2 Marine Macro- and Microalgae: An Overview
Algae most commonly occur in water—marine, freshwater, and brackish—but they can also be
found in almost every other environment on earth, for example, growing on snow in some high mountains
and in the Arctic or living in lichen associations on bare rocks, in desert soils, and in hot springs (Lee
2008). In most habitats, algae are extremely important as they function as primary producers in the food
chains and produce the oxygen necessary for their metabolism and that of the consumer organisms.
Algae also form mutually beneficial partnerships with other organisms in which they provide oxygen and
organic compounds and receive protection and nutrients. This is the case of the zooxanthellae that live
inside the cells of reef-building corals, and of the green algae or cyanobacteria that associate with fungi
to form lichens (Barsanti and Gualtieri 2014).
Algae cells contain the green pigment chlorophyll that captures the sun’s energy for photosynthesis,
that is, the process of building energy-rich compounds from water and carbon dioxide. Photosynthetic
algae will therefore grow only where there is light. In marine environments below low tide level, the
amount and quality of light decreases with increasing depth to a point where the light level is not enough
for algal growth. Some species contain additional pigments that enable them to absorb different light
wavelengths and to use faint levels of light. This is the case of a coralline red algae collected at 268 m
deep, the depth record for marine macrophytes (Littler et al. 1985). Due to these additional pigments,
algae can exhibit different colors, the more common being red or brown. These shades are themselves
extremely variable, with some brown species appearing olive-green and some red species almost black.
Many algae grow attached to the firm substratum (benthic). Benthic algae can grow attached on
stones (epilithic), on sand or mud (epipelic), on animals (epizoic), or on other algae or plants (epiphytic).
In marine environments, they can grow from the littoral zone (encompassing the intertidal area) to the
shallow subtidal to around 200 m in very clear waters (Barsanti and Gualtieri 2014).
Most algae, however, are small single celled and filamentous organisms floating freely in the sea
and constitute the phytoplankton, which forms the base of the marine food chains (Harris 1986; Sournia
2008). Most planktonic species are independent from the coastal and benthic processes, except for the
temporary microscopic stages of the life cycle of the macroalgae (South and Whittick 1987; Falkowski
and Knoll 2007).
Size and shapes
The size of algae is highly variable ranging from the tiny picoplankton which is only 0.2–2.0 μm in
diameter to giant kelps with fronds up to 60 m in length.
Algae occur in dissimilar forms such as microscopic single cells (Fig. 1A), filaments (Fig. 1B),
macroscopic multicellular loose or filmy conglomerations (Fig. 1C), colonies matted or branched (Fig.
1D), and more complex forms encompassing thin foliose sheets (Fig. 1E), tubes (Fig. 1F), sacs or bulbs
(Fig. 1G), leather sheets (Fig. 1H), and gelatinous forms (Fig. 1I), cartilaginous (Fig. 1J) or calcareous
forms, these including bushy (Fig. 1K), foliose (Fig. 1L) and crustose habits (Fig. 1M).
The unicells may or may not be solitary, and may or may not be motile through flagella. They can
also exist as aggregates of single cells in colonial morphs, which can be more or less organized and have
a variable number of cells. A colony is termed coenobium when the number and arrangement of cells are
determined and remain constant. A good example of a motile coenobium is provided by the green alga
Volvox (Fig. 2A).
The filamentous algae are formed by cells divisions along a plane perpendicular to the main axis in
a way that all daughter cells are connected by their end wall. Filaments can be simple, as in Spirogyra
(Fig. 2B), or branched. Branching can be further classified as false (as in the Cyanobacteria Tolypothrix,
Fig. 2C) or true (as in the green Cladophora, Fig. 2D). Filaments can also consist of a single layer of
cells (uniseriate) as in the red Lejolisia (Fig. 2E) or made up of multiple layers (multiseriate) as in
Polysiphonia (Fig. 2F).
A peculiar form occurs in the siphonous algae, which have a coenocytic construction, consisting of
tubular filaments that grow as their nuclei undergo repeated nuclear divisions but in which transverse cell
walls never form. The resulting filament is therefore unicellular but multinucleate (coenocytic). A classic
example of a branched coenocyte thallus is found on the green alga Bryopsis (Fig. 2G).
