Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as marine single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics because they are paraphyletic (lacking a common ancestor for all descendants).
Most protists are too small to be seen with the naked eye. They are highly diverse organisms currently organised into 18 phyla, but not easy to classify. Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting large numbers of eukaryotic microbial communities have yet to be discovered. There has been little research on mixotrophic protists, but recent studies in marine environments found mixotrophic protists contribute a significant part of the protist biomass. Since protists are eukaryotes (and not prokaryotes) they possess within their cell at least one nucleus, as well as organelles such as mitochondria and Golgi bodies. Many protist species can switch between asexual reproduction and sexual reproduction involving meiosis and fertilization.Characteristics of Protists In: Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms. Other marine protist are neither single-celled nor microscopic, such as seaweed.
Protists have been described as a taxonomic grab bag of misfits where anything that does not fit into one of the main biological kingdoms can be placed.Neil A C, Reece J B, Simon E J (2004) Essential biology with physiology Pearson/Benjamin Cummings, Page 291. Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms. This more constrained definition excludes all brown, the multicellular red and green algae, and, sometimes, slime molds (slime molds excluded when multicellularity is defined as "complex").
Background
The ocean represents the largest continuous planetary ecosystem, hosting an enormous variety of organisms, which include microscopic biota such as unicellular eukaryotes (protists). Despite their small size, protists play key roles in marine biogeochemical cycles and harbour tremendous evolutionary diversity. Notwithstanding their significance for understanding the evolution of life on Earth and their role in marine food webs, as well as driving biogeochemical cycles to maintain habitability, little is known about their cell biology including reproduction, metabolism and signaling. Most of the biological knowledge available is based on comparison of proteins from cultured species to homologs in genetically tractable model taxa. A main impediment to understanding the cell biology of these diverse eukaryotes is that protocols for genetic modification are available for only a small number of species that represent neither the most ecologically relevant protists nor the breadth of eukaryotic diversity. Even so, in the decade to 2020, genome and transcriptome sequencing initiatives have resulted in nearly 120 million unigenes being identified in protists, which is facilitating the development of genetic tools for model species.
Trophic modes Protists can be divided broadly into four groups depending on whether their nutrition is plant-like, animal-like, fungus-like, or a mixture of these.
Type of protist Description Example Some other examples Plant-like Autotrophic protists that make their own food without needing to consume other organisms, usually by photosynthesis (sometimes by chemosynthesis) Green algae, Pyramimonas Red and brown algae, diatoms, coccolithophores and some dinoflagellates. Plant-like protists are important components of phytoplankton discussed below. Animal-like Heterotrophic protists that get their food consuming other organisms (bacteria, archaea and small algae) Radiolarian protist as drawn by Haeckel Foraminiferans, and some marine amoebae, ciliates and flagellates. Fungal-like Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel Marine lichen Mixotrophs Mixotrophic and osmotrophic protists that get their food from a combination of the above Euglena mutabilis, a photosynthetic flagellate Many marine mixotrops are found among protists, particularly among ciliates and dinoflagellates
The fungus-like protist saprobes are specialized to absorb nutrients from nonliving organic matter, such as dead organisms or their wastes. For instance, many types of oomycetes grow on dead animals or algae. Marine saprobic protists have the essential function of returning inorganic nutrients to the water. This process allows for new algal growth, which in turn generates sustenance for other organisms along the food chain. Indeed, without saprobe species, such as protists, fungi, and bacteria, life would cease to exist as all organic carbon became "tied up" in dead organisms.Clark M A, Douglas M and Choi J (2018) Biology 2e, 23.4 "Ecology of Protists", OpenStax, Houston, Texas. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
Mixotrophs
Mixotrophs have no single trophic mode. A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.Beware the mixotrophs - they can destroy entire ecosystems 'in a matter of hours' There are two types of eukaryotic mixotrophs: those with their own chloroplasts, and those with endosymbionts—and others that acquire them through kleptoplasty or by enslaving the entire phototrophic cell.Microscopic body snatchers infest our oceans - Phys.org
The distinction between plants and animals often breaks down in very small organisms. Possible combinations are photo- and chemotrophy, litho- and organotrophy, auto- and heterotrophy or other combinations of these. Mixotrophs can be either eukaryotic or prokaryotic. They can take advantage of different environmental conditions.
