Iron and phosphorus effects on the growth of Cryptomonas sp. (Cryptophyceae) and their availability in sediments from the Pearl River Estuary, China

Based on lab-culture experiments analyzing limitation and combination of iron and phosphorus on the growth of Cryptomonas sp. (Cryptophyceae), and the study of accumulation and release of Fe-bound P in sediment cores collected from the marine region of the Pearl River Estuary, China, reasons for the high frequency of phytoplankton bloom therein are discussed. Results show that the combined effect of Fe and P can obviously accelerate algal development, and the optimum culture conditions maintaining maximum growth rate are 0.05 mM Fe and 50 mM P. Cellular contents of Fe and P is consistent and the P:Fe molar ratio is w159:1. The optimum range of the P:Fe molar ratio in culture experiments for cell incubation is 500e1400. The vertical trends of total Fe and total P variations in sediments are parallel. Fe-bound P is the main species of inorganic sedimentary P. Through continuous leaching with agitation, 34e80% of exchangeable P and 4e23% of exchangeable Fe are concurrently released from the surficial sediments. This is a possible way by which nutrients are made available to phytoplankton. These factors might be responsible for a high frequency of harmful algal blooms in the Pearl River Estuary.