Algae most commonly occur in water—marine, freshwater, and brackish—but they can also be
found in almost every other environment on earth, for example, growing on snow in some high mountains
and in the Arctic or living in lichen associations on bare rocks, in desert soils, and in hot springs (Lee
2008). In most habitats, algae are extremely important as they function as primary producers in the food
chains and produce the oxygen necessary for their metabolism and that of the consumer organisms.
Algae also form mutually beneficial partnerships with other organisms in which they provide oxygen and
organic compounds and receive protection and nutrients. This is the case of the zooxanthellae that live
inside the cells of reef-building corals, and of the green algae or cyanobacteria that associate with fungi
to form lichens (Barsanti and Gualtieri 2014).
Algae cells contain the green pigment chlorophyll that captures the sun’s energy for photosynthesis,
that is, the process of building energy-rich compounds from water and carbon dioxide. Photosynthetic
algae will therefore grow only where there is light. In marine environments below low tide level, the
amount and quality of light decreases with increasing depth to a point where the light level is not enough
for algal growth. Some species contain additional pigments that enable them to absorb different light
wavelengths and to use faint levels of light. This is the case of a coralline red algae collected at 268 m
deep, the depth record for marine macrophytes (Littler et al. 1985). Due to these additional pigments,
algae can exhibit different colors, the more common being red or brown. These shades are themselves
extremely variable, with some brown species appearing olive-green and some red species almost black.
Many algae grow attached to the firm substratum (benthic). Benthic algae can grow attached on
stones (epilithic), on sand or mud (epipelic), on animals (epizoic), or on other algae or plants (epiphytic).
In marine environments, they can grow from the littoral zone (encompassing the intertidal area) to the
shallow subtidal to around 200 m in very clear waters (Barsanti and Gualtieri 2014).
Most algae, however, are small single celled and filamentous organisms floating freely in the sea
and constitute the phytoplankton, which forms the base of the marine food chains (Harris 1986; Sournia
2008). Most planktonic species are independent from the coastal and benthic processes, except for the
temporary microscopic stages of the life cycle of the macroalgae (South and Whittick 1987; Falkowski
and Knoll 2007).
Size and shapes
The size of algae is highly variable ranging from the tiny picoplankton which is only 0.2–2.0 μm in
diameter to giant kelps with fronds up to 60 m in length.
Algae occur in dissimilar forms such as microscopic single cells (Fig. 1A), filaments (Fig. 1B),
macroscopic multicellular loose or filmy conglomerations (Fig. 1C), colonies matted or branched (Fig.
1D), and more complex forms encompassing thin foliose sheets (Fig. 1E), tubes (Fig. 1F), sacs or bulbs
(Fig. 1G), leather sheets (Fig. 1H), and gelatinous forms (Fig. 1I), cartilaginous (Fig. 1J) or calcareous
forms, these including bushy (Fig. 1K), foliose (Fig. 1L) and crustose habits (Fig. 1M).
The unicells may or may not be solitary, and may or may not be motile through flagella. They can
also exist as aggregates of single cells in colonial morphs, which can be more or less organized and have
a variable number of cells. A colony is termed coenobium when the number and arrangement of cells are
determined and remain constant. A good example of a motile coenobium is provided by the green alga
Volvox (Fig. 2A).
The filamentous algae are formed by cells divisions along a plane perpendicular to the main axis in
a way that all daughter cells are connected by their end wall. Filaments can be simple, as in Spirogyra
(Fig. 2B), or branched. Branching can be further classified as false (as in the Cyanobacteria Tolypothrix,
Fig. 2C) or true (as in the green Cladophora, Fig. 2D). Filaments can also consist of a single layer of
cells (uniseriate) as in the red Lejolisia (Fig. 2E) or made up of multiple layers (multiseriate) as in
Polysiphonia (Fig. 2F).
A peculiar form occurs in the siphonous algae, which have a coenocytic construction, consisting of
tubular filaments that grow as their nuclei undergo repeated nuclear divisions but in which transverse cell
walls never form. The resulting filament is therefore unicellular but multinucleate (coenocytic). A classic
example of a branched coenocyte thallus is found on the green alga Bryopsis (Fig. 2G).