RNA responses to N- and P-limitation; reciprocal regulation of stoichiometry and growth rate in Brachionus

In this study we address how growth rate in consumers may be regulated by nitrogen (N)- and phosphorus (P)- limitation of ribonucleic acid (RNA), using the rotifer Brachionus calyciflorus as the model organism. Growth rate, RNA and weight-specific concentrations of carbon (C), N and P were analyzed in B. calyciflorus fed algae with different C : N, C : P and N : P ratios. Growth rate correlated negatively with food C : N, but not with C : P or N : P, strongly suggesting N-limited growth. Growth rate also strongly correlated with tissue concentrations of RNA and P, in support of the growth rate hypothesis which states that specific P-content of organisms reflects their RNA-content and thus capacity for protein synthesis. Food C : N rather than C : P regulated the growth, RNA and P in this consumer under the food conditions applied. This suggests that availability of N can also be an important regulator of cellular acquisition of P and build-up of ribosomes in that animals released from N-limitation have an increased demand for the P-rich cellular machinery for protein synthesis. The data suggest a strong reciprocal regulation of consumer demands and their C : N : P stoichiometry via the intimate connections between ribosome allocation and protein synthesis and also lend support to the view that reduced protein synthesis efficiency under high growth rates may affect stoichiometry of the consumer.