The Algae are categorized as Eukaryotic Protists encompassing distantly related groups of mostly aquatic, photoautotrophic organisms containing specific chlorophylls, carotenoids, and pigments that limit them to a particular habitat.

Phylum Dinoflagellata
This group consists of unicellular, free-floating, marine organisms.  The dinoflagellates have a distinctive appearance in that their chloroplasts are golden brown color and they have two flagella, one in an equatorial groove, and the other extends longitudinally.
Pfiesteria piscicida
This dinoflagellate is a single-celled, microscopic organism found along the eastern coast of the United States.  Their life cycle is highly complex and consists of up to two dozen known stages, ranging from cyst form to an amoeboid form, depending on the cells environmental conditions.  P. piscicida can be both plant-like and perform photosynthesis or animal-like in a heterotrophic manner, consuming other organisms such as bacteria and other algae.  When in the presence of fish excrement or secretions, they are stimulated to metamorphose into a toxin producing form.  In this form, they can be harmful to fish by releasing neurotoxins into the water.  Large concentrations of the neurotoxin stuns the fish, allowing the cells to feed on the fish's skin, blood, and tissues.  Fish subjected to this predatory behavior have diagnostic quarter-sized lesions on their carcasses. 

Phylum Phaeophyta
The Phaeophyta are multicellular, marine algaeAlso known as Brown Algae, they get their brownish tinge from the carotenoid, fucoxanthin, which is highly concentrated in their chloroplast.

Macrocystis pyrifera
More commonly known as the Giant Kelp, M. pyrifera is very widespread, located along the eastern coast of the Pacific as well as along the coasts of South America, South Africa, and Australia.  They grow in dense groupings called kelp forests where they provide food and shelter for many marine animals.  Growing up to 180 feet in length and up to two feet per day, this algae is one of the fastest growing organisms on Earth.  Their stalks arise from holdfasts which anchor them to rocky substrate.  The holdfast establishes itself along the substrate using the cement-like properties of alginic acid which the kelp itself produces.  Alginic acid is also harvested commercially and is used as emulsifiers and thickeners in ice cream and cosmetic products.  Each blade of the organism contains a single pneumatocyst, or gas bladder, which provides buoyancy and allows their large structure to float.  When waters become nutrient poor, the kelp can translocate nutrients up the stipe through sieve elements in the same way that vascular plants use their phloems.

Phylum Rhodophyta
The Rhodophyta, or Red Algae, are almost completely multicellular, marine species.  Their characteristic color is due to the photosynthetic pigment, phycoerythrin.  Photosynthetic products are stored as floridean starches.  They do not contain any motile cell at any stage of their life cycle, however, male gametes are amoeboid.

Class Florideophyceae
This group exhibits apical growth with the presence of pit connections and filamentous gonimoblasts.  Their life cycle has three stages consisting of haploid gametes and diploid carpospores and tetrasporophytes.

Corallina officinalis
This species is a red seaweed that is distributed along the Atlantic coasts of Europe, along the coasts of the Indian Ocean, and the western coast of the Pacific where it ranges from the lower to mid-littoral zones.  It is a calcareous algae, possessing the biochemical machinery for the production of calcium carbonate in and around their cell walls.  This ability helps to enhance the formation of coral reefs.  As the algae die, their calcium carbonate persists, forming rocky masses.  The calcium carbonate in the algal cells significantly strengthens their thalli.  This aspect is highly beneficial to the seaweed as its environment is continually subjected to forceful disturbances from waves.  The fronds of the seaweed grow in tufts, close to the substrate, and are segmented.  These attributes provide flexibility for the organism and decrease any negative effects to their structure in the tidal zones.  Additionally, the calcium carbonate in its cells protect it from many predators, making it an undesirable food source for most grazers.  Asexual reproduction in this species is carried out by the formation of spores.

Phylum Bacillariophyta 
The Bacillariophyta, or diatoms, are unicellular organisms that contain complex cell walls made of  pectin and silica.  Their all possess bilateral or radial symmetry and are constructed in two frustules in which the top overlaps the bottom.

Class Bacillariophyceae
All species in this group are radially symmetrical, containing a raphe, or long slit, along their axis.
Cyclotella atomus
C. atomus are small, centric diatoms that are widespread among marine and fresh water environments throughout every continent.  Their valves are short cylinders and can be found in solitary, in pairs, or forming colonies.  The face of the circular valve contains rows of areolae radiating from the central area.  While they primarily perform photosynthesis within their numerous discoid-shaped plastids, they are also able to grow in periods of sustained darkness where they are capable of making use of glucose of other dissolved organic compounds in their environment.  When the cell undergoes binary fission, the daughter cell receives one of the two frustules from the parent cell and thus, each generation becomes smaller and smaller.  Eventually, if asexual reproduction continued, the cell line would vanish.  Sexual reproduction, however, fixes this potential problem.  Within their cell walls, gametes are formed, shed, and fuse to produce a zygote.  The zygote then increases in size substantially before a new cell wall is laid down.

Phylum Oomycota
The Oomycota, or water molds, are filamentous, non-photosynthetic, heterotrophs.  They can be aquatic or terrestrial where they possess flagellated reproductive cells and contain cellulose in their cell walls.

Saprolegnia parasitica
S. parasitica is a fresh water mold that demonstrates a saprotrophic feeding style on dead organic material.  Feeding is accomplished by secreting enzymes that digest large food molecules into smaller molecules that can then be absorbed through the cell wall of the organism.  As the dispersal stage, the flagellated zoospore continually encysts, releasing new spores, until a suitable substrate is found.  Because this species is not tissue specific, such a substrate could be any type of dead tissue as well as compromised organisms where S. parasitica can act as a secondary pathogen by introducing bacteria and other infectious agents.  They are most notably problematic to catfish, salmon, and trout, especially in aquaculture farms where the density of fish is high.  Hairs on the spore's surface allow them to latch onto the substrate.  The mold then spreads across the substrate surface through cellular necrosis, forming masses of individual hyphae that look like fibrous, cotton films.