Difference between revisions of "Diversity and classification of marine benthic algae"

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In the widest sense, algae are defined as oxygenic photosynthesizers other than
 
In the widest sense, algae are defined as oxygenic photosynthesizers other than
embryophyte land plants (Cavalier-Smith, 2007). In recent years our knowledge of
+
embryophyte land plants (Cavalier-Smith, 2007<ref name = Cavalier> Cavalier-Smith, T. (2007). Evolution and relationships of algae: major branches of the tree of life. In Unravelling the algae: the past, present and future of algal systematics, eds J. Brodie and J. Lewis, pp 21-55. CRC Press, Boca Raton, London and New York. </ref>). In recent years our knowledge of these organisms has greatly advanced, thanks to new types of data (mainly electron microscopy observations and DNA sequence data). Based on these data, we know now that algae represent an artificial and unnatural agglomeration of very different
these organisms has greatly advanced, thanks to new types of data (mainly electron
 
microscopy observations and DNA sequence data). Based on these data, we know
 
now that algae represent an artificial and unnatural agglomeration of very different
 
 
organisms, sharing the only common characteristic of living in aquatic habitats. Algae
 
organisms, sharing the only common characteristic of living in aquatic habitats. Algae
 
living in the sea are typically subdivided in plankton (the complex of microscopic
 
living in the sea are typically subdivided in plankton (the complex of microscopic
 
algae not visible with unaided eye, which live floating in the water) and benthos (the
 
algae not visible with unaided eye, which live floating in the water) and benthos (the
 
collectivity of the algae that live attached to the sea bottom).
 
collectivity of the algae that live attached to the sea bottom).
 +
 +
[[image:See weed on a rocky outcrop.JPG|thumb|right|350px|caption|Algal canopy at low tide on the west coast of Irish Sea. Photo © EcoServe]]
  
 
The algae that form the benthos are plant-like organisms, very diverse and very
 
The algae that form the benthos are plant-like organisms, very diverse and very
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deep and substantial that in recent classification schemes they are classified into
 
deep and substantial that in recent classification schemes they are classified into
 
different kingdoms. While green and red algae are considered plants (kingdom
 
different kingdoms. While green and red algae are considered plants (kingdom
Plantae), brown algae belong to a different kingdom called Chromista (Cavalier-
+
Plantae), brown algae belong to a different kingdom called Chromista (Cavalier-Smith, 2007<ref name = Cavalier/>). This separation is due to the evolutionary origin of these groups. All eukaryotic algae originated through a process called endoymbiosis, in which a
Smith, 2007). This separation is due to the evolutionary origin of these groups. All
 
eukaryotic algae originated through a process called endoymbiosis, in which a
 
 
unicellular organism was captured and incorporated by another unicellular organism.
 
unicellular organism was captured and incorporated by another unicellular organism.
  
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==Green algae==
 
==Green algae==
 +
[[Image:Fig1Rindi.jpg|thumb|left|180px|Figure 1: ''Cladophora ordinata''. © Rob Anderson & John Bolton]]
 
The green algae represent a very diverse group, which includes members distributed
 
The green algae represent a very diverse group, which includes members distributed
 
not only in the sea, but also in freshwater and terrestrial habitats. Their classification
 
not only in the sea, but also in freshwater and terrestrial habitats. Their classification
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definition, green algae do not form a homogeneous and coherent entity; they are part
 
definition, green algae do not form a homogeneous and coherent entity; they are part
 
of a larger group called Viridiplantae, in which the land plants are also included
 
of a larger group called Viridiplantae, in which the land plants are also included
(Lewis & McCourt, 2004). However, all marine green algae are classified in a
+
(Lewis & McCourt, 2004<ref> Lewis, L.A. & McCourt, R.M. (2004). Green algae and the origin of land plants. American Journal of Botany, 91: 1535-1556. </ref>). However, all marine green algae are classified in a common class, called Ulvophyceae. The Ulvophyceae are a very diverse group and include about 920 species, which are distributed in all seas of the world.
common class, called Ulvophyceae. The Ulvophyceae are a very diverse group and
+
[[Image:Fig2Rindi.jpg|thumb|right|250px|Figure 2: ''Ulva rigida''. © Katrin Österlund]]
include about 920 species, which are distributed in all seas of the world.
 
