Rj. Lewis et Md. Hanisak, EFFECTS OF PHOSPHATE AND NITRATE SUPPLY ON PRODUCTIVITY, AGAR CONTENTAND PHYSICAL-PROPERTIES OF AGAR OF GRACILARIA STRAIN G-16S, Journal of applied phycology, 8(1), 1996, pp. 41-49
Gracilaria strain G-16S was cultured in various phosphorus (P) supply
rates with low or high nitrogen (N) supply to determine the effects of
nutrient supply on its productivity, agar content and physical proper
ties of the agar. Productivity was reduced after four weeks of growth
in zero P supply as plants reached 0.07% P tissue content (critical le
vel), with fragmentation of these plants by six weeks (0.05% P; minimu
m viable level). Native agar content was higher in low P and high N, o
r low N conditions. Agar content appeared to increase with decreasing
P under high N supply. This increase was not apparent with alkali trea
tment prior to extraction. Agar gel strength was greatly increased by
alkali treatment. The highest gel strengths were obtained under high N
supply at all P supply rates except zero P, and under low N supply at
12 mu M P week(-1). Native agar gel strengths showed a similar patter
n on a lower scale. Melting temperatures were higher in agars with hig
her gel strengths. Dynamic gelling temperatures were generally high fo
r alkali-treated agar, with agar from plants grown in zero P supply sh
owing a slightly elevated gelling temperature. Melting and gelling tem
peratures of native agars with the highest gel strengths were in the s
ame range as bacteriological agar. These results show that P and N sup
ply affects productivity, agar content and agar physical properties, b
ut the tradeoffs between a slightly higher agar quantity under nutrien
t limitation and higher agar quality under nutrient-replete conditions
seem to favor the latter.