain certain features characteristic of monadoid organisms—such as an eyespot, contractile vacuoles, or non-functional flagella known as pseudocilia.

Algae featuring a coccoid type of structure completely lack any characteristics of monadoid Organization. Coccoid forms are unicellular, non-motile, and devoid of an eyespot, flagella, and contractile vacuoles, thereby representing unicellular organisms with a classically plant-like lifestyle strategy.

Monadoid, hemimonadoid, and coccoid algae may exist as solitary cells or aggregate into colonies. Colonies are most commonly formed by holding algal cells together through a common mucilaginous matrix. The number of cells in a colony may either fluctuate during colony growth or remain constant throughout its entire lifespan, regardless of age. Colonies in which The Cell number remains fixed, and size increases solely through the growth of each individual cell, are called cenobia.

In multicellular algae, the morphological Evolution of the body was associated with the gradual functional differentiation of distinct PARTS OF THE thallus.

Thus, in algae with a filamentous (trichal) type of structure, the thallus typically takes the form of a filament composed of sequentially arranged uninucleate cells. The cells in filamentous algae are functionally and morphologically identical, with the exception of the basal and apical cells: the former is sometimes modified into a holdfast cell that may form a basal disc, while the latter can elongate into a Hair with partially reduced Chloroplasts. Filaments in representatives with a filamentous structure are most frequently unbranched (so-called simple), and less commonly branched, though all branches perform identical Functions and do not differ morphologically from one another.

A defining feature of the heterotrichal type of structure is the functional differentiation of filaments. In its simplest form, filaments differentiate into prostrate systems, which function to attach the Organism to the substrate, and erect systems, whose cells carry out Photosynthesis and reproduction. The erect filaments, in turn, may differentiate into supporting branches with thick cell walls and reduced chloroplasts, and assimilatory branches formed predominantly of small cells containing massive chloroplasts. In some algae, certain Branches of the assimilators are modified into protective cortical threads, or form hairs or parenchymal discs. Instances are known where the cells of functionally identical filaments in heterotrichal thalli fuse together to form false tissues. The formation of pseudoparenchymatous thalli is particularly characteristic of many red algae.

Tissue-grade structure arises through the functional differentiation of cell groups originating from specialized meristematic cells. Unlike higher plants, where meristematic cells drive growth, in algae these are predominantly Cells of the cortex or the so-called meristoderm. Algal thalli with a tissue-grade structure can be simple or complex. Simple thalli are composed of only two tissue types—cortex and medulla—whereas complex thalli may contain up to four tissue types: meristoderm, cortex, intermediate tissue, and medulla. Thalli with a tissue-grade structure are widely represented in the Phaeophyceae (brown algae) as well as in certain red algae.

Thalli formed by multinucleate cells are referred to as having a non-Cellular Organization. The term "non-cellular structure" originated in the 19th century when the first multinucleate algae were discovered. By that time, the fundamental tenets of Cell Theory had already been formulated, particularly the proposition that the elementary structural unit of Living matter is the uninucleate cell. The contradiction between this theoretical tenet and empirical observations led to the Introduction of the concept of "non-cellular structure," even though the cell remained the fundamental unit. From the early 20th century to the present day, the term "non-cellular structure" has been used to denote a specific multinucleate cellular architecture. Algae exhibit Two Types of non-cellular body structures: siphonous and siphonocladous.

In algae with a siphonous type of structure, the entire thallus constitutes a single multinucleate cell, often of considerable size. In many siphonous algae, thalli can exhibit quite complex differentiation—specifically, forming rhizoid-like attachment structures and upright assimilatory parts (for example, in the xanthophyte Vaucheria and the chlorophyte Bryopsis). However, these parts are not separated from one another by transverse cross-walls, and thus even such complexly differentiated thalli remain merely a single giant multinucleate cell.

Thalli of the siphonocladous type of structure are multicellular, with each individual cell being multinucleate. Siphonocladous thalli develop through a specialized mode of division known as segregative Cell Division, in which nuclear division (karyokinesis) outpaces cytoplasmic division (cytokinesis). Siphonocladous thalli frequently display complex functional differentiation, such as into prostrate and erect parts, rhizoids and supporting multinucleate threads, and so on.

In algal systematics, morphological structural types serve as an important taxonomic feature across various divisions at the rank of families and orders, and in some cases even classes. Different types of morphological structures have arisen multiple times independently across various phyla, with the monadoid type serving as the ancestral structural foundation in the majority of algal divisions.