Aspects of structure, growth and morphogenesis in a new filamentous red alga (Ceramiaceae, Rhodophyta)

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.