The Eukaryotes: Fungi, Algae, Protozoa, and Helminths



Algae are unicellular, filamentous, or multicellular (thallic).

Most algae live in aquatic environments.



Characteristics of Algae

All algae are eukaryotic photoautotrophs that produce oxygen (except for the water molds, which are fungus-like chemoheteroptrophs).

The thallus (body) of multicellular algae usually consists of a stipe, a holdfast, and blades.

Algae reproduce asexually by cell division and fragmentation.

Many algae reproduce sexually.

Algae are classified according to their structures and pigments.

Selected Divisions of Algae


Brown algae (kelp) may be harvested for algin.


Red algae grow deeper in the ocean than other algae because their red pigments can absorb the blue light that penetrates to deeper levels.

Irish moss (Chondrus crispus) produces carrageenan, used as a food thickener.

Gracilaria can be used for food but some species are toxic.


Green algae have cellulose and chlorophyll a and b and store starch.



Diatoms are unicellular and have pectin and silica cell walls: some produce a neurotoxin, domoic acid, that causes a rare syndrome known as amnesic shellfish poisoning (ASP). The toxin concentrates in shellfish, primarily in mussels.

Symptoms start with gastrointestinal distress within 24 hours and may include dizziness, headache, disorientation, and permanent short-term memory loss. In severe cases, seizures, focal weakness, paralysis, and death may occur.



Dinoflagellates produce neurotoxins that cause ciguatera, neurotoxic shellfish poisoning (NSP), and paralytic shellfish poisoning (PSP).

Ciguatera is caused by the concentration of Gambierdiscus toxicus in reef fish that aren’t normally toxic. Barracuda, grouper, sea bass, snapper, mullet, and a number of other fish found between latitude 35° N and 35° S (i.e. South Pacific islands, the Virgin Islands, and Puerto Rico) have caused the disease.

Symptoms include nausea, vomiting, diarrhea, cramps, excessive sweating, headache, and muscle aches.

Less commonly, patients may experience a sensation of burning or "pins-and-needles," weakness, itching, dizziness, reversal of temperature sensation in their mouth (hot surfaces feeling cold and cold, hot), unusual taste sensations, nightmares, or hallucinations. Ciguatera poisoning is rarely fatal and symptoms usually clear in 1 to 4 weeks.

Neurotoxic shellfish poisoning is caused by a different dinoflagellate (Gymnodinium breve) with a different toxin that may accumulate in shellfish from the southern Atlantic coast, the Gulf of Mexico, and the Caribbean.

Gymnodinium is typically found during the algal blooms commonly known as "red tides", although a number of other algae can be responsible for these blooms (the color may vary as well, depending on the pigmentation of the predominant algal species involved).

Symptoms include numbness, tingling in the mouth, arms and legs, incoordination, gastrointestinal upset and sometimes the same reversal of temperature sensation reported by ciguatera patients. Like ciguatera poisoning, NSP is rarely fatal and symptoms usually clear in 2 to 3 days.

Paralytic shellfish poisoning is caused by saxitoxins produced by several species of Alexandrium. Like Gymnodinium, Alexandrium is also associated with red tides.

The toxins concentrate in shellfish that usually live in colder coastal waters although the syndrome has been reported in Central America.

Symptoms are generally mild, beginning with numbness or tingling of the face, arms, and legs followed by headache, dizziness, nausea, muscular incoordination and sometimes a floating sensation. Severe poisoning may cause muscle paralysis and respiratory failure leading to death in 2 to 25 hours.



Water molds - decomposers, resemble fungi, produce asexual spores in a sac but their spores have flagella and fungi don't.

Usually seen as a white fuzzy mass on dead algae or animals in fresh water.  Terrestrial forms are often plant pathogens.

Phytophthora infestans – caused the Irish potato famine, infects soybeans and cocoa also.



Roles of Algae in Nature

Algae are the primary producers in aquatic food chains.

Planktonic algae produce most of the molecular oxygen in the earth’s atmosphere.

Most petroleum is the fossilized remains of planktonic algae.

Unicellular algae are symbionts in such animals as Tridacna (giant clam).