 
 
 
In the green seaweeds, the body of the alga shows a great range of variation of
 
In the green seaweeds, the body of the alga shows a great range of variation of
 
forms, but usually its morphology is quite simple. Thin filaments, either branched or
 
forms, but usually its morphology is quite simple. Thin filaments, either branched or
not, are a common growth form and are found in Cladophora (Figure 1) and Chaetomorpha, two widespread genera
+
not, are a common growth form and are found in ''Cladophora'' (Figure 1) and ''Chaetomorpha'', two widespread genera.  
[[Image:Fig1Rindi.jpg|thumb|left|150px|Figure 1: Cladophora ordinata]]
+
Sheets formed by two layers of cells are typical of ''Ulva'' (Figure 2). Because of their appearance, species of this genus are popularly called sea lettuce. These algae are well known for their fast growth and high capacity to adsorb nutrients from seawater.  
Blades formed by two layers of cells are
+
[[Image:Fig5Rindi.jpg|thumb|right|180px|Figure 5: ''Halimeda cuneata''. © Heroen Verbruggen]]
typical of Ulva (Figure 2). [[Image:Fig2Rindi.jpg|thumb|right|250px|Figure 2: Ulva rigida]] Because of their appearance, species of this genus are
+
[[Image:Fig3Rindi.jpg|thumb|left|250px|Figure 3: ''Caulerpa fergusonii''. © Michael Guiry]]
popularly called sea lettuce. These algae are well known for their fast growth and
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[[Image:Fig4Rindi.jpg|thumb|left|250px|Figure 4: ''Caulerpa taxifolia''. © Eduardo Infantes Oanes]]
high capacity to adsorb nutrients from seawater. An abundant growth of Ulva is a
+
An abundant growth of ''Ulva'' is a common phenomenon in eutrophic waters; when such growth becomes uncontrolled,
common phenomenon in eutrophic waters; when such growth becomes uncontrolled,
+
the accumulation of large masses of ''Ulva'' produces the so-called green tides, which
the accumulation of large masses of Ulva produces the so-called green tides, which
 
 
may require mechanical removal of the algal biomass. A type of body organization
 
may require mechanical removal of the algal biomass. A type of body organization
 
which is unique to the green seaweeds is the so-called siphonalean organization (or
 
which is unique to the green seaweeds is the so-called siphonalean organization (or
 
coenocytic organization). Siphonalean green algae are classified in two orders,
 
coenocytic organization). Siphonalean green algae are classified in two orders,
Bryopsidales and Dasycladales, and are among the most ecologically successful
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Bryopsidales and Dasycladales, and are among the most ecologically successful seaweeds.  
seaweeds. The body of these algae is formed by one single giant cell, which contains
+
The body of these algae is formed by one single giant cell, which contains
 
numerous nuclei. The best-known example of siphonalean seaweeds is represented
 
numerous nuclei. The best-known example of siphonalean seaweeds is represented
by the genus Caulerpa. Species of this genus consist of a creeping stolon (that grows
+
by the genus ''Caulerpa''. Species of this genus consist of a creeping stolon (that grows attached to the rocky bottom), from which numerous erect frond of variable shape arise (Figure 3). Species of ''Caulerpa'' are distributed in tropical and warm-temperate
attached to the rocky bottom), from which numerous erect frond of variable shape
 
arise (Figure 3). Species of Caulerpa are distributed in tropical and warm-temperate
 
 
seas. Because of their beautiful habit, they are very popular among aquarium
 
seas. Because of their beautiful habit, they are very popular among aquarium
hobbyists and are widely used in tropical aquaria (Stam et al., 2006). Unfortunately,
+
hobbyists and are widely used in tropical aquaria (Stam et al., 2006<ref> Stam, W., Olsen, J.L., Zaleski, S.F., Murray, S.N., Brown, K.R. & Walters, L.J. (2006). A forensic and phylogenetic survey of Caulerpa species (Caulerpales, Chlorophyta) from the Florida coast, local aquarium shops and e-commerce: establishing a proactive baseline for early detection. Journal of Phycology, 42: 1113-1124. </ref>). Unfortunately,
 
algae of this genus have often the tendency to grow in aggressive and uncontrolled
 
algae of this genus have often the tendency to grow in aggressive and uncontrolled
manner. The spread of Caulerpa taxifolia (Figure 4) in the Mediterranean Sea, which
+
manner. The spread of ''Caulerpa taxifolia'' (Figure 4) in the Mediterranean Sea, which
 
occurred after accidental release from the Monaco Oceanographic Museum, has
 
occurred after accidental release from the Monaco Oceanographic Museum, has
 
represented one of the most spectacular events of invasion by a marine organism. In
 
