Effect of Orientation on Light Absorption by Colonial Diatoms

Many marine phytoplankton, especially diatoms, form elongate cells and colonies with high aspect ratios. Recent observations suggest that shear and turbulence in the ocean can affect the orientation of these particles relative to incident light. In certain orientations pigment packaging effects (i.e. self-shading) can be greatly reduced, thereby maximizing absorption per unit chlorophyll, growth, and fitness of elongate forms in light limited environments (e.g. deep chlorophyll maxima or thin layers). In this study we examine light absorption by naturally oriented, elongate diatom colonies to assess the potential effects on light capture and growth. Orientation was determined using an in situ holographic microscope capable of imaging cells and colonies without disturbing the local flow field. The total light absorbed by cells and colonies was estimated based on their observed size and orientation. Results suggest populations with predominantly horizontal orientation can have greatly enhanced light capture per unit chlorophyll compared to vertical or randomly oriented populations. If orientation is a common phenomenon throughout the world’s oceans, it may have a significant impact on rates of primary production and growth of large phytoplankton which transport significant amounts of carbon to the interior of the deep ocean. Orientation may also help explain why phytoplankton have been able to evolve many diverse shapes and sizes.