Influence of bacteria on phytoplankton cell mortality with phosphorus or nitrogen as the algal-growth-limiting nutrient

The effects of bacteria on phytoplankton mortality were studied with phosphorus or nitrogen as the algal-growth-limiting nutrients. Experiments were performed with the diatom Ditylum brightwellii using batch cultures, steady state continuous cultures and batch-mode cultures which were starved for the limiting nutrient after being preconditioned in a chemostat. With phosphorus limiting algal growth, the specific death rates of D. brightwellii generally increased upon bacterial inoculation. Bacteria enhanced algal mortality very likely due to competition with D. brightwellii for the limiting phosphate. With nitrogen as the algal-growth-limiting nutrient, however, the presence of bacteria had either no pronounced effect on or led to a reduction of the specific death rates of D. brightwellii. Remineralized ammonium was probably partly utilized by N-starved cells of D. brightwellii, leading to reduced death rates of the algal cells. Bacteria thus indirectly prolonged survival of D. brightwellii populations under N starvation. This study shows that bacteria can affect phytoplankton survival, which in turn may influence algal species succession. The degree of bacterial influence on algal death kinetics depends largely on the type of nutrient limiting algal growth and on the culture conditions.