represented one of the most spectacular events of invasion by a marine organism. In
subsequent years, a population of Caulerpa racemosa var. cylindracea introduced in an unknown way from Australia has also invaded aggressively the Mediterranean
+
subsequent years, a population of ''Caulerpa racemosa'' var. ''cylindracea'' introduced in an unknown way from Australia has also invaded aggressively the Mediterranean
(Verlaque et al., 2003; Piazzi et al., 2005). Other common genera with siphonalean
+
(Verlaque et al., 2003<ref> Verlaque, M., Durand, C., Huisman, J. M., Boudouresque, C. F. & Parco, Y. (2003). On the identity and origin of the Mediterranean invasive Caulerpa racemosa (Caulerpales, Chlorophyta). European Journal of Phycology, 38: 325–339. </ref>; Piazzi et al., 2005<ref> Piazzi, L., Meinesz, A., Verlaque, M., Açali, B., Antolic, B., Argyrou, M., Balata, D., Ballesteros, E., Calvo, S., Cinelli, F., Cirik, S., Cossu, A., d'Archino, R., Djellouli, A.S., Javel, F., Lanfranco, E., Mifsud, C., Pala, D., Panayotidis, P., Peirano, A., Pergent, G., Petrocelli, A., Ruitton, S., Zuljevic, A. & Ceccherelli, G. (2005). Invasion of Caulerpa racemosa var. cylindracea (Caulerpales, Chlorophyta) in the Mediterranean Sea: an assessment of the spread. Cryptogamie, Algologie, 26: 189-202. </ref>). Other common genera with siphonalean
organization are Codium, Acetabularia, Halimeda, Udotea and Valonia. Halimeda
+
organization are ''Codium'', ''Acetabularia'', ''Halimeda'', ''Udotea'' and ''Valonia''. ''Halimeda''
 
has a peculiar appearance; it is a branched alga, whose body is formed by many
 
has a peculiar appearance; it is a branched alga, whose body is formed by many
 
connected segments with the shape of coins, wedges or sausages (Figure 5). This
 
connected segments with the shape of coins, wedges or sausages (Figure 5). This
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==Red algae==
 
==Red algae==
 +
[[Image:Fig6Rindi.jpg|thumb|left|200px|Figure 6: ''Delesseria sanguinea''. © Michael Guiry]]
 
The red algae are one of the most ancient groups of eukaryotic algae (fossils of
 
The red algae are one of the most ancient groups of eukaryotic algae (fossils of
Bangiomorpha pubescens, believed to be the oldest red alga, are 1.2 billions years
+
''Bangiomorpha pubescens'', believed to be the oldest red alga, are 1.2 billions years
 
old). A distinctive characteristic of this group is that they are the only seaweeds
 
old). A distinctive characteristic of this group is that they are the only seaweeds
 
lacking flagella (as well as centrioles and other structures typical of the flagellar
 
lacking flagella (as well as centrioles and other structures typical of the flagellar
apparatus) at any stage of their life histories (Maggs et al., 2007). Their colour, which
+
apparatus) at any stage of their life histories (Maggs et al., 2007<ref> Maggs, M.A., Verbruggen, H. & De Clerck (2007). Molecular systematics of red algae: building future structures of firm foundations. In Unravelling the algae: the past, present and future of algal systematics, eds J. Brodie & J. Lewis, pp. 103-121. CRC Press, Boca Raton, London & New York. </ref>). Their colour, which
 
ranges from pink to bright red, purple or sometimes dark brown, is due to the
 
ranges from pink to bright red, purple or sometimes dark brown, is due to the
 
presence of pigments called phycobilins. At present, about 6,000 red algal species
 
presence of pigments called phycobilins. At present, about 6,000 red algal species
 
are recognized. They have a complex life history, which usually involves the
 
are recognized. They have a complex life history, which usually involves the
 
alternation of three generations (gametophyte, carposporophyte and
 
alternation of three generations (gametophyte, carposporophyte and
tetrasporophyte). Their sexual reproductive apparatus is a very sophisticated
+
tetrasporophyte).  
 +
[[Image:Fig7Rindi.jpg|thumb|right|250px|Figure 7: ''Porphyra umbilicalis''. © Michael Guiry]]
 +
Their sexual reproductive apparatus is a very sophisticated
 
structure, whose arrangement has been used for a long time as the main criterion for
 
structure, whose arrangement has been used for a long time as the main criterion for
 
taxonomy at ordinal level. Molecular data produced in the last two decades have
 
taxonomy at ordinal level. Molecular data produced in the last two decades have
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seven classes (Cyanidiophyceae, Bangiophyceae, Compsopogonophyceae,
 
seven classes (Cyanidiophyceae, Bangiophyceae, Compsopogonophyceae,
 
Florideophyceae, Porphyridiophyceae, Rhodellophyceae and Stylonematophyceae)
 
Florideophyceae, Porphyridiophyceae, Rhodellophyceae and Stylonematophyceae)
and 33 orders (Saunders & Hommersand, 2004; Yoon et al., 2006).
+
and 33 orders (Saunders & Hommersand, 2004<ref> Saunders, G.W. & Hommersand, M.H. (2004). Assessing red algal supraordinal diversity and taxonomy in the context of contemporary systematic data. American Journal of Botany, 91, 1494-1507. </ref>; Yoon et al., 2006<ref> Yoon, H.S., Müller, K.M., Sheath, R.G., Ott, F.D. & Bhattacharya, D. (2006). Defining the major lineages of red algae. Journal of Phycology, 42: 482-492. </ref>).
 
