Marine protists

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Marine protists
Alga (phytoplankton)
Protozoan (zooplankton)
Protists are usually one-celled microorganisms. They include algae (autotrophs which make their own food) and protozoans (heterotrophs which eat the algae for food). In recent years, researchers have discovered many protists are mixotrophs, which can function in both modes.

Marine protists are defined by their habitat as

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.

Golgi bodies. Many protist species can switch between asexual reproduction and sexual reproduction involving meiosis and fertilization.[6]

In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually

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.[7] 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 doesn't fit into one of the main biological kingdoms can be placed.[8] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.[9][10] 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").[11]

Background

"Marine protists are a

polyphyletic group of organisms playing major roles in the ecology and biogeochemistry of the oceans, including performing much of Earth's photosynthesis and driving the carbon, nitrogen, and silicon cycles. In addition, marine protists occupy key positions in the tree of life, including as the closest relatives of metazoans [animals]... Unicellular eukaryotes are often lumped as 'protists', a term that is useful despite its taxonomic irrelevance and origin as a definition by exclusion — a protist being any eukaryote that's not a plant, animal, or fungus".[12]

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

unigenes being identified in protists,[21] which is facilitating the development of genetic tools for model species.[22]

Phylogenetic and symbiogenetic tree of living organisms, showing a schematic view of the central position occupied by the protista (protists)
Schematic view of the eukaryotic tree of life with effigies of main marine protist representatives [22]

Trophic modes

Protists can be divided broadly into four groups depending on whether their nutrition is plant-like, animal-like, fungal-like,[23] or a mixture of these.[24]

Protists according to how they get food
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
Protozoans
(see below
)
Heterotrophic protists that get their food consuming other organisms (bacteria, archaea and small algae)
Radiolarian protist as drawn by Haeckel
amoebae, ciliates and flagellates
.
Fungal-like
slime nets
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
Various
(see below)
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[5]
micrograph
cell schematic
Choanoflagellates, unicellular "collared" flagellate protists, are thought to be the closest living relatives of the animals.[25]
  • Single-celled and microscopic protists
  • Diatoms are a major algae group generating about 20% of world oxygen production.[26]
    Diatoms are a major algae group generating about 20% of world oxygen production.[26]
  • Fossil diatom frustule from 32 to 40 mya
    Fossil diatom frustule from 32 to 40
    mya
  • Radiolarian
    Radiolarian
  • Single-celled alga, Gephyrocapsa oceanica
    Single-celled alga, Gephyrocapsa oceanica
  • Two dinoflagellates
  • A single-celled ciliate with green zoochlorellae living inside endosymbiotically
    A single-celled
    endosymbiotically
  • Euglenoid
    Euglenoid
  • This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.
    This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.
External videos
video icon How microscopic hunters get their lunch
video icon Euglenoids: Single-celled shapeshifters
video icon How do protozoans get around?
  • Ciliate ingesting a diatom
  • Amoeba engulfing a diatom

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.[27][28]

Mixotrophs

Mixotrophic radiolarians

heterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.[29] 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.[30]

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.[31] They can take advantage of different environmental conditions.[32]

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.[5]

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.[33] 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.[34] As a result, Phaeocystis is an important contributor to the marine carbon[35] and sulfur cycles.[36] Phaeocystis species are endosymbionts to acantharian radiolarians.[37][38]

Mixotrophic plankton that combine phototrophy and heterotrophy – table based on Stoecker et al., 2017[39]
General types Description Example Further examples
Bacterioplankton
Photoheterotrophic bacterioplankton
Vibrio cholerae Roseobacter spp.
Erythrobacter spp.
Gammaproteobacterial clade OM60
Widespread among bacteria and archaea
Phytoplankton Called constitutive mixotrophs by Mitra et al., 2016.[40] Phytoplankton that eat: photosynthetic protists with inherited plastids and the capacity to ingest prey. Ochromonas species
Prorocentrum
minimum
Zooplankton Called nonconstitutive mixotrophs by Mitra et al., 2016.[40] 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
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

amoeboids. Other protists are not motile
, and consequently have no movement mechanism.

