Effect of growth conditions on flow-induced inhibition of population growth of a red-tide dinoflagellate

The population growth of some dinoflagellates is known to be reduced by exp osure. to fluid flow. The red-tide dinoflagellate Lingulodinium polyedrum w as used to examine the effect of growth conditions on flow-induced inhibiti on of population growth. Three factors were tested: time of exposure relati ve to the light:dark (LD) cycle, illumination level, and culture growth pha se (early vs. late exponential phase). Cultures maintained on a 12:12 h LD cycle were exposed to one of two flow conditions: quantified laminar shear produced by Couette flow or unquantified flow generated in shaken flasks. T he duration of exposure to flow was 1 h d(-1) for 5-8 d in all experiments; the shear stress in Couette shear experiments was 0.004 N m(-2). There wer e many qualitative similarities in the pattern of response to how in the tw o hydrodynamic conditions. In both cases, exposure to flow in the last hour of the dark phase resulted in greater reduction of net growth than exposur e during the light phase. Cultures grown under lower illumination had propo rtionally greater reductions in net growth than cultures under higher light . Finally, late exponential phase cultures exhibited much greater reduction s in net growth following a given flow exposure than early exponential phas e cultures. The higher sensitivity of late exponential phase cultures did n ot appear to be linked to nutrient limitation or changes in pH of the mediu m; it may be partially attributed to exudates from late exponential phase c ells. These results suggest that the response of red-tide dinoflagellate po pulation growth to in situ turbulence may depend on both environmental cond itions and the physiologic:al state of the cells.