+
[[Image:Fig8Rindi.jpg|thumb|left|250px|Figure 8: ''Lithothamnion glaciale''. © Colin Bates]]
 +
[[Image:Fig9Rindi.jpg|thumb|right|220px|Figure 9: ''Gelidium capense''. © Rob Anderson]]
 
The red algae show a great range of morphological variation. The simplest forms
 
The red algae show a great range of morphological variation. The simplest forms
consist of single cells like Porphyridium or thin filaments like Bangia. The habit of
+
consist of single cells like ''Porphyridium'' or thin filaments like ''Bangia''. The habit of
 
expanded blades is found in many genera, including some of the most spectacular,
 
expanded blades is found in many genera, including some of the most spectacular,
such as Delesseria (Figure 6), Polyneura and Halymenia. A widespread and
+
such as ''Delesseria'' (Figure 6), ''Polyneura'' and ''Halymenia''. A widespread and
economically important genus with blade-like habit is Porphyra (Figure 7). Porphyra
+
economically important genus with blade-like habit is ''Porphyra'' (Figure 7). ''Porphyra yezoensis'' and similar species are popularly known with the japanese name of nori and have been used as food for many centuries in eastern Asia; they are the
yezoensis and similar species are popularly known with the japanese name of nori
 
and have been used as food for many centuries in eastern Asia; they are the
 
 
seaweeds used as wrap for sushi. In other red algae, the body of the alga is formed
 
seaweeds used as wrap for sushi. In other red algae, the body of the alga is formed
 
by a crust which grows attached to the rocky bottom. A typical example is
 
by a crust which grows attached to the rocky bottom. A typical example is
 
represented by species of the order Corallinales, in which the cell walls accumulate
 
represented by species of the order Corallinales, in which the cell walls accumulate
 
calcium carbonate in the form of aragonite, conferring to their body a robust and
 
calcium carbonate in the form of aragonite, conferring to their body a robust and
coriaceous consistence. Algae of this order, such as Lithophyllum, Lithothamnion
+
coriaceous consistence. Algae of this order, such as ''Lithophyllum'', ''Lithothamnion''
(Figure 8) and Phymatolithon, look like pink or red calcified crusts, which are very
+
(Figure 8) and ''Phymatolithon'', look like pink or red calcified crusts, which are very
 
resistent to grazing and mechanical dislodgement; they often thrive on very exposed
 
resistent to grazing and mechanical dislodgement; they often thrive on very exposed
 
rocky shores, where seaweeds with soft tissues would be easily dislodged by the
 
rocky shores, where seaweeds with soft tissues would be easily dislodged by the
 
violence of the waves. Many other species of red seaweeds have a branched plantlike
 
violence of the waves. Many other species of red seaweeds have a branched plantlike
 
shape and look like small bushes or trees. Species with this habit include many
 
shape and look like small bushes or trees. Species with this habit include many
common genera, such as Chondrus, Gelidium (Figure 9), Gigartina, Gracilaria,
+
common genera, such as ''Chondrus'', ''Gelidium'' (Figure 9), ''Gigartina'', ''Gracilaria'',
Hypnea and Laurencia. Species of Eucheuma and Kappahycus are the most important source of carrageenans (compounds widely used in the food industry), and
+
''Hypnea'' and ''Laurencia''. Species of ''Eucheuma'' and ''Kappahycus'' are the most important source of carrageenans (compounds widely used in the food industry), and
 
for this purpose they are farmed on large scale in tropical regions, especially
 
for this purpose they are farmed on large scale in tropical regions, especially
 
Philippines and Indonesia.
 
Philippines and Indonesia.
  
 
==Brown algae==
 
==Brown algae==
 +
[[Image:Fig10Rindi.jpg|thumb|right|290px|Figure 10: ''Sargassum hemiphyllum''. © Yukihiko Serisawa]]
 
The brown algae are represented by about 1,780 species, currently classified in the
 
The brown algae are represented by about 1,780 species, currently classified in the
 
class Fucophyceae (or Phaeophyceae) of the phylum Ochrophyta (De Reviers et al.,
 
class Fucophyceae (or Phaeophyceae) of the phylum Ochrophyta (De Reviers et al.,
2007); the class includes 17 orders. Brown seaweeds are not close relatives of red
+
2007<ref> De Reviers, B., Rousseau, F. & Draisma, S.G.A. (2007). Classification of the Phaeophyceae from past to present and current challenges. In Unravelling the algae: the past, present and future of algal systematics, eds J. Brodie & J. Lewis, pp. 267-284. CRC Press, Boca Raton, London & New York. </ref>); the class includes 17 orders. Brown seaweeds are not close relatives of red
 
and green seaweeds, although they are macroscopically similar and live mixed
 
and green seaweeds, although they are macroscopically similar and live mixed
 
together on rocky shores. They belong to a different kingdom (Chromista) and their
 
together on rocky shores. They belong to a different kingdom (Chromista) and their
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Laminariales and Desmarestiales) are entirely confined to polar and cold-temperate
 