Protists according to how they move
Type of protist Movement mechanism Description Example Other examples
Motile 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
foraminiferans and Apicomplexa
)
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 bursaria
click to see cilia
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.[43]
Amoeba Found in every major protist
fungi.[44][45]
Not motile
none
Diatom Coccolithophores, most diatoms, and non‐motile species of Phaeocystis[42] Among protozoans the parasitic Apicomplexa are non‐motile.
Difference of beating pattern of flagellum and cilium

Flagella

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.

Eukaryotic flagella—those of animal, plant, and protist cells—are complex cellular projections that lash back and forth. Eukaryotic flagella are classed along with eukaryotic motile cilia as undulipodia[46] to emphasize their distinctive wavy appendage role in cellular function or motility. Primary cilia are immotile, and are not undulipodia.

Cryptaulax, Abollifer, Bodo, Rhynchomonas, Kiitoksia, Allas, and Metromonas[47]
Cilia performs powerful forward strokes with a stiffened flagellum followed by relatively slow recovery movement with a relaxed flagellum

Ciliates generally have hundreds to thousands of cilia that are densely packed together in arrays. Like the flagella, the cilia are powered by specialised molecular motors. An efficient forward stroke is made with a stiffened flagellum, followed by an inefficient backward stroke made with a relaxed flagellum. During movement, an individual cilium deforms as it uses the high-friction power strokes and the low-friction recovery strokes. Since there are multiple cilia packed together on an individual organism, they display collective behaviour in a metachronal rhythm. This means the deformation of one cilium is in phase with the deformation of its neighbor, causing deformation waves that propagate along the surface of the organism. These propagating waves of cilia are what allow the organism to use the cilia in a coordinated manner to move. A typical example of a ciliated microorganism is the Paramecium, a one-celled, ciliated protozoan covered by thousands of cilia. The cilia beating together allow the Paramecium to propel through the water at speeds of 500 micrometers per second.[48]

  • Flagellate, ciliates and amoeba
  • Green algal flagellate (Chlamydomonas)
  • Paramecium feeding on bacteria
    Paramecium feeding on bacteria
  • The ciliate Oxytricha trifallax with cilia clearly visible
    The ciliate
    Oxytricha trifallax
    with cilia clearly visible
  • Amoeba with ingested diatoms
    Amoeba with ingested diatoms
External videos
video icon Paramecium: The White Rat of Ciliates

Marine algae

euglenophytes, dinoflagellates and diatoms
.

Dinoflagellates and diatoms are important components of marine algae and have their own sections below.

Euglenophytes
are a phylum of unicellular flagellates with only a few marine members.

Not all algae are microscopic. Green, red and brown algae all have multicellular macroscopic forms that make up the familiar

multicellular and including many seaweeds such as kelp
. Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as
macroalgae
.

multicellular. Microalgae are important components of the marine protists discussed above, as well as the phytoplankton discussed below. They are very diverse. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.[53]
Depending on the species, their sizes range from a few micrometers (µm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.

  • Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left
    Chlamydomonas globosa, a unicellular green alga with two
    flagella
    just visible at bottom left
  • Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate[54]
    endosymbiosis with a ciliate[54]
  • Centric diatom
    Centric diatom
  • Dinoflagellates
    Dinoflagellates

multicellular and more visible types of algae, commonly called seaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by a holdfast. Like microalgae, macroalgae (seaweeds) can be regarded as marine protists
since they are not true plants. But they are not microorganisms, so they are not within the scope of this article.

Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions.

