Phytoplankton represent the base of the marine food web and are responsible for roughly half of the photosynthesis on earth. A major factor regulating the growth and composition of phytoplankton communities is the ability of different species grow and reproduce at limiting concentrations of nutrients. Unfortunately, due to the lack of sensitivity of standard analytical methods, relationships between concentrations of major inorganic nutrients (N and P) and the growth rate of marine phytoplankton were not known, preventing a comparison of competitive capabilities of different species for limiting nutrients. We used a new fluorometric method for ammonium and chemostat culture experiments to establish relationships among ammonium concentration, ammonium uptake rates, cellular N:C ratios, and specific growth rate in 13 marine algal isolates representing different phytoplankton groups, cell sizes, adaptations to limiting nutrients, and grazing susceptibilities. Interesting patterns emerged in which larger species showed severe diffusion limitation of uptake, which greatly restricted their growth and reproduction rates at low (20-80 nM) ammonium concentrations. However, these larger-celled species are also less susceptible to grazing by zooplankton, and thus have lower grazing-linked mortality rates. Among the smaller species, ammonium-limited growth rates and ammonium uptake rates differed, with well-defended, poorly grazed species showing lower rates of ammonium uptake and growth. We hypothesize that the large carbon and energy expense these algae incur for their robust grazing defenses (and consequent lower grazing mortality rates) restricts the carbon available to support cellular growth and reproduction. Hence, there appear to be fundamental tradeoffs among cellular traits that promote high rates of growth and reproduction (small size and high investment in the cellular machinery and energy needed for nutrient uptake and growth) and those that promote low grazing mortality rates (large cell size and high cellular investment in grazing defenses). Many of the same trade off patterns are seen in terrestrial plants, and thus ecological principles that regulate competition among species in the two major groups of phototrophs (land plants and marine phytoplankton) may be similar.
William (Bill) Sunda is an adjunct professor in the Marine Sciences Department at the University of North Carolina, Chapel Hill, NC, USA. He recently retired (2014) as a research scientist from the Beaufort Laboratory, NOAA, Beaufort, NC where he worked for 39 years. He received his PhD in Chemical Oceanography in 1975 from the newly formed Joint Program in Oceanography, sponsored by the Woods Hole Oceanographic Institution and Massachusetts Institute of Technology. He has published 103 scientific papers on wide range of topics in marine trace chemistry, physiological ecology of phytoplankton, and biogeochemical cycling of trace metals and the climatically active gas dimethylsulfide (DMS). Recently he has focused much his attention on phytoplankton ecology, especially the factors and mechanisms that promote the formation and toxicity of harmful algal blooms. He has won several awards, including a Citation Classics Award for his PhD research on trace metal biological availability, an Administrator Award from NOAA for his research on DMS, a Career Achievement Award from NOAA, and the Clair Patterson Award in environmental geochemistry from the Geochemical Society for his research on the biogeochemistry of trace metals in the ocean. He is also a Fellow in the American Geophysical Union, the American Association for the Advancement of Science, and the Geochemical Society.