Laminariales and Desmarestiales) are entirely confined to polar and cold-temperate
 
waters. Despite of this general trend, however, this group includes also some genera
 
waters. Despite of this general trend, however, this group includes also some genera
which are very common and diverse in tropical seas, such as Sargassum (Figure 10)
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which are very common and diverse in tropical seas, such as ''Sargassum'' (Figure 10)
and Turbinaria.
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and ''Turbinaria''.
 
+
[[Image:Fig11Rindi.jpg|thumb|left|250px|Figure 11: ''Pylaiella littoralis''. © Colin Bates]]
 +
[[Image:Fig12Rindi.jpg|thumb|right|270px|Figure 12: belt of ''Fucus'' at low tide (Black Head, Co. Clare, Ireland). © Michael Guiry]]
 +
[[Image:Fig13Rindi.jpg|thumb|left|250px|Figure 13: ''Cystoseira humilis''. © Fabio Rindi]]
 
The diversity of forms and shapes of the brown seaweeds is not inferior to that of the
 
The diversity of forms and shapes of the brown seaweeds is not inferior to that of the
 
green and red seaweeds. Filamentous species consisting of thin branched threads,
 
green and red seaweeds. Filamentous species consisting of thin branched threads,
such as Ectocarpus and Pylaiella (Figure 11), grow on rock or on larger seaweeds in
+
such as ''Ectocarpus'' and ''Pylaiella'' (Figure 11), grow on rock or on larger seaweeds in
 
the intertidal zone of many regions of the world. Most brown algae, however, have a
 
the intertidal zone of many regions of the world. Most brown algae, however, have a
 
bigger size and look like branched ribbons, bushes or small trees. Members of the
 
bigger size and look like branched ribbons, bushes or small trees. Members of the
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in the intertidal zone of many rocky shores of temperate seas. For this reason, they
 
in the intertidal zone of many rocky shores of temperate seas. For this reason, they
 
produce a type of habitat that supports a great biological diversity and are considered
 
produce a type of habitat that supports a great biological diversity and are considered
keystone species; the belts of Fucus (Figure 12) in the northern Atlantic and
+
keystone species; the belts of ''Fucus'' (Figure 12) in the northern Atlantic and
Cystoseira (Figure 13) in the Mediterranean are well known-examples. This order
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''Cystoseira'' (Figure 13) in the Mediterranean are well known-examples. This order
 
also includes the only example of a macroalga which lives floating permanently and
 
also includes the only example of a macroalga which lives floating permanently and
 
is never attached to the rocky bottom: the Sargasso Sea, located in the middle of the
 
is never attached to the rocky bottom: the Sargasso Sea, located in the middle of the
 
North Atlantic Ocean, is an area delimited by oceanic currents in which large masses
 
North Atlantic Ocean, is an area delimited by oceanic currents in which large masses
of Sargassum float permanently. The largest known algae are also species of brown
+
of ''Sargassum'' float permanently.  
seaweeds; they belong to the order Laminariales and are designated with the term
+
[[Image:Fig14Rindi.jpg|thumb|right|200px|Figure 14: ''Macrocystis pyrifera''. © Erasmo Macaya]]
kelps. The giant Pacific kelp Macrocystis pyrifera (Figure 14) is the seaweed with the
+
The largest known algae are also species of brown seaweeds; they belong to the order Laminariales and are designated with the term
 +
kelps. The giant Pacific kelp ''Macrocystis pyrifera'' (Figure 14) is the seaweed with the
 
largest size. It can reach 60 metres in length and forms dense forests on the Pacific
 
largest size. It can reach 60 metres in length and forms dense forests on the Pacific
 
shores of Canada and U.S.A. Similar submerged forests are produced in other parts
 
shores of Canada and U.S.A. Similar submerged forests are produced in other parts
of the world by species of Ecklonia, Eisenia, Laminaria and Lessonia. The kelps are
+
of the world by species of ''Ecklonia'', ''Eisenia'', ''Laminaria'' and ''Lessonia''. The kelps are
 
also the most complex algae from an anatomic and morphological point of view, as
 
also the most complex algae from an anatomic and morphological point of view, as
 
their tissues contain types of cells and structures with complexity comparable to
 
their tissues contain types of cells and structures with complexity comparable to
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financial support received under the National Marine Biodiscovery Program. I am
 
financial support received under the National Marine Biodiscovery Program. I am
 
also thankful to Michael Guiry, Rob Anderson, John Bolton, Olivier De Clerck, Katrin
 
also thankful to Michael Guiry, Rob Anderson, John Bolton, Olivier De Clerck, Katrin
Österlund, Erasmo Macaya, Eduardo Infantes Oanes, Colin Bates and Heroen
+
Österlund, Erasmo Macaya, Eduardo Infantes Oanes, Colin Bates, Yukihiko Serisawa and Heroen
 
Verbruggen for use of pictures, obtained from AlgaeBase (www.algaebase.org).
 