Mermaid's wineglass, a genus of subtropical green algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studying cell biology.[55] Another single-celled algae, Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an invasive species known colloquially as killer algae.[56]

Diatoms

Diatoms come in many shapes and sizes

Diatoms are photosynthetic unicellular algae populating the oceans and other waters around the globe. They form a (disputed) phylum containing about 100,000 recognised species. Diatoms generate about 20 per cent of all oxygen produced on the planet each year,[26] and take in over 6.7 billion metric tons of silicon each year from the waters in which they live.[57] They produce 25–45% of the total primary production of organic material in the oceans,[58][59][60] owing to their prevalence in open-ocean regions when total phytoplankton biomass is maximal.[61][62]

Diatoms are enclosed in protective silica (glass) shells called

particulate carbon exported to ocean depths.[60][65][62]

  • Diatoms are one of the most common types of phytoplankton
    Diatoms are one of the most common types of phytoplankton
  • Their protective shells (frustles) are made of silicon
    Their protective shells (frustles) are made of silicon
Diatom shapes
          Drawings by Haeckel 1904 (click for details)
External videos
video icon Diatoms: Tiny factories you can see from space
video icon Diatom 3D interference contrast
Structure of a centric diatom frustule[66]

Physically driven seasonal enrichments in surface nutrients favour

diatom blooms. Anthropogenic climate change will directly affect these seasonal cycles, changing the timing of blooms and diminishing their biomass, which will reduce primary production and CO2 uptake.[67][62] Remote sensing data suggests there was a global decline of diatoms between 1998 and 2012, particularly in the North Pacific, associated with shallowing of the surface mixed layer and lower nutrient concentrations.[68][62]

  • Silicified frustule of a pennate diatom with two overlapping halves
    Silicified frustule of a pennate diatom with two overlapping halves
  • Guinardia delicatula, a diatom responsible for diatom blooms in the North Sea [69]
    Guinardia delicatula, a diatom responsible for
    diatom blooms in the North Sea [69]
  • There are over 100,000 species of diatoms accounting for 25–45% of the ocean's primary production
    There are over 100,000 species of diatoms accounting for 25–45% of the ocean's primary production
  • Linked diatoms
    Linked diatoms
  • Pennate diatom from an Arctic meltpond, infected with two chytrid-like fungal pathogens. Scale bar = 10 µm.[70]
    Pennate diatom from an Arctic
    meltpond, infected with two chytrid-like fungal pathogens. Scale bar = 10 µm.[70]

Coccolithophores

Coccolithophores
...have plates called coccoliths
...extinct fossil
Coccolithophores build calcite skeletons important to the marine carbon cycle[71]

Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by calcium carbonate shells covered with ornate circular plates or scales called coccoliths. The term coccolithophore derives from the Greek for a seed carrying stone, referring to their small size and the coccolith stones they carry. Under the right conditions they bloom, like other phytoplankton, and can turn the ocean milky white.[72]

Fossil of Coccolithus pelagicus, about 10 μm across
Diverse coccolithophores from the Maldives [73]
The fossil coccolithophore Braarudosphaera bigelowii has an unusual shell with a regular dodecahedral structure about 10 micrometers across.[74]

Dinoflagellates

Dinoflagellate shapes
Unarmored dinoflagellates Kofoid (1921)
Haeckel Peridinea (1904)

red tides. Excavates may be the most basal flagellate lineage.[41]

By trophic orientation dinoflagellates are all over the place. Some dinoflagellates are known to be

mixotrophic
and could also be classified as phytoplankton.

The toxic dinoflagellate Dinophysis acuta acquire chloroplasts from its prey. "It cannot catch the cryptophytes by itself, and instead relies on ingesting ciliates such as the red Mesodinium rubrum, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)".[39]

  • Gyrodinium, one of the few naked dinoflagellates which lack armour
    Gyrodinium, one of the few naked dinoflagellates which lack armour
  • The dinoflagellate Protoperidinium extrudes a large feeding veil to capture prey
    The dinoflagellate Protoperidinium extrudes a large feeding veil to capture prey
  • Nassellarian radiolarians can be in symbiosis with dinoflagellates
    Nassellarian
    radiolarians can be in symbiosis with dinoflagellates
  • The dinoflagellate Dinophysis acuta
    The dinoflagellate Dinophysis acuta
Lingulodinium polyedrum
Suggested explanation for glowing seas[77]
Dinoflagellates
        Armoured
        Unarmoured
Traditionally dinoflagellates have been presented as armoured or unarmoured

