''Mediolabrus comicus'', known previously as ''Minidiscus comicus'', is a species of

nanophytoplankton Nanophytoplankton are particularly small phytoplankton with sizes between 2 and 20 μm. They are the autotrophic part of nanoplankton. Like other phytoplankton, nanophytoplankton are microscopic organisms that obtain energy through the proce ...

ic centric

diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

s within the family

Thalassiosiraceae Thalassiosiraceae is a family of diatoms in the order Thalassiosirales. The family of Thalassiosiraceae have the unique quality of having a flat valve face. These diatoms are common in brackish, nearshore, and open-ocean habitats, with approximat ...

. Its cells have diameters as small as 1.9 ''μ''m, which makes ''M.'' ''comicus'' one of the smallest known diatoms and brings it near to the theoretical lower size limit for photosynthetic eukaryotes.

Taxonomy

''Mediolabrus comicus'' was originally described as '' Minidiscus comicus'', but in 2020, it was transferred based on

molecular phylogenetics Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

to a newly erected genus ''

Mediolabrus ''Mediolabrus'' is a genus of centric diatoms within the family Thalassiosiraceae. Known ''Mediolabrus'' species live planktonically in brackish and fresh water. This genus, together with the related '' Minidiscus'', includes some of the smalles ...

'', as its

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

Morphology and ultrastructure

Cells of ''M.'' ''comicus'' have diameters between 1.9 and 6.0 μm. The larger cells are discoidal with flat valve faces (ends of the siliceous shell), while the smaller ones are spherical or even oblong. Valve margins are very narrow and marginal areolae (small regularly repeated pores) terminate close to the valve edge. There are 6–8 areolae per 1 μm. Each valve face has one central rimoportula (lip-like pore) surrounded by two to four fultoportulae (tube-like pores). Only a single rimoportula near the valve center is a diagnostic feature of the genus ''

''. External fultoportula tubes are well developed as fluted cylinders, rising to over 0.5 ''μ''m in height above the mantle (side of the valve). The central rimoportula tube can reach up to 2.4 ''μ''m in height in large cells. The copulae that protect dividing cells are very thin and not discernible in non-dividing cells. The fultoportulae extrude long threads (presumably of

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

) that connect cells within the small colonies.

Life cycle and adaptations to small size

''M.'' ''comicus'', like other diatoms, undergoes a multi-year life cycle during which its cells get increasingly smaller in diameter with each asexual division, until they reach a point of size restoration, presumably connected to a sexual process. David Jewson and colleagues studied samples of ''M.'' ''comicus'' collected in the western

Mediterranean The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern ...

by RV ''Sarmiento de Gamboa'' between March and September 2009. This allowed them to explore the diversity of shapes and sizes of cells during different phases of

algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in fresh water or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompass ...

development, as well as the life cycle of the algae. They found a strong relationship between cell diameter and shape. The largest cells, with diameters above 4'' μ''m, are cylindrical with flat or slightly domed valves. As the cell diameter declines with each consecutive division, doming of the valve faces becomes more pronounced, until they become spherical at around 3 ''μ''m (this is also the size at which size restoration starts occurring) and some cells even become oblong at around 2 ''μ''m. This allows ''M.'' ''comicus'' to keep its cell volume relatiovely stable, even though the diameter changes dramatically.

Ecology and distribution

''Mediolabrus comicus'' was first described from a

red tide A harmful algal bloom (HAB), or excessive algae growth, sometimes called a red tide in marine environments, is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, water deoxygenation, ...

Tokyo Tokyo, officially the Tokyo Metropolis, is the capital of Japan, capital and List of cities in Japan, most populous city in Japan. With a population of over 14 million in the city proper in 2023, it is List of largest cities, one of the most ...

harbour on September 17, 1980, forming large aggregated flocks. Since then it has usually been found as solitary cells or forming short chains of 2 or 3 cells. It is widespread along the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...

and

Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...

coasts and in the

Adriatic The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkans, Balkan Peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Se ...

and

seas, suggesting a cosmopolitan distribution.

Role in algal blooms and carbon cycle

In 2013, scientists aboard the research vessel '' Téthys II'' studied a large annual spring

phytoplankton bloom Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. Phyt ...

in the

Gulf of Lion The Gulf of Lion or Gulf of Lions is a wide embayment of the Mediterranean coastline of Catalonia in Spain with Languedoc-Roussillon and Provence in France, extending from Begur in the west to Toulon in the east. The chief port on the gulf is ...

s (northwestern

Mediterranean Sea The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...

) and found that contrary to expectations, it was dominated by two species of nanophytoplanktonic diatoms: ''M.'' ''comicus'' and ''Minidiscus'' ''trioculatus''''.'' Such intense blooms were thought to be caused by larger chain-forming diatom genera and the tiny '' Minidiscus'' and ''

'' have never been documented as their dominant constituents in the

before. The same study also found out that these smallest

s can reach the seafloor at high sinking rates, potentially contributing to carbon export much more substantially than previously expected.

References

External links

*
''Mediolabrus comicus'' in NCBI Taxonomy

''Mediolabrus comicus'' in DiatomBase

''Mediolabrus comicus'' in OBIS
{{Taxonbar, from=Q134168694 Thalassiosirales Diatom species Protists described in 1981