- an algal epiphyte
The orange velvety covering on this Elder tree (Sambucus nigra) is an epiphyte (a plant or alga growing on another plant). It is the filamentous alga Trentepohlia. When viewed under the microscope. A closer examination shows the filamentous nature of this alga. This alga can add a beautiful splash of rustic color to winter woodlands and some species prefer to grow on rocks, walls etc. accounting for the spectacular 'Red-stone Valley' phenomenon in China (external link).
The body or thallus of this alga consists of two types of filament: creeping horizontal (prostrate) filaments that grow over the substrate and put out erect filaments as branches; the overall effect is to form a small cushion of filaments. This alga belongs to the Chlorophyta, or green algae, since it has green chloroplasts but orange beta-carotene (as naked droplets in the cytoplasm) mask the green color of the chlorophyll, giving the alga a yellow, orange or red coloration. The carotenoids combine with polyhydroxyalcohols to form the orange-red pigment haematochrome. Specifically, Trentepohlia belongs to a group of the green algae called the Trentepohliophyceae or Trentopohliaceae (depending on classification used: the former is a class, the latter a family). In this group the algae consist of filaments of cells, each cell possessing a single nucleus.
The cells are connected by a group of plasmodesmata that pierce the center of the cross-wall between adjacent cells. Plasmodesmata also connect neighbouring cells in plants: they are channels that allow water and small molecules to cross from one cell to another and may also allow neighbouring cells to communicate. Their presence in Trentepohlia shows it to be a true multicellular organism.
The black ball-like structure shown above is probably a sporangium or spore-containing capsule. These sporangia may detach before they release spores and be dispersed by wind, rain or by animals.The erect filaments produce these sporangia which release asexual zoids that resemble Chlamydomonas but possess four flagella (they are quadriflagellate) rather than two, all four emerging from one pole of the cell.
The creeping filaments form gametangia (capsules that release gametes / sexual zoids) which open via a single exit pore through which haploid biflagellate (possessing two flagella) gametes emerge: these zoids are gametes with the male and female gametes being identical in size and structure (so-called isogametes). Both flagella emerge from one pole of the cell. Gametes differ from spores in that a male-female pair of gametes must fuse prior to developing into a new individual, whereas a single spore is capable of developing into an individual without fusion (fertilization).
Individual cells can also form dormant spores called akinetes - thick-walled cells that can survive harsh conditions before germinating under more favorable conditions.
Flagella are anchored within the zoids and gametes by microtubules that function as rootlets. Two rootlets anchor each flagellum, one made of 2 microtubules and the other made of a variable number, x, of microtubules, specific to the species, for example 4 in Chlamydomonas. A pair of flagella is anchored by four such rootlets,two per flagellum, forming an x-2-x-2 arrangement or 4-2-4-2 in Chlamydomonas. Trentepohlia and its group are unusual in having a 6-4-6-4 arrangement. The pattern of locomotion is best studied in Chlamydomonas. The flagella are at the pole of the cell that usually travels forwards, with the flagella propelling the cell forwards in a breast-stroke fashion. However, Chlamydomonas can swim in reverse by undulating its flagella.
Above: a biflagellate zoid of Trentepohlia as seen in longitudinal section under the transmission electron microscope. Each flagellum ends in a basal body with associated cytoskeletal structures. The two flagella are held apart at a wide angle (close to 180 degrees). Note that the two flagella cores cross-over where they enter the cell. Biflagellate zoids include gametes but possibly also haploid spores that give rise to haploid individuals (the life-cycle is poorly known but may consist of haploid and diploid individuals, both of which may be capable of asexual reproduction). Ch: chloroplast; Mi: mitochondrion (one behind the microtubule rootlet is redrawn in detail); MR-4: microtubule rootlet with 4 microtubules); MR-6: microtubule rootlet with 6 microtubules; MLB: multi-layered body; Nu: nucleus; Pi: pigment droplet (orange, carotene); SG: starch grain (inside chloroplast); TC:terminal cap at base of basal body of flagellum. (Based upon: Graham and McBride, 1975 and Van den Hoek et al. 1995). Note that each of the MR-6 rootlets passes over a mitochondrion. Golgi apparatus is present only in developing zoids prior to their escape from the sporangia / gametangia. The body of the zoid is about 8 micrometers long (8 thousandths of a millimeter) and are depicted as dorsiventrally flattened in shape by some sources.
Above: the basal apparatus of the pair of flagella as seen looking down on the front pole of the biflagellate zoid of Trentepohlia. Each basal body ends in a terminal cap (TC-1 or TC in the first diagram) which gives of a rootlet of 6 microtubules (MR-6) which extends deep into the cell. associated with each basal body is also a second type of terminal cap (TC-2) which is a multi-layered body (MLB) with the topmost layer giving rise to a second rootlet of 4 microtubules (MR-4). These four rootlets give the 6-4-6-4 configuration characteristic of the group. also characteristic is the 11 o'clock to 5 o'clock arrangement: this means that the two basal bodies are displaced anticlockwise with respects to one-another. If 12 o'clock was on the left of the diagram, then the lower BB in the diagram is situated at about 11 o'clock, whilst the upper BB is at about 5 o'clock. (Based on: Van den Hoek et al. 1995). Note the small (and non-striated) connective between the two basal bodies. Such connectives presumably resist hydrodynamic forces and so maintain the constant angle between the two flagella.
In contrast, the better known green alga Chlamydomonas has a 4-2-4-2 rootlet arrangement and a cross-like (cruciate) 1 o'clock - 7 o'clock arrangement of basal bodies (a clockwise displacement). The angle between the two flagella in Chlamydomonas is also narrower at about 90 degrees (50 degrees in Chlamydomonas reinhardi) and the connectives between the pair of basal bodies are striated and well-developed. These striated connectives are contractile and contain the contractile protein centrin. There is clearly considerable scope to advance our understanding of cell biomechanics by studying algal cytoskeletons.
Graham, L.E. and McBride, G.E. 1975. The ultrastructure of multilayered structures associated with flagellar bases in motile cells of Trentepohlia aurea. J. Phycol. 11: 86-96.
Van den Hoeak, C.; Mann, D.G. and Jahns, H.M. 1995. Algae: an introduction to phycology. Cambridge University Press.