Aspects of structure, growth and morphogenesis in a new filamentous red alga (Ceramiaceae, Rhodophyta)
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Date
1993
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Abstract
Pteroceramium, a descriptive name given to an undescribed winged species closely
related to Ceramium, has uniaxial filamentous thallus construction with
pseudodichotomous branching. Alternate branches become dominant. This pattern of
growth is referred to as cellulosympodial growth. All growth is from an apical cell
which cuts off subapical cells. The subapical cells develop into axial cells. Each axial
cell cuts off six pericentral cells in a ring around its apical pole. The pericentral cells
divide further to form the cortical band. Pc1 always forms on the outer face of the
thallus as determined by the preceding pseudodichotomy and gives rise to the larger
outer wing which is a lateral expansion of the cortical band. The smaller inner wing
forms from Pc6 on the inner face. The other pericentral cells give rise apically to
uniseriate spines. The pericentral cells also give rise to rhizoids and adventitious lateral
branches.
Each axial cell has a large central vacuole with a few peripheral chloroplasts,
mitochondria and floridean starch granules. The smaller wing cells have a much denser
cytoplasm with fewer small vacuoles, many chloroplasts which are more closely packed
together and more floridean starch granules than axial cells. Chloroplasts have a typical
Rhodophyta ultrastructure with single, evenly spaced thylakoids with phycobilisomes.
Pit connections have a plug core but no plug cap.
Pteroceramium has a typical Polysiphonia-type triphasic life history. The
carposporophyte is naked and tetraspores are produced in a characteristic decussate
cruciate arrangement.
The effect of a number of physical and chemical factors on growth and morphogenesis
was investigated. Pteroceramium grew best at irradiance levels between 79 μmol m⁻² S¯¹
and 129 μmol m⁻²
S¯¹ with growth being limited at 30 μmol m⁻² S-I. The largest axial
cells and wings were obtained from the material grown at 79 μmol m⁻² S¯¹ and the
smallest measurements for material grown at 129 μmol m⁻² S¯¹. Monochromatic light
fields of red, green and blue caused reduced growth rates compared to the control
replicates grown in a white light from both incandescent and fluorescent lights. Light quality had no effect on morphogenesis. The critical daylength for maximum rates of
cell elongation was 10 hours or longer, although 16 hours light caused a decrease in
final axial cell volume. Optimum temperatures for growth of Pteroceramium were
between 20°C and 25°C with growth decreasing at 15°C and 30°C. Axial cell volume
was reduced and wing size was stunted at these two extreme temperatures tested.
Scouring by sand caused axial cells to decrease in volume although the wings were
unaffected. Smothering by sand did not prevent growth although axial cells and wings
were greatly decreased in size, with the wings consisting of only one or two other cells.
Tumbling to disrupt gravity did not affect the angle of each pseudodichotomy.
Decreased levels of nitrogen and phosphorus limited growth but had little effect on
axial cell volume and wing development. Pteroceramium was stenohaline with
maximum growth at 34°/[00] and reduced growth at 300/[00] and 40°/[00]. Pteroceramium grew
best at pH 7.5 and pH 8.5 with decreased growth at pH 6.5 and pH 5.5. The various
pHs tested had little effect on morphogenesis.
The best photosynthetic responses were obtained from material preconditioned at
80 μmol m⁻²
S¯¹ compared with that at 30 μmol m⁻² S¯¹ and 150 μmol m⁻² S¯¹. There was
a decrease in pigment content with increasing irradiance at which the alga was grown.
Phycoerythrin was the dominant pigment. Exposure to a high irradiance
(3000 μmol m⁻²
S¯¹) for 30 minutes or longer inhibited photosynthesis. Plants did not
fully recover even 24 hours later, indicating that this damage was permanent.
Pteroceramium was able to acclimatize slowly over a week to temperature changes
within the range of 15°C to 25°C. Rapid increases of 5°C within this temperature
range increased photosynthetic performance and a rapid drop of 5°C decreased
photosynthetic performance. However, a 10°C increase or drop reduced
Pteroceramium's photosynthetic performance. Photosynthetic rates were decreased in
alkaline conditions and increased in acidic conditions.
Pteroceramium has well defined developmental patterns with basal band growth of axial
cells and tip growth in the rhizoids. The pericentral cells are formed in a set sequence
similar to Ceramium species with Pcl forming on the outer face, Pc2 and Pc3 forming
on the lower and upper surface nearest to Pel respectively, Pc4 and PcS forming on the lower and upper surface respectively farthest from Pel, and Pc6 forming on the
inner face. This sequence is unaffected by the direction of illumination or gravity.
Exogenous application of plant hormones (IAA, GA3 and kinetin) in the concentration
range of 10[-9] M to 10[-5] M had no effect on growth and morphogenesis in
Pteroceramium. Application of polyamines and their precursors caused a decrease in
growth and a reduction in cell size at concentrations higher than 10[-4] M. Polyamine
inhibitors caused a reduction in growth and cell size at concentrations higher than
10[-5] M. Arginine increased growth at concentrations 10[-5] M and 10[-6] M. High power
liquid chromatography (HPLC) separation of Pteroceramium extracts indicated that
spermidine was present in Pteroceramium at approximately 38 μg spermidine g¯¹ fresh
weight. The apical tip exerts an apical dominance effect on the subordinate branches,
suppressing their elongation. Removal of the dominant apical tip increased adventitious
branch formation. This effect was not reversed by application of exogenous IAA at
concentrations of 10[-9] M to 10[-4] M.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1993.
Keywords
Red algae., Marine algae., Theses--Botany.