Gromia
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''Gromia'' is a genus of protists, closely related to
foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
, which inhabit marine and freshwater environments. It is the only genus of the
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
Gromiidae. ''Gromia'' are
ameboid An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and r ...
, producing filose
pseudopodia A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...
that extend out from the cell's proteinaceous
test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film) ...
through a gap enclosed by the cell's oral capsule. The test, a shell made up of protein that encloses the cytoplasm, is made up of several layers of membrane, which resemble honeycombs in shape – a defining character of this genus. ''Gromia'' were first discovered in shallow waters, with members of the best-characterized species '' Gromia oviformus'' often found inhabiting rock surfaces, sediments, or
seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of ''Rhodophyta'' (red), '' Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...
holdfasts. However, research from the 1990s and early 2000s identified gromiids inhabiting depths up to 4,392 m, leading to several new deep-sea ''Gromia'' species being described and recognized. A recent study of the deep-sea species ''
Gromia sphaerica ''Gromia sphaerica'' is a large spherical testate amoebae, testate amoeba, a single-celled Eukaryote, eukaryotic organism and the largest of its genus, ''Gromia''. The genus itself contains about 13 known species, 3 of which were discovered as la ...
'' revealed that it produces traces on the seafloor which resemble fossil traces formerly attributed to early
Bilateria Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left ...
(animals with bilateral symmetry); this now calls into question whether such fossil traces are reliable as documentation of early multicellular animal diversification in the
Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...
era. Deep-sea gromiids have also been shown to be important for
carbon cycling The carbon cycle is a part of the biogeochemical cycle where carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth. Other major biogeochemical cycles include the nitrogen cycle and the water cycl ...
and
denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitr ...
.


History of study

''Gromia'' were first described in the 1835, with ''G. oviformis'' gaining prominence because it was often found in the intertidal zones on the British coast. Initially, ''Gromia'' were regarded as members of
Foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
or Filosea, as noted in a review by Cifelli (1990). ''Gromia'' became better characterized throughout the 1960s, when electron microscopy revealed more details on their morphology, including their honeycomb The first molecular studies involving ''Gromia'', which sampled ''G. oviformis'', used small subunit (SSU) ribosomal RNA genes and concluded that Gromia were members of Cercozoa, a large group of amoebae with tests and filose pseudopodia. Follow-up studies on this group placed ''Gromia'' within the
Gromiidea Gromiida is an order of Endomyxa Endomyxa is a group of eukaryotic organisms in the supergroup Rhizaria. They were initially a subphylum of Cercozoa and later a subphylum of Retaria, but several analyses have proven they are a phylogenetical ...
class, again based on SSU rRNA genes Eventually, when molecular studies combined data from several genes – actin, polyubiquitin, RNA polymerase II and small subunit rRNA genes – ''Gromia'' was shown to be a sister group to
Foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
. Moreover, within the genus ''Gromia'', studies of the small subunit ribosomal RNA genes of various deep-sea gromiids has revealed species diversity within ''Gromia'', with molecular data tending to correlate with distinct morphologies of the various species’ tests. ''Gromia'' were long thought to only inhabit shallow waters, until samples from the Arabian Sea from depths below 1,000 m revealed the first deep-sea gromiid – ''
Gromia sphaerica ''Gromia sphaerica'' is a large spherical testate amoebae, testate amoeba, a single-celled Eukaryote, eukaryotic organism and the largest of its genus, ''Gromia''. The genus itself contains about 13 known species, 3 of which were discovered as la ...
''. Additional species of deep-sea ''Gromia'' protists were later described in waters from the Arabian sea, the European Arctic sea, and off the coast of Antarctica, among other locations, and characterized both morphologically and through molecular studies of their small subunit rRNA genes.