Verbruggen for use of pictures, obtained from AlgaeBase (www.algaebase.org).
  
 +
 +
==Related articles==
 +
:[[Seaweed (macro-algae) ecosystem services]]
 +
:[[Kelp forests]]
 +
 +
 +
== References ==
 +
<references/>
  
  
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|AuthorFullName=Rindi, Fabio
 
|AuthorFullName=Rindi, Fabio
 
|AuthorName=Fabio}}
 
|AuthorName=Fabio}}
 +
 +
[[Category:Coastal and marine ecosystems]] 
 +
[[Category:Eutrophication]]
 +
[[Category:Marine Biodiversity‏‎]]

Latest revision as of 21:17, 12 February 2024

Introduction

Chlorophyllian photosynthesis is responible of almost all primary production at global level and represents the most fundamental process for the support and maintenance of life on our planet. Organisms performing photosynthesis occur in every habitat in which sunlight is available. Whereas vascular plants are largely responsible of photosynthesis on land, in aquatic habitats this process is mainly perfomed by the wide and heterogeneous complex of organisms called algae.

In the widest sense, algae are defined as oxygenic photosynthesizers other than embryophyte land plants (Cavalier-Smith, 2007[1]). In recent years our knowledge of these organisms has greatly advanced, thanks to new types of data (mainly electron microscopy observations and DNA sequence data). Based on these data, we know now that algae represent an artificial and unnatural agglomeration of very different organisms, sharing the only common characteristic of living in aquatic habitats. Algae living in the sea are typically subdivided in plankton (the complex of microscopic algae not visible with unaided eye, which live floating in the water) and benthos (the collectivity of the algae that live attached to the sea bottom).

Algal canopy at low tide on the west coast of Irish Sea. Photo © EcoServe

The algae that form the benthos are plant-like organisms, very diverse and very different in size, shape and colour; they are are typically designated with the term seaweeds. Despite of the undeserved negative connotation associated with such a name, seaweeds play a fundamental role marine ecosystems, where they have a multitude of beneficial effects. Although unrecognized, they also have an important part in our everyday life. Substances extracted from seaweeds occur in toothpastes, shampoos, cosmetics, drugs, soups, ice creams, soft drinks, beer, jellies, marmelades, salad dressings, chewing gums and many other products of large use. In China, Korea and Japan some species of seaweeds are among the most popular foods, and for this reason they have been farmed for many centuries.

Seaweeds occur on any shore where a hard bottom or any other type of stable surfaces are present. From a systematic point of view they are divided in three groups, whose empirical distinction is based on the colour: green algae, red algae and brown algae. The distinction of these three groups, however, is much more substantial than indicated by this simple designation. Besides the colour, they differ in their photosynthetic pigments, storage compounds, composition of cell walls, presence/absence of flagella, ultrastructure of mitosis, fine structure of the chloroplasts and several other characters. The separation of these three groups is so deep and substantial that in recent classification schemes they are classified into different kingdoms. While green and red algae are considered plants (kingdom Plantae), brown algae belong to a different kingdom called Chromista (Cavalier-Smith, 2007[1]). This separation is due to the evolutionary origin of these groups. All eukaryotic algae originated through a process called endoymbiosis, in which a unicellular organism was captured and incorporated by another unicellular organism.

Instead than being killed and digested, the organism incorporated remained alive in its host and established a symbiotic relationship with it. Green algae and red algae were created by primary endosymbiosis. In this process, a unicellular photosynthetic cyanobacterium was incorporated by a larger non-photosythetic eukaryote. Performing photosynthesis, the cyanobaterium had a beneficial effect on its host and was maintained alive; gradually in time, the cyanobacterium established a strict metabolic relationship with the host and underwent deep modifications, becoming the chloroplast of the modern green and red algae. Conversely, brown algae originated through secondary endosymbiosis. In this process, a unicellular red alga was incorporated by a unicellular, non-photosyntetic eukaryote. Once again, the red alga was kept alive by his host and established a strict metabolic relationship with it; the symbiosis of the two organisms produced the ancestral brown alga.

