Effect of naturally occurring apatites on growth and morphology of algae

Crystals (30--100 micrometer) of selected naturally occurring apatite (Ca10 (PO4)6(OH, F)2) samples were added to P-free (less than 0.001 microgram/ml total P) Bristol's medium (1-1000 microgram/ml of apatite) as the sole source of ortho-PO43-. The media were inoculated with washed, non-axenic cells of three chlorophycean algal species cultivated under PO43--deficient conditions. Phase-contrast and scanning electron microscopy revealed that at low slurry densities (1-10 microgram/ml of apatite), Ankistrodesmus braunii (ATCC 2744) cells were morphologically distorted. At concentrations of 100 and 1000 microgram/ml of apatite, more than 85% of the cells had undergone autospore formation within 7--10 days of incubation at 20 degrees C. Most autospores formed failed to germinate under high nutrient conditions. Scenedesmus longus (No. 1236) formed colonies when cultivated in Bristol's medium but daughter cells displayed a Chodatella-like unicellular morphology when grown in apatite media. Test algal species (Chlamydomonas dysosmos, S. longus, A. braunii) showed a marked preference for growth on apatite crystals over non-nutritive surfaces. Unialgal and mixed-algal cultures produced an extensive matrix of extracellular fibrous material in response to growth on crystals at concentrations greater than 10 microgram/ml of apatite.