•   These hyphae have cellulose cells walls and are analogous with the hyphae of true fungi (with chitin cell walls).

•     Unlike fungi, the diploid stage dominates in oomycotes and they have biflagellated cells.

•     These filamentous bodies have extensive surface area, enhancing absorption of nutrients.

•      In the Oomycota, the “egg fungi”, a relatively large egg cell is fertilized by a smaller “sperm nucleus,” forming a resistant zygote.

•      Water molds are important decomposers, mainly in fresh water.

•     They form cottony masses on dead algae and animals.

•     Some water molds are parasitic, growing on the skin and gills of injured fish.

•      White rusts and downy mildews are parasites of terrestrial plants.

•     They are dispersed by windblown spores.

•     One species of downy mildew threatened French vineyards in the 1870’s and another species causes late potato blight, which contributed to the Irish famine in the 19th century.

•      The photosynthetic stramenopile taxa are known collectively as the heterokont algae.

•     “Hetero” refers to the two different types of flagella.

•      The plastids of these algae evolved by secondary endosymbiosis.

•     They have a three-membrane envelope and a small amount of eukaryotic cytoplasm within the plastid.

•     The probable ancestor was a red alga.

•      The heterokont algae include diatoms, golden algae, and brown algae.

•      Diatoms (Bacillariophyta) have unique glasslike walls composed of hydrated silica embedded in an organic matrix.

•     The wall is divided into two parts that overlap like a shoe box and lid.

•      Most of the year, diatoms reproduce asexually by mitosis with each daughter cell receiving half of the cell wall and regenerating a new second half.

•      Some species form cysts as resistant stages.

•      Sexual stages are not common, but sperm may be amoeboid or flagellated, depending on species.

•      Diatom are abundant members of both freshwater and marine plankton.

•     Diatoms store food reserves in a glucose polymer, laminarin, and a few store food as oils.

•     Massive accumulations of fossilized diatoms are major constituents of diatomaceous earth.

•      Golden algae ( Chrysophyta), named for the yellow and brown carotene and xanthophyll pigments, are typically biflagellated.

•      Some species are mixotrophic and many live among freshwater and marine plankton.

•      While most are unicellular,
some are colonial.

•      At high densities, they can form resistant cysts that remain viable for decades.

•      Brown algae (Phaeophyta) are the largest and most complex algae.

•     Most brown algae are multicellular.

•     Most species are marine.

•      Brown algae are especially common along temperate coasts in areas of cool water and adequate nutrients.

•      They owe their characteristic brown or olive color to accessory pigments in the plastids.

•      Some brown algae have floats to raise the blades toward the surface.

•     Giant brown algae, known as kelps, form forests in deeper water.

•     The stipes of these plants
may be 60 m long.

Structural and biochemical adaptations help seaweeds survive and reproduce at the ocean’s margins

•      The largest marine algae, including brown, red, and green algae, are known collectively as seaweeds.

•      Seaweeds have a complex multicellular anatomy, with some differentiated tissues and organs that resemble those in plants.

•     These analogous features include the thallus or body of the seaweed.

•     The thallus typically consists of a rootlike holdfast and a stemlike stipe, which supports leaflike photosynthetic blades.

•      Many seaweeds have biochemical adaptations for intertidal and subtidal conditions.

•     The cells walls, composed of cellulose and gel-forming polysaccharides, help cushion the thalli against agitation by waves.

•      Many seaweeds are eaten by coastal people, including Laminaria (“kombu” in Japan) and Porphyra (Japanese “nori”) for sushi wraps.

•      A variety of gelforming substances are extracted in commercial operations.

•     Algin from brown algae and agar and carageenan from red algae are used as thickeners in food, lubricants in oil drilling, or culture media in microbiology.

Some algae have life cycles with alternating multicellular haploid and diploid generations

•      The multicellular brown, red, and green algae show complex life cycles with alternation of multicellular haploid and multicellular diploid forms.

•     A similar alternation of generations evolved convergently in the life cycle of plants.

•      The life cycle of the brown alga Laminaria is an example of alternation of generations.

•      The diploid individual, the sporophyte, produces haploid spores (zoospores) by meiosis.

•      The haploid individual,the gametophyte, produces gametes by mitosis that fuse to
form a diploid zygote.

•      In Laminaria, the sporophyte and gametophyte are structurally different, called heteromorphic.

•      In other algae, the alternating generations look alike (isomorphic), but they differ in the number of chromosomes.

Rhodophyta: Red algae lack flagella

•      Unlike other eukaryotic algae, red algae have no flagellated stages in their life cycle.

•      The red coloration visible in many members is due to the accessory pigment phycoerythrin.

•     Coloration varies among species and depends on the depth which they inhabit.

•      The plastids of red algae evolved from primary endosymbiosis of cyanobacteria.

•      Some species lack pigmentation and are parasites on other red algae.

 

•      Red algae (Rhodophyta) are the most common seaweeds in the warm coastal waters of tropical oceans.

•     Others live in freshwater, still others in soils.

•      Some red algae inhabit deeper waters than other photosynthetic eukaryotes.

•     Their photosynthetic pigments, especially phycobilins, allow some species to absorb those wavelengths (blues and greens) that penetrate down to deep water.

•   One red algal species has been discovered off Bahamas at a depth of over 260m.

•      Most red algae are multicellular, with some reaching a size to be called “seaweeds”.

•     The thalli of many species are filamentous.

•     The base of the thallus is usually differentiated into a simple holdfast.

•      The life cycles of red algae are especially diverse.

•     In the absence of flagella, fertilization depends entirely on water currents to bring gametes together.

•     Alternation of generation (isomorphic and especially heteromorphic) is common in red algae.

