Station 3: Supergroup SAR (Stramenopiles, Alveolates and Rhizarians)

Diatoms

Diatoms are a major group of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of the Earth's biomass: they generate about 20 to 50 percent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of silicon each year from the waters in which they live, and constitute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half-mile (800 m) deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by transatlantic winds from the African Sahara.

Diatoms are unicellular: they occur either as solitary cells or in colonies, which can take the shape of ribbons, fans, zigzags, or stars. Individual cells range in size from 2 to 200 micrometers. In the presence of adequate nutrients and sunlight, an assemblage of living diatoms doubles approximately every 24 hours by asexual multiple fission; the maximum life span of individual cells is about six days. Diatoms have two distinct shapes: a few are radially symmetric, while most are broadly bilaterally symmetric. A unique feature of diatom anatomy is that they are surrounded by a cell wall made of silica, called a frustule. These frustules have structural coloration prompting them to be described as "jewels of the sea" and "living opals". Movement in diatoms primarily occurs passively as a result of both water currents and wind-induced water turbulence but there are exceptions. Similar to plants, diatoms convert light energy to chemical energy by photosynthesis, although this shared autotrophy evolved independently in both lineages. 

Diatoms are used to monitor past and present environmental conditions, and are commonly used in studies of water quality. Diatomaceous earth (diatomite) is a collection of diatom shells found in the earth's crust. They are soft, silica-containing sedimentary rocks which are easily crumbled into a fine powder and typically have a particle size of 10 to 200 μm. Diatomaceous earth is used for a variety of purposes including for water filtration, as a mild abrasive, in cat litter, and as a dynamite stabilizer.

Brown Algae

The brown algae are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Most brown algae live in marine environments, where they play an important role both as food and as a potential habitat. For instance, Macrocystis, a kelp of the order Laminariales, may reach 60 m (200 ft) in length and forms prominent underwater kelp forests. Kelp forests like these contain a high level of biodiversity. Another example is Sargassum, which creates unique floating mats of seaweed in the tropical waters of the Sargasso Sea that serve as the habitats for many species. 

Brown algae belong to a group (Heterokontophyta) distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among heterokonts in developing into multicellular forms with differentiated tissues, but they reproduce by means of flagellated spores and gametes that closely resemble cells of other heterokonts. Genetic studies show their closest relatives to be the yellow-green algae.

Therefore, despite the presence of photosynthesis and a similarity in external appearance these are not plants and are not even in the same supergroup as plants (Archaeplastida). 

Genus Sargassum

Sargassum is a genus of brown macroalgae (seaweed) in the order Fucales. Numerous species are distributed throughout the temperate and tropical oceans of the world, where they generally inhabit shallow water and coral reefs, and the genus is widely known for its planktonic (free-floating) species. Most species within the group are predominantly cold-water organisms that benefit from nutrients upwelling, but the genus Sargassum appears to be an exception. The Atlantic Ocean's Sargasso Sea was named after the algae, as it hosts a large amount of Sargassum.

Dinoflagellates

The alveolates (meaning "with cavities") are a group of protists and are also called Alveolata.

The most notable shared characteristic is the presence of cortical (outer-region) alveoli (sacs). These are flattened vesicles (sacs) packed into a continuous layer just under the membrane and supporting it, typically forming a flexible pellicle (thin skin). In dinoflagellates they often form armor plates. Alveolates have mitochondria with tubular cristae (ridges), and their flagella or cilia have a distinct structure.

The largest group of Alveolates and one of the largest groups of photosynthetic algaes (although still much smaller than diatoms) are the dinoflagellates. Dinoflagellates contain both freshwater and marine species (most are marine) and while all of them are photosynthetic a number of dinoflagellate species are mixotrophic, combining photosynthesis and ingestion of prey.

One of the well known characteristics of dinoflagellates is that some species can accumulate rapidly resulting in a visible discoloration of the water, colloquially known as "red tide", which can cause shellfish poisoning if human eat contaminated shellfish.  Another characteristic of many dinoflagellates is that they can produce biolumenescence in large amounts giving water an appearance of sparkling light.

Genus: Ceratium

The genus Ceratium includes species of freshwater and marine dinoflagellates. Most species of Ceratium are unicellular organisms that are characterized by their armored plates, two flagella, and horns. Species of Ceratium are found worldwide and are of concern due to their blooms.  Species of Ceratium have other structures called chromatophores, which contain red, brown, and yellow pigments used for photosynthesis.

Ciliates

Genus: Paramecium

Paramecium is a genus of unicellular ciliates, commonly studied as a representative of the ciliate group. Paramecia are widespread in freshwater, brackish, and marine environments and are often very abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, it has been widely used in classrooms and laboratories to study biological processes. Its usefulness as a model organism has caused one ciliate researcher to characterize it as the "white rat" of the phylum Ciliophora.

Genus: Stentor

Stentor, sometimes called trumpet animalcules, are a genus of filter-feeding, heterotrophic ciliates.  They are usually horn-shaped, and reach lengths of two millimeters; as such, they are among the biggest known extant unicellular organisms. They reproduce asexually through binary fission.

Apicomplexans

Species: Plasmodium falciparum

Plasmodium falciparum is a unicellular protozoan parasite of humans, and the deadliest species of Plasmodium that cause malaria in humans. It is transmitted through the bite of a female Anopheles mosquito. It is responsible for roughly 50% of all malaria cases. It causes the most dangerous form of the disease called falciparum malaria. It is therefore regarded as the deadliest parasite in humans, causing a conservative estimate of one million deaths every year.

Life Cycle of Malaria

The natural history of malaria involves cyclical infection of humans and female Anopheles mosquitoes. In humans, the parasites grow and multiply first in the liver cells and then in the red cells of the blood. In the blood, successive broods of parasites grow inside the red cells and destroy them, releasing daughter parasites (“merozoites”) that continue the cycle by invading other red cells.

The blood stage parasites are those that cause the symptoms of malaria. When certain forms of blood stage parasites (gametocytes, which occur in male and female forms) are ingested during blood feeding by a female Anopheles mosquito, they mate in the gut of the mosquito and begin a cycle of growth and multiplication in the mosquito. After 10-18 days, a form of the parasite called a sporozoite migrates to the mosquito’s salivary glands. When the Anopheles mosquito takes a blood meal on another human, anticoagulant saliva is injected together with the sporozoites, which migrate to the liver, thereby beginning a new cycle.

Thus the infected mosquito carries the disease from one human to another (acting as a “vector”), while infected humans transmit the parasite to the mosquito, In contrast to the human host, the mosquito vector does not suffer from the presence of the parasites.

Below is an illustration of the Plasmodium life cycle.

Foraminiferans

The Rhizaria are a species-rich group clade of mostly unicellular eukaryotes. Being described mainly from rDNA sequences, they vary considerably in form, having no clear morphological distinctive characters (synapomorphies), but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, and these make up the vast majority of protozoan fossils. Nearly all have mitochondria with tubular cristae.   

Foraminifera (Latin for "hole bearers") are members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an external shell (called a "test") of diverse forms and materials. Tests of chitin (found in some simple genera, and Textularia in particular) are believed to be the most primitive type. Most foraminifera are marine, the majority of which live on or within the seafloor sediment (i.e., are benthic), while a smaller variety float in the water column at various depths (i.e., are planktonic).

Foraminifera typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are commonly made of calcium carbonate (CaCO3) or agglutinated sediment particles.