Research Project: The Enigma that is Phaeocystis
The phytoplankton genus Phaeocystis is one of only a few acknowledged key taxa in marine pelagic ecosystems. Over a large and geochemically salient portion of the global ocean, Phaeocystis spp. are common, frequent, or dominant organisms, capable of sequestering tremendous amounts of carbon and nutrients. Phaeocystis does so in the often acknowledged form of gelatinous colonies but also as individual cells, and undergoes transformations between these states. During such transitions, food web structure and material fluxes are thought to be altered dramatically. Despite a century of observation and decades of study, fundamental questions remain unanswered, including "Why do colonies sometimes form massive blooms?"; "What is the role of life cycle phenomena in the success of Phaeocystis ?"; "Does Phaeocystis provide an unusually inefficient trophic link?"; and "What determines whether Phaeocystis or diatoms dominate?".
The issue of grazing on Phaeocystis remains contradictory and, accordingly, simulation of Phaeocystis food webs is seriously hindered by misunderstanding of how to parameterize essential aspects of carbon or nitrogen flows. In a continuing attempt to better address the questions listed above, experiments have been conducted to investigate the susceptibilities of solitary and colonial Phaeocystis to grazing by proto- and metazoan zooplankton. Grazing was compared to that on other potential food items (diatoms, bacteria, detritus), with the goal of determining functional relationships. Additional data included rates of detritus formation, and transitions between solitary and colonial cells of Phaeocystis . Equations fit to the data are being used to constrain a detailed time-dependent food web model incorporating these and other relevant stocks and flows. While not designed to simulate a specific ecosystem, model results generally mimic the magnitude and timing of plankton community development in cold waters where Phaeocystis occurs, e.g. Norwegian fjords and coastal waters. The growth of solitary cells, their grazing by microzooplankton, colony formation from solitary cells, and microzooplankton predation by macrozooplankton are important regulators of Phaeocystis colony blooms. Sensitivity analysis suggests that Phaeocystis may be successful in certain environments or situations by altering its trophic relationships before competitive exclusion and/or grazer control can be achieved. Such changes reverberate through the pelagic food web influencing carbon and nitrogen cycling and vertical export.
Phaeocystis produces prodigious blooms of gelatinous colonies, releases copious amounts of DMS, and significantly alters material flows among trophic levels and export from the upper ocean. A potentially salient property is the ability of Phaeocystis to transform between solitary cell and gelatinous colonial life cycle stages, a process which changes organism biovolume by 6-9 orders of magnitude, and which is hypothesized to be mediated by chemical communication. The colony skin confers protection against grazers, viruses, and parasitoids. Phaeocystis utilizes chemistry and/or changes in size as defenses against predation, and its ability to create refuges from biological attack is known to stabilize predator-prey dynamics in model systems. The life cycle form in which it occurs determines whether primary production flows through the traditional "great fisheries" food chain or the more regenerative microbial food web.