Dinoflagellates often live in

nassellarian radiolarians house dinoflagellate symbionts within their tests.[78] The nassellarian provides ammonium and carbon dioxide for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders.[79] There is evidence from DNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as with foraminifera.[80]

Some dinoflagellates are

cytoplasmic bodies which contain dinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes called the phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.[83]

Marine protozoans

Protozoans are protists which feed on organic matter such as other microorganisms or organic tissues and debris.[87][88] Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.[89][90] Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy
.

Marine protozoans include

.

Radiolarians

Radiolarian shapes
          Drawings by Haeckel 1904 (click for details)

ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.[91]

  • Like diatoms, radiolarians come in many shapes
    Like diatoms, radiolarians come in many shapes
  • Also like diatoms, radiolarian shells are usually made of silicate
    Also like diatoms, radiolarian shells are usually made of silicate
  • However acantharian radiolarians have shells made from strontium sulfate crystals
    However
    acantharian radiolarians have shells made from strontium sulfate
    crystals
  • Cutaway schematic diagram of a spherical radiolarian shell
    Cutaway schematic diagram of a spherical radiolarian shell
Turing and radiolarian morphology
Shell of a spherical radiolarian
Shell micrographs
Computer simulations of Turing patterns on a sphere
closely replicate some radiolarian shell patterns[92]
External videos
video icon Radiolarian geometry
video icon Ernst Haeckel's radiolarian engravings
  • Cladococcus abietinus
    Cladococcus abietinus
  • Cleveiplegma boreale
    Cleveiplegma boreale

Foraminiferans

Foraminiferan shapes
          Drawings by Haeckel 1904 (click for details)

Like radiolarians, foraminiferans (forams for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often called tests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made of agglutinated sediment particles or chiton, and (rarely) of silica. Most forams are benthic, but about 40 species are planktic.[93] They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.[91]

Foraminiferans
...can have more than one nucleus
...and defensive spines
Foraminiferans are important unicellular zooplankton protists, with calcium tests
External videos
video icon foraminiferans
video icon Foraminiferal networks and growth
  • section showing chambers of a spiral foram
    section showing chambers of a spiral foram
  • Live Ammonia tepida streaming granular ectoplasm for catching food
    Live Ammonia tepida streaming granular ectoplasm for catching food
  • Group of planktonic forams
    Group of planktonic forams
  • Fossil nummulitid forams of various sizes from the Eocene
    Fossil nummulitid forams of various sizes from the Eocene
  • The Egyptian pyramids were constructed from limestone that contained nummulites.[94]
    The
    Egyptian pyramids were constructed from limestone that contained nummulites.[94]

A number of forams are mixotrophic (see below). These have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates.[93] Mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters.[95] Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.[96]

Amoeba

Shelled and naked amoeba
testate
) or naked
Amoeba engulfing a diatom
External videos
video icon Amoebas
video icon Testate amoebas
video icon Feeding amoebas

Ciliates

Ciliate shapes
          Drawings by Haeckel 1904 (click for details)

Marine ciliates are major grazers of the phytoplankton.[97][98]

Phytoplankton primary production supports higher

export production.[101][102] Grazing on particulate primary production in the global ocean surface is ~10–15% for mesozooplankton and 59–75% for microzooplankton,[103][104][105][106] with estimates for coastal and estuarine systems usually in the a lower range.[106][98]

Ciliates constitute an important component of the microzooplankton community with preference for small-sized preys, in contrast to mesozooplankton, and many ciliate species are also grazed by mesozooplankton.[107] Thus, ciliates can be an important link between small cells and higher trophic levels.[108] Besides their significant role in carbon transfer, ciliates are also considered high quality food, as a source of proteinaceous compounds with a low C:N ratio in comparison to phytoplankton.[109][110][98]

Conjugation of two Coleps sp.
Two similar-looking but sexually distinct partners connected at their front ends exchange genetic material via a plasma bridge.

