Regulation of Carbon Flow by Nitrogen and Light in the Red Alga, Gelidium coulteri

Abstract

The red alga Gelidium coulteri Harv. photosynthetically fixed 11'4C bicarbonate at high rates under defined conditions in unialgal laboratory culture. The fixation rate and flow of photosynthate into various end products were dependent on the nitrogen status of the tissue. Plants fed luxury levels of nitrogen (approximately 340 micromolar) showed fixation rates several-fold higher than those seen for plants starved for nitrogen. algal species for both agar and the related polysaccharide, carrageenan (2,17). This effect appears to involve altered allocation of photosynthate as well as changes in overall growth rate.To assess the nature of N regulation of agar biosynthesis, it became of interest to determine the pathways of carbon flow from primary fixation products into agar and other end products. If these pathways were known in detail, the effect of N on the physiological levels ofparticular metabolites could be determined and the branch points for photosynthate flow and enzymic sites of regulation could be pinpointed (7). Previous work in this area with a variety of red algae indicates that carbon is fixed via the reductive pentose phosphate pathway in light (10, 12). Fixation in the dark appears to be primarily via PEP2 carboxykinase (1 1). For Gelidium robustum, a significant fraction of the photosynthate appears in the compound floridoside (2-0-glycerol-a-Dgalactopyranose) (3, 9). However, previous work utilized plants harvested from field populations with limited control for their nutrient levels, environmental condition or reproductive status. Plant to plant variation in field-collected material can significantly affect quantitation and subsequent conclusions. A detailed assessment of the metabolic pathways of carbon flow under defined conditions in the red algae has not been reported.Since it has been possible to grow cloned strains of G. coulteri under defined conditions, experiments were undertaken to characterize carbon fixation and subsequent partitioning of photosynthate into various end products under a variety of environmental conditions. The results reported here detail some of the effects of N and light on carbon fixation and flow.

Seasonal nitrogen metabolism in an intertidal population ofGelidium latifolium(Gelidiaceae, Rhodophyta)

“…Carbon and nitrogen metabolism are related by a demonstrable inverse relationship between agar synthesis and carbon metabolism on one hand, and protein synthesis and nitrogen metabolism on the other (Hanisak, 1983), that has been shown indirectly for some species of Gelidium in culture (Macler, 1986(Macler, , 1988Fredriksen & Rueness, 1989, Torres et al, 1991. In addition, populations of red algae, including members of the Gelidiales, around the world show a characteristic summer 'bleaching', described as a loss of pigmentation related to environmental conditions (Neish et al, 1977;Santelices, 1988).…”

“…In red algae, research has concentrated on the economically important species of Gracilaria and Chondrus (DeBoer, 1978;Ryther et al, 1978;Rosenberg & Ramus, 1982;Bird et al, 1982;Bird, 1984;Friedlander & Dawes, 1985;Fujita, 1985;Dawes & Koch, 1990;Friedlander et al, 1990;Hanisak, 1990) and tank cultures of Palmaria palmata (Morgan & Simpson, 1981a, b) and Chondrus crispus (Neish et al, 1977). Information about metabolism in species of the economically important order Gelidiales is limited and confined to descriptions of seasonal changes in agar content (Establier, 1964;Garda, 1988;Mouradi-Givemaud et al, 1992), assimilation of ammonium and nitrate (Bird, 1976) and short-term pulse-chase experiments (Macler, 1986).…”

Photosynthesis and Photosynthate Partitioning in N2-Fixing Soybeans

“…For example, the hexose/ protein and hexose/Chl ratios in Nod+/+ were diminished by as much as 6 times less than the Nod+/-plants (Table II). This same trend has been demonstrated previously in a study of photosynthate partitioning in algae (20). Clearly, when inorganic nitrogen assimilation and N2 fixation concurrently exert a major control upon foliar photosynthate partitioning, as it does in soybean, then both the carbohydrate assimilates and the nitrogen assimilates must be examined before a complete understanding of control points can be elucidated.…”

“…Nitrogen deficiency can bring about lower net photosynthetic rates in nonnodulated legumes and in other plants (5,28). Nitrogen limitations will bring about changes in carbohydrate to protein ratios in algae, with carbohydrate synthesis favored by low nitrogen availability while protein synthesis is favored by high nitrogen availability (20). Protein usually is not considered to be stored photosynthate.…”

Primitive red alga <i>Cyanidioschyzon merolae</i> accumulates storage glucan and triacylglycerol under nitrogen depletion

“…There have been reports showing that red algae accumulate sugar under nitrogen-depleted conditions (Macler, 1986;Razaghi et al, 2014); however, they were not focused on the relationship to TAG accumulation. We showed that the primitive red alga C. merolae accumulates not only TAG but also starch under nitrogen-depleted conditions.…”