Kinetids, intracellular systems that are diverse but conserved in various
protoctist taxa, provide clues to identification of cells and whole organisms.
Tardigrades, rotifers, acarid mites, and insects (mainly springtails and flies) complete the range of groups found, together with some heterotrophic
protoctists traditionally considered as protozoans.
Although bacteria can exchange genetic material and unicellular
protoctists can merge to form larger cells, in neither of these cases is the exchange of genes necessary for reproduction to occur.
(For three who do not, see note 31.) Commonly, all unicells (other than unicellular bacteria), including the tens of thousands of species of the ill-defined phylum of "protozoa" (proto-animals), are assigned to a kingdom of "protists" or "
protoctists" (proto-organisms).
The Eucarya (meaning "true nucleus") include
protoctists, animals, plants, and fungi.
Only some forms of bacteria, some
protoctists (all are algae), and plants can perform photosynthesis.
This build-up of CO2 may have been the cause of the apparent release of calcium carbonate during the Cretaceous period in shallow sea areas where many corals and mollusks lived, all in a symbiotic relationship with photosynthetic
protoctists. These corals and mollusks ended up forming enormous fossil masses.
Dead plants are colonized by several types of organism, including bacteria, fungi,
protoctists, and small animals such as nematodes.
The silicoflagellates are medium-sized flagellated
protoctists (20-70 microns), with a thin membrane, numerous chromatophores and a highly distinctive internal reticulate skeleton, generally showing four-fold or six-fold symmetry.
Various heterotrophic
protoctists also produce external skeletons that persist after the death of the more active part.
Filter feeders like sponges, tunicates, and some bivalve mollusks always have a large number of internal commensal organisms, including such diverse groups as
protoctists, amphipods, copepods, lamellibranchs, crabs, worms, and even fish.
They contain a very special flora and fauna, consisting mainly of cyanobacteria, diatoms, benthic
protoctists, and some harpacticoid crustaceans, as well as insect larvae.
Ongoing research to find natural toxins from marine organisms that serve to prevent biological succession that leads to fouling (such as glycoproteins from bacteria, diatoms,
protoctists, algae, and invertebrates), began a decade ago.
The soil is occupied by
protoctists, nematodes, oligochaeta (earthworms), acarina (mites and ticks), collembola (springtails), and diptera larvae.
Overall biological activity can be measured by the amount of carbon dioxide emitted and, more analytically, by drawing up an inventory and count of 1) the mesofauna (nematodes, annelid worms, microarthropods, arachnids, apterygote and pterygote insects, myriapods, isopod and decapod crustaceans, rotifers, tardigrades, molluscs), 2) the microbiota (
protoctists and the microorganisms that perform the essential processes of denitrification, ammonification, and nitrification; the other bacteria; and microsymbionts like Rhizobium, Klebsiella, Spirillum, or Azospirillum), and 3) the mycota (fungi and lichens).