Green algae

Figure 1: Cladophora ordinata. © Rob Anderson & John Bolton

The green algae represent a very diverse group, which includes members distributed not only in the sea, but also in freshwater and terrestrial habitats. Their classification has been revised in recent years, based on DNA sequence data. In the current definition, green algae do not form a homogeneous and coherent entity; they are part of a larger group called Viridiplantae, in which the land plants are also included (Lewis & McCourt, 2004[2]). However, all marine green algae are classified in a common class, called Ulvophyceae. The Ulvophyceae are a very diverse group and include about 920 species, which are distributed in all seas of the world.

Figure 2: Ulva rigida. © Katrin Österlund

In the green seaweeds, the body of the alga shows a great range of variation of forms, but usually its morphology is quite simple. Thin filaments, either branched or not, are a common growth form and are found in Cladophora (Figure 1) and Chaetomorpha, two widespread genera. Sheets formed by two layers of cells are typical of Ulva (Figure 2). Because of their appearance, species of this genus are popularly called sea lettuce. These algae are well known for their fast growth and high capacity to adsorb nutrients from seawater.

Figure 5: Halimeda cuneata. © Heroen Verbruggen
Figure 3: Caulerpa fergusonii. © Michael Guiry
Figure 4: Caulerpa taxifolia. © Eduardo Infantes Oanes

An abundant growth of Ulva is a common phenomenon in eutrophic waters; when such growth becomes uncontrolled, the accumulation of large masses of Ulva produces the so-called green tides, which may require mechanical removal of the algal biomass. A type of body organization which is unique to the green seaweeds is the so-called siphonalean organization (or coenocytic organization). Siphonalean green algae are classified in two orders, Bryopsidales and Dasycladales, and are among the most ecologically successful seaweeds. The body of these algae is formed by one single giant cell, which contains numerous nuclei. The best-known example of siphonalean seaweeds is represented by the genus Caulerpa. Species of this genus consist of a creeping stolon (that grows attached to the rocky bottom), from which numerous erect frond of variable shape arise (Figure 3). Species of Caulerpa are distributed in tropical and warm-temperate seas. Because of their beautiful habit, they are very popular among aquarium hobbyists and are widely used in tropical aquaria (Stam et al., 2006[3]). Unfortunately, algae of this genus have often the tendency to grow in aggressive and uncontrolled manner. The spread of Caulerpa taxifolia (Figure 4) in the Mediterranean Sea, which occurred after accidental release from the Monaco Oceanographic Museum, has represented one of the most spectacular events of invasion by a marine organism. In subsequent years, a population of Caulerpa racemosa var. cylindracea introduced in an unknown way from Australia has also invaded aggressively the Mediterranean (Verlaque et al., 2003[4]; Piazzi et al., 2005[5]). Other common genera with siphonalean organization are Codium, Acetabularia, Halimeda, Udotea and Valonia. Halimeda has a peculiar appearance; it is a branched alga, whose body is formed by many connected segments with the shape of coins, wedges or sausages (Figure 5). This genus is widespread in tropical seas and is particularly important in the atolls. Its cell walls accumulate calcium carbonate in the form of calcite; after the death of the alga, this material is released and contributes to produce the white sand typical of atoll beaches.

Red algae

Figure 6: Delesseria sanguinea. © Michael Guiry

The red algae are one of the most ancient groups of eukaryotic algae (fossils of Bangiomorpha pubescens, believed to be the oldest red alga, are 1.2 billions years old). A distinctive characteristic of this group is that they are the only seaweeds lacking flagella (as well as centrioles and other structures typical of the flagellar apparatus) at any stage of their life histories (Maggs et al., 2007[6]). Their colour, which ranges from pink to bright red, purple or sometimes dark brown, is due to the presence of pigments called phycobilins. At present, about 6,000 red algal species are recognized. They have a complex life history, which usually involves the alternation of three generations (gametophyte, carposporophyte and tetrasporophyte).

Figure 7: Porphyra umbilicalis. © Michael Guiry

Their sexual reproductive apparatus is a very sophisticated structure, whose arrangement has been used for a long time as the main criterion for taxonomy at ordinal level. Molecular data produced in the last two decades have revolutionised the classification of these organisms, which belong to a single phylum, the Rhodophyta, subdivided in two subphyla (Cyanidiophytina and Rhodophytina), seven classes (Cyanidiophyceae, Bangiophyceae, Compsopogonophyceae, Florideophyceae, Porphyridiophyceae, Rhodellophyceae and Stylonematophyceae) and 33 orders (Saunders & Hommersand, 2004[7]; Yoon et al., 2006[8]).

