1United Graduate School of Agricultural Science, Ehime University, Matsuyama, Ehime 790-0905
In eutrophic coastal environments in Japan, red tides have caused mass mortality of cultured and wild fish. Some of the red tide phytoplanktons can produce alkaline phosphatase (AP) under low ambient phosphate concentration, which suggests that such phytoplankton can utilize phosphatase-hydrolyzable phosphorus in the water as a phosphorus source. In the present study, the distribution and variation of AP activity and AP-hydrolyzable phosphorus (APHP) in coastal seawater was examined to elucidate the contribution of organic phosphorus to the growth of red tide phytoplankton.
Total AP activity in surface seawater of Uranouchi Inlet was 1.2-70.1 nmol/L/min, and increased in temperate seasons (spring-autumn). The contribution of planktonic AP to total AP activity was larger (average 65%) than free and bacterial AP in temperate seasons.
APHP concentration in coastal seawaters (Uranouchi, Nomi and Urado Inlet, and Hiroshima Bay) were <0.02-0.29 μM (average 0.04-0.10 μM except APHP non-detectable samples), and there were no significant differences among these concentrations. The ratio of APHP to AP producing algae-available phosphorus (AAP: the sum of dissolved inorganic phosphate and APHP) was high (average 0.45) in surface seawater of Uranouchi Inlet.
These results suggest that AP-producing phytoplankton can grow by utilizing APHP as a phosphorus source in coastal eutrophic seawaters.