Chlorophyta: Green algae and plants evolved from a common photoautotrophic ancestor

•      Green algae (chlorophytes and charophyceans) are named for their grass-green chloroplasts.

•     These are similar in ultrastructure and pigment composition to those of plants.

•     The common ancestor of green algae and plants probably had chloroplasts derived from cyanobacteria by primary endosymbiosis.

•      The charophyceans are especially closely related to land plants.

•      Most of the 7,000 species of chlorophytes live in freshwater.

•     Other species are marine, inhabit damp soil or snow, or live symbiotically within other eukaryotes.

•   Some chlorophytes live symbiotically with fungi to form lichens, a mutualistic collective.

•      Chlorophytes range in complexity, including:

•     biflagellated unicells that resemble gametes and zoospores

•     colonial species and filamentous forms

•     multicellular forms large enough to qualify as  seaweeds.

•      Large size and complexity in chlorophytes has evolved by three different mechanisms:

(1) formation of colonies of individual cells (Volvox)

(2) the repeated division of nuclei without cytoplasmic division to form multinucleate filaments (Caulerpa)

(3) formation of true multicellular forms by cell division and cell differentiation (Ulva).

•      Most green algae have both sexual and asexual reproductive stages.

•     Most sexual species have biflagellated gametes with cup-shaped chloroplasts.

A diversity of protists use pseudopodia for movement and feeding

•      Three groups of protists use pseudopodia, cellular extensions, to move and often to feed.

•      Rhizopods (amoebas) are all unicellular and use pseudopodia to move and to feed.

•      Pseudopodium emerge from anywhere in the cell surface.

•     To move, an amoeba extends a pseudopod, anchors its tip, and then streams more cytoplasm into the pseudopodium.

•      Amoeboid movement is driven by changes in microtubules and microfilaments in the cytoskeleton.

•      Pseudopodia activity is not random but in fact directed toward food.

•      In some species pseudopodia extend out through openings in a protein shell around the organism.

•      Amoebas inhabit freshwater and marine environments

•     They may also be abundant in soils.

•      Most species are free-living heterotrophs.

•      Some are important parasites.

•     These include Entamoeba histolytica which causes amoeboid dysentery in humans.

•   These organisms spread via contaminated drinking water, food, and eating utensils.

•      Actinopod (heliozoans and radiolarians), “ray foot,” refers to slender pseudopodia (axopodia) that radiate from the body.

•     Each axopodium is reinforced by a bundle of microtubules covered by a thin layer of cytoplasm.

•      Most actinopods are planktonic.

•     The large surface area created by axopodia help them to float and feed.

•     Smaller protists and other microorganisms stick to the axopodia and are phagocytized by the thin layer of cytoplasm.

•     Cytoplasmic streaming carries the engulfed prey into the main part of the cell.

•      Most heliozoans (“sun animals”) live in fresh water.

•     Their skeletons consist of unfused siliceous (glassy) or chitinous plates.

•      The term radiolarian refers to several groups of mostly marine actinopods.

•     In this group, the siliceous skeleton is fused into one delicate piece.

•     After death, these skeleton accumulate as an ooze that may be hundreds of meters thick in some seafloor locations.

•      Foraminiferans, or forams, are almost all marine.

•     Most live in sand or attach to rocks or algae.

•     Some are abundant in the plankton.

•     Forams have multichambered, porous shells, consisting of organic materials hardened with calcium carbonate.

•      Pseudopodia extend through the pores for swimming, shell formation, and feeding.

•   Many forams form symbioses with algae.

•      Over ninety percent of the described forams are fossils.

•     The calcareous skeletons of forams are important components of marine sediments.

•     Fossil forams are often used as chronological markers to correlate the ages of sedimentary rocks from different parts of the world.

Mycetozoa: Slime molds have structural adaptations and life cycles that enhance their ecological roles as decomposers

•      Mycetozoa (slime molds or “fungus animals”) are neither fungi nor animals, but protists.

•     Any resemblance to fungi is analogous, not homologous, for their convergent role in the decomposition of leaf litter and organic debris.

•      Slime molds feed and move via pseudopodia, like amoeba, but comparisons of protein sequences place slime molds relatively close to the fungi and animals.

•      The plasmodial slime molds (Myxogastrida) are brightly pigmented, heterotrophic organisms.

•      The feeding stage is an amoeboid mass, the plasmodium, that may be several centimeters in diameter.

•     The plasmodium is not multicellular, but a single mass of cytoplasm with multiple nuclei.

 

•      The diploid nuclei undergo synchronous mitotic divisions, perhaps thousands at a time.

•      Within the cytoplasm, cytoplasmic streaming distributes nutrients and oxygen throughout the plasmodium.

•      The plasmodium phagocytises food particles from moist soil, leaf mulch, or rotting logs.

•      If the habitat begins to dry or if food levels drop, the plasmodium differentiates into stages that lead to sexual reproduction.

•      The cellular slime molds (Dictyostelida) straddle the line between individuality and multicellularity.

•     The feeding stage consists of solitary cells.

•     When food is scarce, the cells form an aggregate (“slug”) that functions as a unit.

•   Each cell retains its identity in the aggregate.

•      The dominant stage in a cellular slime mold is the haploid stage.

•     Aggregates of amoebas form fruiting bodies that produce spores in asexual reproduction.

•     Most cellular slime molds lack flagellated stages.

Multicellularity originated independently many times

•      The origin of unicellular eukaryotes permitted more structural diversity than was possible for prokaryotes.

•      This ignited an explosion of biological diversification.

•      The evolution of multicellular bodies and the possibility of even greater structural diversity, triggered another wave of diversification.