Figure 8: Lithothamnion glaciale. © Colin Bates
Figure 9: Gelidium capense. © Rob Anderson

The red algae show a great range of morphological variation. The simplest forms consist of single cells like Porphyridium or thin filaments like Bangia. The habit of expanded blades is found in many genera, including some of the most spectacular, such as Delesseria (Figure 6), Polyneura and Halymenia. A widespread and economically important genus with blade-like habit is Porphyra (Figure 7). Porphyra yezoensis and similar species are popularly known with the japanese name of nori and have been used as food for many centuries in eastern Asia; they are the seaweeds used as wrap for sushi. In other red algae, the body of the alga is formed by a crust which grows attached to the rocky bottom. A typical example is represented by species of the order Corallinales, in which the cell walls accumulate calcium carbonate in the form of aragonite, conferring to their body a robust and coriaceous consistence. Algae of this order, such as Lithophyllum, Lithothamnion (Figure 8) and Phymatolithon, look like pink or red calcified crusts, which are very resistent to grazing and mechanical dislodgement; they often thrive on very exposed rocky shores, where seaweeds with soft tissues would be easily dislodged by the violence of the waves. Many other species of red seaweeds have a branched plantlike shape and look like small bushes or trees. Species with this habit include many common genera, such as Chondrus, Gelidium (Figure 9), Gigartina, Gracilaria, Hypnea and Laurencia. Species of Eucheuma and Kappahycus are the most important source of carrageenans (compounds widely used in the food industry), and for this purpose they are farmed on large scale in tropical regions, especially Philippines and Indonesia.

Brown algae

Figure 10: Sargassum hemiphyllum. © Yukihiko Serisawa

The brown algae are represented by about 1,780 species, currently classified in the class Fucophyceae (or Phaeophyceae) of the phylum Ochrophyta (De Reviers et al., 2007[9]); the class includes 17 orders. Brown seaweeds are not close relatives of red and green seaweeds, although they are macroscopically similar and live mixed together on rocky shores. They belong to a different kingdom (Chromista) and their closest relatives are microscopic algae which live in the plankton of seas and lakes (diatoms, chrysophytes, xanthophytes). Brown seaweeds are distributed in all seas of the world, but they are most diverse and abundant in cold seas; in particular their largest-sized and most spectacular representatives (species of the orders Laminariales and Desmarestiales) are entirely confined to polar and cold-temperate waters. Despite of this general trend, however, this group includes also some genera which are very common and diverse in tropical seas, such as Sargassum (Figure 10) and Turbinaria.

Figure 11: Pylaiella littoralis. © Colin Bates
Figure 12: belt of Fucus at low tide (Black Head, Co. Clare, Ireland). © Michael Guiry
Figure 13: Cystoseira humilis. © Fabio Rindi

The diversity of forms and shapes of the brown seaweeds is not inferior to that of the green and red seaweeds. Filamentous species consisting of thin branched threads, such as Ectocarpus and Pylaiella (Figure 11), grow on rock or on larger seaweeds in the intertidal zone of many regions of the world. Most brown algae, however, have a bigger size and look like branched ribbons, bushes or small trees. Members of the order Fucales are algae of particular ecological importance, as they form dense belts in the intertidal zone of many rocky shores of temperate seas. For this reason, they produce a type of habitat that supports a great biological diversity and are considered keystone species; the belts of Fucus (Figure 12) in the northern Atlantic and Cystoseira (Figure 13) in the Mediterranean are well known-examples. This order also includes the only example of a macroalga which lives floating permanently and is never attached to the rocky bottom: the Sargasso Sea, located in the middle of the North Atlantic Ocean, is an area delimited by oceanic currents in which large masses of Sargassum float permanently.

Figure 14: Macrocystis pyrifera. © Erasmo Macaya

The largest known algae are also species of brown seaweeds; they belong to the order Laminariales and are designated with the term kelps. The giant Pacific kelp Macrocystis pyrifera (Figure 14) is the seaweed with the largest size. It can reach 60 metres in length and forms dense forests on the Pacific shores of Canada and U.S.A. Similar submerged forests are produced in other parts of the world by species of Ecklonia, Eisenia, Laminaria and Lessonia. The kelps are also the most complex algae from an anatomic and morphological point of view, as their tissues contain types of cells and structures with complexity comparable to those of vascular plants.


ACKNOWLEDGEMENTS. I am very grateful to the Marine Institute of Ireland for financial support received under the National Marine Biodiscovery Program. I am also thankful to Michael Guiry, Rob Anderson, John Bolton, Olivier De Clerck, Katrin Österlund, Erasmo Macaya, Eduardo Infantes Oanes, Colin Bates, Yukihiko Serisawa and Heroen Verbruggen for use of pictures, obtained from AlgaeBase (www.algaebase.org).


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References

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The main author of this article is Rindi, Fabio
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Citation: Rindi, Fabio (2024): Diversity and classification of marine benthic algae. Available from http://www.coastalwiki.org/wiki/Diversity_and_classification_of_marine_benthic_algae [accessed on 31-10-2024]