Recent studies of marine microzooplankton found 30–45% of the ciliate abundance was mixotrophic, and up to 65% of the amoeboid, foram and radiolarian biomass was mixotrophic.
Phaeocystis is an important algal genus found as part of the marine phytoplankton around the world. It has a polymorphic life cycle, ranging from free-living cells to large colonies. It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size during blooms. As a result, Phaeocystis is an important contributor to the marine carbon and sulfur cycles. Phaeocystis species are endosymbionts to acantharian radiolarians.
General types Description Example Further examples Bacterioplankton Photoheterotrophic bacterioplankton Vibrio cholerae Roseobacter spp.Erythrobacter spp.Gammaproteobacterial clade OM60Widespread among bacteria and archaea Phytoplankton Called constitutive mixotrophs by Mitra et al., 2016. Phytoplankton that eat: photosynthetic protists with inherited plastids and the capacity to ingest prey. Ochromonas species Ochromonas spp.Prymnesium parvumDinoflagellate examples: Fragilidium subglobosum,Heterocapsa triquetra,Karlodinium veneficum,Neoceratium furca,Prorocentrum minimum Zooplankton Called nonconstitutive mixotrophs by Mitra et al., 2016. Zooplankton that are photosynthetic: microzooplankton or metazoan zooplankton that acquire phototrophy through chloroplast retentiona or maintenance of algal endosymbionts. Generalists Protists that retain chloroplasts and rarely other organelles from many algal taxa Most oligotrich ciliates that retain plastidsa Specialists 1. Protists that retain chloroplasts and sometimes other organelles from one algal species or very closely related algal species Dinophysis acuminata Dinophysis spp.Mesodinium rubrum 2. Protists or zooplankton with algal endosymbionts of only one algal species or very closely related algal species Noctiluca scintillans Metazooplankton with algal endosymbiontsMost mixotrophic Rhizaria (Acantharea, Polycystinea, and Foraminifera)Green Noctiluca scintillans aChloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm.
Protist locomotion
Another way of categorising protists is according to their mode of locomotion. Many unicellular protists, particularly protozoans, are motile and can generate movement using flagella, cilia or pseudopods. Cells which use flagella for movement are usually referred to as flagellates, cells which use cilia are usually referred to as ciliates, and cells which use pseudopods are usually referred to as amoeba or amoeboids. Other protists are not motile, and consequently have no movement mechanism.
Type of protist Movement mechanism Description Example Other examplesMotile Flagellates A flagellum (Latin for whip) is a lash-like appendage that protrudes from the cell body of some protists (as well as some bacteria). Flagellates use from one to several flagella for locomotion and sometimes as feeding and sensory organelle. Cryptophytes All dinoflagellates and nanoflagellates (choanoflagellates, silicoflagellates, most green algae) Ciliates A cilium (Latin for eyelash) is a tiny flagellum. Ciliates use multiple cilia, which can number in many hundreds, to power themselves through the water. Paramecium bursariaclick to see cilia Foraminiferans, and some marine amoebae, ciliates and flagellates. Amoebas(amoeboids) Pseudopods (Greek for false feet) are lobe-like appendages which amoebas use to anchor to a solid surface and pull themselves forward. They can change their shape by extending and retracting these pseudopods. Amoeba Found in every major protist lineage. Amoeboid cells occur among the protozoans, but also in the algae and the fungi. Not motile Diatom Coccolithophores, most diatoms, and non‐motile species of Phaeocystis Among protozoans the parasitic Apicomplexa are non‐motile.
Flagella are used in prokaryotes (archaea and bacteria) as well as protists. In addition, both flagella and cilia are widely used in eukaryotic cells (plant and animal) apart from protists.
The regular beat patterns of eukaryotic cilia and flagella generates motion on a cellular level. Examples range from the propulsion of single cells such as the swimming of spermatozoa to the transport of fluid along a stationary layer of cells such as in a respiratory tract. Though eukaryotic flagella and motile cilia are ultrastructurally identical, the beating pattern of the two organelles can be different. In the case of flagella, the motion is often planar and wave-like, whereas the motile cilia often perform a more complicated three-dimensional motion with a power and recovery stroke.