To survive, an algal species must maximize its reproduction rate via efficient nutrient acquisition and assimilation while minimizing rates of mortality from grazing or other sources. In a survey of 11 well studied marine algal species (including 3 racial variants of a single species, Emiliania huxleyi) in ammonium-limited cyclostat cultures, we observed variations in ammonium uptake rates and associated growth rates, which were linked to cell size and, apparently, also to grazing defense. The 5 largest species (10-12 µm cell diameter) had similar low ammonium uptake rates and associated ammonium-limited growth rates owing to diffusion limitation of uptake. Diffusive flux per unit of cell volume decreases with the inverse square of the cell diameter; hence, cells can minimize diffusion limitation and increase nutrient uptake by becoming smaller. However, this comes at the cost of higher susceptibility to grazing mortality. The 7 smallest isolates (2.7-4.6 µm diameter) had higher rates of ammonium uptake and growth as predicted from theory, but there was considerable variation for similarly sized cells. All 4 of the small algal species or isolates that are known to be poorly grazed or poorly assimilated by grazers had unusually low ammonium uptake rates and associated growth rates for their size, which we hypothesize to be due to the high cost of grazing defense. The results suggest the existence of evolutionary tradeoffs between large cell size and grazing defenses (which decrease grazing mortality) and small size and minimal defenses (which promote growth and reproduction). Such tradeoffs could increase the species diversity of algal communities, which in turn promotes ecosystem stability.KEY WORDS: Ammonium · Phytoplankton · Nitrogen uptake · Growth rate · Grazing defense · Cell size · Evolutionary tradeoffs Resale or republication not permitted without written consent of the publisher

Zooplankton grazers are capable of influencing food-web dynamics by exerting topdown control over phytoplankton prey populations. Certain toxic or unpalatable algal species have evolved mechanisms to disrupt grazer control, thereby facilitating the formation of massive, monospecific blooms. The harmful algal bloom (HAB)-forming dinoflagellate Karenia brevis has been associated with lethal and sublethal effects on zooplankton that may offer both direct and indirect support of bloom formation and maintenance. Reductions in copepod grazing on K. brevis have been attributed to acute physiological incapacitation and nutritional inadequacy. To evaluate the potential toxicity or nutritional inadequacy of K. brevis, food removal and egg production experiments were conducted using the copepod Acartia tonsa and K. brevis strains CCMP 2228, Wilson, and SP-1, characterized using liquid chromatography-mass spectrometry (LC-MS) as having high, low, and no brevetoxin levels, respectively. Variable grazing rates were found in experiments involving mixtures of toxic CCMP 2228 and Wilson strains. However, in experiments with toxic CCMP 2228 and non-toxic SP-1 strains, A. tonsa grazed SP-1 at significantly higher rates than the toxic alternative. Additionally, A. tonsa experienced significantly greater mortality when exposed to toxic K. brevis strains, particularly after prolonged exposure. Egg production rates of copepods fed toxic K. brevis strains were similar to those of starved copepods, while those of copepods fed non-toxic SP-1 and the nutritious Rhodomonas salina were significantly higher. