You should be able to:
In the 5-kingdom taxonomy of life (on Earth), the Protoctista is the kingdom containing primitive eukaryotes. These organisms are mainly unicellular, although some (e.g. diatoms) form chains of undifferentiated cells, and others (some of the red, green and brown algae) form multicellular seaweeds. According to the Serial Endosymbiosis Theory (SET), the earliest members of the kingdom appeared about 1.5 billion years ago, when a large bacterium developed the genetically-coded ability to separate its DNA behind a nuclear membrane and began a career of phagotrophy, feeding on ingested particles. Many of those particles were living bacteria. According to the SET, some of these bacteria became perpetual symbionts, giving rise to mitochondria, chloroplasts and cytokinetic organelles (flagellae, cilia and mechanisms for nuclear division mechanism). Succesful symbioses occurred on a number of occasions, as evidenced, for example, by the existence of three main lines of protoctistan algae - those with green chloroplasts, those with yellow-brown chloroplasts, and those with red-blue chloroplasts. Some protoctistan lines lost chloroplasts and hence the power to photosynthesise - and then regained them in new symbioses.
some definitions:
The kingdom Protoctista contains at least 10 phyla that have species that are heterotrophic or myxotrophic and are found in the plankton. Three phyla will be considered here in detail.
This phylum, whose members may be called 'zooflagellates', is placed by analysis of ribosomal RNA close to the Archaeobacteria and hence may be considered as amongst the earliest eukaryotes. Most members have flagella, but some have no mitochondria,and none are thought to be primarily photosynthetic. These flagellates reproduce by binary fission and do not appear to have sexual reproduction or alternation of generations. Examples from coastal plankton include the genus Bodo and the choanoflagellate genus Monosiga. These small flagellates feed on bacteria.
Also called Dinophyta or Pyrrophyta. A typical dinoflagellate has a body divided into two parts by a grove or girdle, containing a transverse flagellum. Another grove, or sulcus, containing a longitudinal flagellum, runs along the hinder body lobe. Many dinoflagellates have a cell wall made up of cellulose plates, called an armour. Autotrophic cells usually contain numerous yellow, brown or reddish chloroplasts, containing a reddish xanthophyll pigment called peridinin as well as green chlorophyll and yellow-brown-orange carotenoids. Heterotrophs often contain pink oil droplets.
Ribosomal RNA analysis shows dinoflagellates to be evolved away from the original protoctists, but not closely related to the Plant, Animal or Fungal kingdoms. Many dinoflagellates have a life cycle that involves alternation of generations and, in some cases, the formation of a cyst, often a thickwalled cell that overwinters on the sea-bed. Typical pelagic dinoflagellates are haploid, and reproduce by binary division, although their chromosomes are unusual in remaining condensed throughout the cell cycle. Sexual reproduction may be stimulated by shortage of food or mineral nutrients (in the case of autotrophic dinoflagellates), and leads to the formation of a diploid zygote which forms a cyst. When the cyst hatches it releases a swimming cell which undergoes meiosis to produce haploid cells.
Because their cysts fossilize well, dinoflagellates are known to have existed for at least 600 million years. The first dinoflagellates were probably autotrophic and armoured and may have possessed the power of emitting light (bioluminescence). Modern dinoflagellate species have lost none, some, or all of these properties. Two groups are common in Scottish coastal waters:
Heterotrophic dinoflagellates eat bacteria, algae and protozoa. This does not entirely depend on size, as some dinoflagellates have specialised methods of dealing with larger prey. These methods include:
From here it is a small step to dinoflagellates becoming parasitic on other protoctists, and in some cases on animal zooplankton.
You may view a movie of dinoflagellates from Loch Striven feeding on phytoplanktonic algae. NB: The movie is a large file, and will take a long time to dowload over a telephone link - probably not advised. You need a MPEG viewer - part of most current internet browsers. Click here for the link.
Ciliates have many flagella-like structures capable of moving in synchrony, and usually called cilia. RNA analysis shows that ciliates are related to dinoflagellates, but are a primarily heterotrophic and 'animal'-like group without cellulose in their cell wall. A few members are functionally photosynthetic as a result of recent symbioses. Classification depends in part on the extent of coverage of the body by cilia. Common marine pelagic forms belong to three groups:
Small oligotrich and tintinnid ciliates feed on bacteria and perhaps small zooflagellates; larger ciliates can ingest algae, including diatom chains. Microscopic observations of living ciliates show the cells mocing rapidly, drawn through the water by their rototating oral cilia. Exactly how they ingest their prey when they encounter them is not clear; observations suggest that most contacts are unfruitful!
The heterotrophic pelagic protozoa have a variety of modes of nutrition:
Research during the previous two decades has shown that these pelagic protozoa consume a large part of phytoplanktonic primary production. The microbial food chain starts with bacteria using dissolved organic matter excreted by phytoplankton, then passes by way of small and large protozoa to mesozooplankton. Because there can be several steps in the microbial food chain, it may be thought that the consequences of a microbially-dominated marine ecosystem is a very low ecological transfer efficiency from primary producers to animal zooplankton. This would indeed be the case if the only function of the microbial food chain was to pass organic carbon to higher trophic levels. However, it also has another function, that of recycling mineral nutrients, as implied by the term microbial loop. The microbial food web is thought to be most important during summer in temperate seas. At this time, nutrients are in short supply. Although each stage in the microbial food web involves consumption of organic matter, it also release ammonium and phosphate into the water, and these recyled nutrients can be taken up by algae, stimulating additional primary production. Because small autotrophs grow best under low-nutrient conditions, but cannot easily be captured by copepods, the microbial loop provides an effective means for the exploitation of primary production under these conditions. The pelagic crustaceans can feed on the larger protozoans, which provide them with nutritious and high-energy food, having done the hard work of capturing smaller organisms and pre-processing their contents. A dinoflagellate such as Protoperidinium, rich in oil droplets and nitrogen, is the ideal diet for a larger zooplankter such as an euphausiid.
In the Firth of Clyde, armoured heterotrophic dinoflagellates, especially bioluminescent species of Protoperidinium, are most abundant during summer. This is shown by the maximum occurrence of bioluminescence at this time, and is as expected according the the microbial loop hypothesis. In contrast, small ciliates in Loch Creran showed a different pattern when studied during 1979 and 1980: there was no obvious seasonal variation, suggesting that these protozoans were feeding on bacteria that were in turn supported by dissolved organic matter that had originated in the waste from a seaweed processing factory.
This lecture expands on material that is not well covered by most Marine Biology texts. Protozoa and the microbial loop are briefly dealt with by Nybakken in chapter 2.