Irradiance-induced variability in light scatter from marine phytoplankton in culture

A series of laboratory experiments are reported that illustratethe response of beam attenuation ( = 660 nm) and single-celllight scatter ( = 488 nm) properties of several species of marinephytoplankton to light intensity. When unialgal cultures weresubjected to an increase in light intensity, the particle-scatteringcomponent of beam attenuation and near-forward single-cell lightscatter were found to increase rapidly in response. Cell abundanceincreased only slightly over the course of the experiments,leading to the conclusion that the response in beam attenuationwas due to irradiance-induced changes in the single-cell opticalproperties. The percent hourly increase in beam attenuation,normalized to cell abundance, and single-cell light scatterrangedfrom 5% for a culture of the coccolithophore Emiliania huxleyito 25% for a culture of Thalassiosira pseudonana. In a separateset of experiments, carbon-specific beam attenuation (c^*_c; thepaniculate material component of beam attenuation normalizedto the concentration of paniculate organic carbon) was foundto be species specific and, to some extent, sensitive to irradiance.The positive response in phytoplankton light scatter, both atthe population and at the single-cell level, to an increasein light intensity is similar to diet patterns in beam attenuationreported for the near-surface ocean. If a component of the observeddiel pattern in beam attenuation is due to irradiance-induced,carbon-independent optical variability in the phytoplanktonassemblage, as the results of the high-light experiments suggest,neglecting such variability can result in either an overestimationor an underestimation of primary production, depending on theresponse in cc^*_c. Natural variability in cc^*_c is poorly understoodand responses to environmental factors, such as irTadiance,have yet to be addressed outside of the laboratory.