Although many ciliates are heterotrophs, a number of pelagic species are

phagotrophic and phototrophic nutrition (Stoecker, 1998). The recognition of mixotrophy in the marine plankton food web has challenged the classical understanding of pelagic food webs, as autotrophy and heterotrophy are not necessarily two distinct functional compartments.[111] Classical understanding of ecological interactions among plankton, such as competition for nutrients, indicates that nutrient uptake affinity decreases with organism size,[112] favoring smaller sizes under resource limiting conditions. Mixotrophy is advantageous to organisms under nutrient limited conditions, allowing them to reduce direct competition by grazing on smaller prey and increase direct ingestion of nutrients.[113] Modeling results suggest that mixotrophy favors larger organisms, and therefore enhances trophic transfer efficiency.[113][114] On top of that, mixotrophy appears to be important over both, space and time, in marine systems.[115] stressing the need for ecological field studies to further elucidate the role of mixotrophy.[98]

  • Several taxa of ciliates interacting
  • Blepharisma americanum swimming in a drop of pond water with other microorganisms
External videos
video icon Peritrich Ciliates
video icon Conjugating protists

Macroscopic protists

Planktonic protists

Interactome

Planktonic protist interactome[118]
Bipartite networks, providing an overview of the interactions represented by a manually curated Protist Interaction DAtabase (PIDA).

Interaction between microbial species has played important roles in evolution and speciation.

mitochondria, chloroplasts, and other metabolic capacities in the eukaryotic cell,[119][120][121][122] Microbial interactions guarantee ecosystem function, having crucial roles in, for instance, carbon channeling in photosymbiosis, control of microalgae blooms by parasites, and phytoplankton-associated bacteria influencing the growth and health of their host.[118]

Despite their importance, understanding of microbial interactions in the ocean and other aquatic systems is rudimentary, and the majority of them are still unknown.

high throughput sequencing (HTS)[128][129][130] of environmental DNA or RNA has transformed understanding of microbial diversity [131] and evolution,[132] as well as generating hypotheses on microbial interactions based on correlations of estimated microbial abundances over spatiotemporal scales.[133][134][135][136][118]

The diagram on the right is an overview of the interactions between planktonic protists recorded in a manually curated Protist Interaction DAtabase (PIDA). The network is based on 2422 ecological interactions in the PIDA registered from ~500 publications spanning the last 150 years. The nomenclature and taxonomic order of Eukaryota is based on Adl et al. 2019.[137] The nomenclature and taxonomic order of Bacteria is based on Schultz et al. 2017.[138][118]

The nodes are grouped (outer circle) according to eukaryotic supergroups (or Incertae sedis), Bacteria and Archaea. All major protistan lineages were involved in interactions as hosts, symbionts (mutualists and commensalists), parasites, predators, and/or prey. Predation was the most common interaction (39%), followed by symbiosis (29%), parasitism (18%), and unresolved interactions (14%, where it is uncertain whether the interaction is beneficial or antagonistic). Nodes represent eukaryotic and prokaryotic taxa and are colored accordingly. Node size indicates the number of edges/links that are connected to that node. Each node/taxon is assigned a number, which corresponds with the numbers for taxa in B, C and D. Edges represent interactions between two taxa and are colored according to ecological interaction type: predation (orange), symbiosis (green), and parasitism (purple).[118]

The network is undirected, meaning that a node can contain both parasites/symbionts/prey and hosts/predators. To avoid cluttering of the figure, "Self-loops", which represent cases where both interacting organisms belong to the same taxon (e.g., a dinoflagellate eating another dinoflagellate) are not shown as edges/links in this figure, but are considered in the size of nodes. The outermost circle groups taxa in the different eukaryotic ‘supergroups’ or the prokaryotic domains Bacteria and Archaea.

