Brown Algae: Characteristics, Classification, and Ecology, Summaries of Botany and Agronomy

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BIOLOGY OF BROWN

ALGAE

• (^) Brown algae are algal species characterized by being multicellular and having brown or greenish brown color. The color is due to the predominance of brown pigments, e.g. fucoxanthin.
• (^) in addition to the green pigments (chlorophyll a and c). Many of them are macroscopic_._
• (^) Fucus is another macroscopic brown algal species. They thrive in intertidal zones of rocky seashores. They are harvested, dried, and processed for the commercial production of soap, glass, etc. They are also used as fertilizers. Synonyms: Phaeophyta; Phaeophyceae; Fucophyceae.

(1) cell walls composed of cellulose, alginic acid, and various polysaccharides; (2) cellular inclusions of polyphenolic polymers, known as physodes; (3) chloroplasts with thylakoids in stacks of three, enclosed by a girdle lamella; and (4) their main storage product in laminarin, a β-1,3-glucan. Some species also produce mannitol, sucrose, glycerol, or oils as storage reserves. Unlike other members of the phylum, all members of the Phaeophyceae are multicellular; none are unicellular in the vegetative phase, the predominant morphology in other golden-brown groups. Most species have an alternation of haploid and diploid generations, which may be either isomorphic or heteromorphic. Out of the estimated 1836 species in ~285 genera described for the fewer than 1% are known from freshwater habitats.

Diversity and Classification

• (^) A Diversity and Classification Freshwater brown algae are undoubtedly the least diverse of all groups of freshwater algae. While some species can at times form substantial or even dominant expanses in streams, only a few freshwater habitats are known that have more than one brown algal species in a single location.

Sub-groups Phaophyceae is comprised of the following taxonomic orders(4):

• (^) Ascoseirales
• (^) Asterocladales
• (^) Chordales
• (^) Desmarestiales
• (^) Dictyotales
• (^) Discosporangiales
• (^) Ectocarpales
• Fucales
• (^) Ishigeales
• Laminariales
  • Nemodermatales
  • (^) Onslowiales
  • Phaeophyceae ordo incertae sedis
  • (^) Phaeosiphoniellales
  • Ralfsiales
  • (^) Scytothamnales
  • (^) Sphacelariales
  • Sporochnales
  • (^) Stschapoviales
  • Syringodermatales
  • (^) Tilopteridales

• (^) Current classification of freshwater members of the Phaeophyceae is therefore still largely based on:
• (^) morphological and reproductive structures:
  • (^) growth habit,
  • (^) branching pattern,
  • (^) plastid shape and number,
  • and type of sporangia
  • (^) Based on these criteria, the present scheme recognizes seven freshwater species in six genera , within four orders.

ECOLOGY AND DISTRIBUTION

• (^) Ecological Factors Ecology of freshwater phaeophytes is not extensive, although an increasing number of autecological and bioassessment studies have appeared that shed some light on their ecological limits and requirements.
• (^) Habit and Substrata All species of freshwater brown algae adopt a benthic habit, but the range of substrata or hosts and habitats they occupy is quite diverse. - Heribaudiella fluviatilis (Areschoug) Svedelius colonizes stones in clear streams or lakes.
• (^) In this encrusting species, there appears to be a preference for more resistant rocks, such as basalt, quartz, schist, and gneiss

• (^) Current Velocity A preference for flowing water has not been clearly established for any freshwater phaeophyte; nearly all species are known from both river and lake habitats.

Early divergence events

• (^) The orders Discosporangiales and Ishigeales branched from the other brown algal lineages early in the evolutionary history of the brown algae sometime at the beginning of the Mesozoic Era.
• (^) Discosporangiales exhibit uniseriate, branched filaments with apical meristematic cells, but lack the heterotrichous growth pattern (prostrate and upright thalli projections) common in many other brown algal order.
• (^) Ishigeales include branched, upright or flattened parenchymatous thalli up to 10–20 cm high.

Mid-Mesozoic diversification

• (^) Sometime during the Mid-Mesozoic (approximate timeframe for the Jurassic period, 200–145 Ma; Figure 3), the SSDO clade split from the lineage that gave rise to the remaining extant brown algal orders and diversified into what are now four orders: Sphacelariales, Syringodermatales, Dictyotales, and Onslowiales
• (^) Dictyotales , which currently encompasses a great deal of brown algal species diversity.
• Sphacelariales has been characterized by a thallus structure
• (^) Onslowiales is a small order comprising two genera which were traditionally classified in Sphacelariales.
• (^) Syringodermatales have fan-shaped thalli that develop by lateral cohesion of filaments arising from a marginal meristem.

• (^) Fucales is a large order of more than 500 species and 9 families, members of which are major components of coastal ecosystems globally, including cold water regions of the Northern ( Fucus, Ascophyllum, Pelvetiopsis , Silvetia , etc.) and Southern Hemisphere ( Durvillaea, Cystophora, etc.), as well as in warm temperate to tropical coastal ecosystems ( Cystoseira s.l. , Sargassum, Turbinaria , etc.)
• (^) Asterocladales comprise a single genus Asterocladon, which is distributed from tropical to temperate coasts.

• (^) Ectocarpales , on the other hand, is the most speciose brown algal order, with more than 750 species in more than 100 genera and 5 to 6 families, though the taxonomy at the genus and family rank is far from resolved
• (^) Laminariales the Laminariales includes the largest marine macroalgae, and often form large ‘kelp forests,’ which provide habitats for a wide range of other taxa.
• (^) Chordales is a small order, sister to the Laminariales, with nine known species in three genera and three families distributed in temperate to cold-water regions of the Northern Hemisphere.

• (^) Ralfsiales is composed of 36 species from 7 genera distributed from tropical to cold water regions.
• (^) Nemodermatales consists of two crustose monotypic genera from temperate coasts, Nemoderma (Nemodermataceae) and Zeacarpa (Zeacarpaceae)