IDENTIFICATION OF A UNICELLULAR, NONPIGMENTED ALGA THAT MEDIATES GROWTH-INHIBITION IN ANURAN TADPOLES - A NEW SPECIES OF THE GENUS PROTOTHECA (CHLOROPHYCEAE, CHLOROCOCCALES)

A non-pigmented, unicellular alga isolated from the faeces of British anuran tadpoles and which is associated with growth inhibition in thes e tadpoles, was described and identified using cytological, ultrastruc tural, nutrient assimilation and immunological studies. The alga posse ssed all the distinctive morphological features of the genus Protothec a, it grew weakly on Prototheca Isolation Medium (PIM), it required th iamine for continued growth and replication, and it could assimilate t he five major substrates used to speciate the protothecans. All of the se characteristics, together with previous nucleic acid hybridisation studies, indicated that the microorganism belonged to the genus Protot heca. There are currently five species recognised as valid (Pore, 1985 & 1986): Prototheca zopfii Kruger, 1884, P. wickerhamii Tubaki & Sone da, 1959, P. moriformis Kruger, 1884, P. stagnora Cooke, 1968 and P. u lmea Pore, 1986. The immunology showed that the new species was relate d to two of the protothecans, but overall it showed that the alga was antigenically distinct from the other protothecans tested in the immun oassay. This, together with its inability to grow strongly on PIM, its ability to assimilate a wide rage of carbon substrates and its abilit y to mediate growth inhibition in anuran tadpoles, indicated a new spe cies of Prototheca. We therefore propose the name Prototheca richardsi sp. n.