Habitat and ecology

Gromiids inhabit sediments or surfaces of flora in both shallow waters and the deep sea. The best characterized species of shallow-water ''Gromia'' is ''G. oviformis''. It inhabits intertidal zones and other regions of shallow waters; it is often found attached to rocks, kelp, weeds, Cladophora algae, or within sediments. ''G. oviformis'' has been shown to tolerate a temperature range of 0–30 °C. Deep-sea gromiids have been found in the Arabian sea, off the coast of Antarctica and in the water of the Northwest Atlantic Ocean. They were often collected from the 1,000–3,100 m range. Oxygen levels in gromiid habitats tend to exceed 0.2 mL/L and are therefore not limiting to the organisms’ growth. The temperature tolerance of deep-sea Gromia is uncertain. ''Gromia'' are thought to acquire nutrients from the organic matter in sediments on the sea floor, as they are often found in areas with abundant phytodetritus. Their apertures face down on sediment surfaces and they use their pseudopodia to feed. Gromiids found in the deep sea near Oman and Pakistan are often found with Foraminifera, filamentous prokaryotes and bacteria living on their cell surface. Gromiids provide substrates and serve as a surface for attachment to their epibionts.


Description

''Gromia'' members are quite large, ranging from 0.4 mm to 30 mm. Their proteinaceous tests vary in shapes, from spherical (e.g. ''G. oviformis''), “sausage shaped”, “grape-shaped”, or pear-shaped (e.g. ''G. pyriformis''). Test shape is often used for classifying ''Gromia'' species, and their morphology tends to align with the molecular data used to differentiate species. The interior of the test is layered with membranes with a honeycomb pattern. These honeycomb membranes are a unique feature of ''Gromia''. An oral complex containing an aperture (an opening in the test) allows the filose pseudopodia to extend out. The pseudopodia are non-granular, and can form connections to make net-like structures. Gromia use their pseudopodia to crawl along the surface of sediments. Waste pellets (“stercomata”) and mineral grains accumulate inside the cell – another characteristic feature of ''Gromia''.


Life cycle

''Gromia'' have been observed to undergo both asexual and sexual reproduction. In sexual reproduction observed in ''G. oviformis'', the shells of adult organisms fuse. Gametogenesis and fertilization follow, after which the zygotes mature into amoebulae and exit the parental shells.


Practical importance

Gromiids are hypothesized to be important for carbon cycling, as they are often found in carbon-rich sediments and feed on detritus. In addition, gromiids have been shown to store high levels of intracellular nitrate, suggesting a role for gromiids in denitrification. Gromiids have also enriched our understanding of evolutionary history. The ability of the giant, deep-sea species ''G. sphaerica'' to produce tracks on the sea floor has been used to propose a re-evaluation of the use of fossils with similar traces as evidence for dating the origins of animals with bilateral symmetry.


List of species

Sources: *'' Gromia oviformis'' Dujardin, 1835 *''
Gromia appendiculariae ''Gromia appendiculariae'' is a unicellular, and parasitic, organism in the genus '' Gromia'', which closely resembles ''Gromia sphaerica''. A specimen of ''G. appendiculariae'' was discovered as a parasite Parasitism is a Symbiosis, cl ...
'' Brooks & Kellner, 1908 *'' Gromia dubia'' Gruber, 1884 *'' Gromia dujardinii'' Schultze, 1854 *'' Gromia fluvialis'' Dujardin, 1837 *'' Gromia granulata'' Schulze, 1875 *'' Gromia solenopus'' Zarnik, 1907 *'' Gromia granulata'' Schulze, 1875 *'' Gromia hyalina'' Schlumberger, 1845 *'' Gromia paludosa'' Cienkowski, 1876 *'' Gromia pyriformis'' Gooday & Bowser, 2005 *'' Gromia schulzei'' Norman, 1892 *''
Gromia sphaerica ''Gromia sphaerica'' is a large spherical testate amoebae, testate amoeba, a single-celled Eukaryote, eukaryotic organism and the largest of its genus, ''Gromia''. The genus itself contains about 13 known species, 3 of which were discovered as la ...
'' Gooday, Bowser, Bett & Smith 2000


References


External links

* {{Authority control Rhizaria genera Amoeboids