PHYSIOLOGICAL-RESPONSES TO PHOSPHORUS LIMITATION IN BATCH AND STEADY-STATE CULTURES OF DUNALIELLA-TERTIOLECTA (CHLOROPHYTA) - A UNIQUE STRESS PROTEIN AS AN INDICATOR OF PHOSPHATE DEFICIENCY

A protein unique to phosphorus stress observed in Dunaliella tertiolec ta Butcher was studied in the context of phosphate-limited cell physio logy and is a potential diagnostic indicator of phosphate deficiency i n this alga. Cells were grown over a range of limited, steady-state gr owth rates and at maximum (replete) and zero (phosphate-starved) growt h rates. The stress protein, absent in nutrient-replete cells, was pro duced under all steady-state phosphate-limited conditions and increase d in abundance with increasing limitation (decreasing growth rate), Ce llular carbon : phosphorus ratios and the maximum uptake rate of phosp hate (V-m) increased with increasing limitation, whereas the ratio of chlorophyll a: carbon decreased. Alkaline phosphatase activity did not respond to limitation but was measurable in starved, stationary-phase cells. F-v/F-m, a measure of photochemical efficiency, was a nonlinea r, saturating function of mu, as commonly observed under N limitation. The maximum F-v/F-m of 0.64 was measured in nutrient-replete cells gr owing at mu(max), and a value of zero was measured in stationary-phase starved cells. When physiological parameters were compared, the P-str ess protein abundance and F-v/F-m were the most sensitive indicators o f the level of deficiency. The stress protein was not produced under N - or Fe-limited conditions. It is of high molecular weight (>200) and is associated with internal cell membranes. The stress protein has sev eral characteristics that make it a potential diagnostic indicator: it is 1) unique to phosphorus limitation (i.e. absent under all other co ndititions), 2) present under limiting as well as starved conditions, 3) sensitive to the level of limitation, and 4) observable without tim e-course incubation of live samples.