Project: The role of phytoplankton ballast material in deterring copepod grazing

Many groups of marine protists (algae and protozoa) are "armored" with thickened cell walls, coatings of scales, hard "cases" (tests, loricas), or latticework "skeletons". The inferred evolutionary function of these mineral deposits is to deter grazing. However, to date there are no direct measurements of grazing rates on protists as a function of their mineral content. With the recent development of silica specific stains and state of the art flow cytometry techniques we are poised to make these measurements. In this study, we directly test the relationship between the per cell minerals quota and the ingestion rates of copepods. Using well controlled algal rearing techniques, we will create phytoplankton cells that differ in the relative degree of mineral armor. Mineral load will be determined chemically (chemical digestion), visually (SEM) and photometrically (mineral and cell surface specific dyes and flow cytometry). The grazing protection conferred by biogenic minerals will be examined against copepod predation with detailed examination (microcinematography) of the behavioral mechanisms that underlie the selective process.

Selective grazing is a manifestation of the evolutionary "watery arms race" among the plankton. Our preliminary grazing experiments show that copepods have a strong preference for cells with low biogenic mineral content. This suggests that heavily fortified cells are less likely to be packaged into fecal pellets thus uncoupling the mineral content of plankton from what is exported to the deep ocean. This implies that global biogeochemical cycles are structured, in part, by the ecological and evolutionary constraints of predator prey interactions. In this study we measure the mineral content of the ingested particles and the mineral content in the fecal pellets of copepods. We hypothesize that a higher mineral content in the fecal pellet will increase the density of the pellet and therefore, lead to a higher settling velocity. The role of the biological pump in sequestering atmospheric CO2 is driven, in part, by the rapid sinking rates of fecal pellets. Experiments outlined in this proposal will link the mineral content of the copepod's diet with the mineral content of the fecal pellet. Subsequently, through direct video observations we will measure the sinking rates of fecal pellets as a function of their mineral load.