Alveolata, St Stramenopiles, Ha Haptista, Cy Cryptista, Ap Archaeplastida, Ex Excavata, Ob Obazoa, Am Amoebozoa, Cu CRuMS, An Ancryomonadidae, Is Incertae sedis.[118]

B: Predator–prey interactions in PIDA. The node numbers correspond to taxa node numbers in a. Abbreviations for supergroups are described above. Background and nodes are colored according to functional role in the interaction: Prey are colored light orange (left part of figure), while predators are depicted in dark orange (right part of figure). The size of each node represents the number of edges connected to that node.[118]

C. Symbiont–host interactions included in PIDA. The node numbers correspond to node numbers in A. Abbreviations for supergroups are described above. Symbionts are to the left, colored light green, and their hosts are to the right in dark green. The size of each node represents the number of edges connected to that node.[118]

D: Parasite–host interactions included in PIDA. The node numbers correspond to node numbers in A. Abbreviations for supergroups are described above. Parasite taxa are depicted in light purple (left), hosts in dark purple (right).[118]

It was found that protist predators seem to be "multivorous" while parasite–host and symbiont–host interactions appear to have moderate degrees of specialization. The SAR supergroup (i.e., Stramenopiles, Alveolata, and Rhizaria) heavily dominated PIDA, and comparisons against a global-ocean molecular survey (Tara expedition) indicated that several SAR lineages, which are abundant and diverse in the marine realm, were underrepresented among the recorded interactions.[118]

Protist shells

Many protists have protective shells or

ocean sediments, and in the global cycling
of elements and nutrients.

Diatom shells are called

ultraviolet light by nanostructured silica protects the DNA in the algal cells, and this may be an evolutionary cause for the formation of the glass cages.[140][141]

Coccolithophores are protected by a shell constructed from ornate circular plates or scales called coccoliths. The coccoliths are made from calcium carbonate or chalk. The term coccolithophore derives from the Greek for a seed carrying stone, referring to their small size and the coccolith stones they carry.[72]

Diatoms
Diatoms, major components of marine plankton, have glass skeletons called frustules. "The microscopic structures of diatoms help them manipulate light, leading to hopes they could be used in new technologies for light detection, computing or robotics.[142]
SEM images of pores in diatom frustules[140]
Coccolithophores
Coccolithophores are armoured with chalk plates or stones called coccoliths. The images above show the size comparison between the relatively large coccolithophore Scyphosphaera apsteinii and the relatively small but ubiquitous Emiliania huxleyi[143]
Benefits of having shells
Benefits in coccolithophore calcification [144] – see text below
Costs of having shells
Energetic costs in coccolithophore calcification [144]

There are benefits for protists that carry protective shells. The diagram on the left above shows some benefits coccolithophore get from carrying coccoliths. In the diagram, (A) represents accelerated photosynthesis including carbon concentrating mechanisms (CCM) and enhanced light uptake via scattering of scarce photons for deep-dwelling species. (B) represents protection from photodamage including sunshade protection from

ultraviolet light (UV) and photosynthetic active radiation (PAR) and energy dissipation under high-light conditions. (C) represents armour protection includes protection against viral/bacterial infections and grazing by selective and nonselective grazers.[144]

There are also costs for protists that carry protective shells. The diagram on the right above shows some of the energetic costs coccolithophore incur from carrying coccoliths. In the diagram, the energetic costs are reported in percentage of total photosynthetic budget. (A) represents transport processes include the transport into the cell from the surrounding seawater of primary calcification substrates Ca2+ and HCO3− (black arrows) and the removal of the end product H+ from the cell (gray arrow). The transport of Ca2+ through the cytoplasm to the coccolith vesicle (CV) is the dominant cost associated with calcification. (B) represents metabolic processes include the synthesis of coccolith-associated

Golgi complex (white rectangles) that regulate the nucleation and geometry of CaCO3 crystals. The completed coccolith (gray plate) is a complex structure of intricately arranged CAPs and CaCO3 crystals. (C) Mechanical and structural processes account for the secretion of the completed coccoliths that are transported from their original position adjacent to the nucleus to the cell periphery, where they are transferred to the surface of the cell.[144]

See also

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Further references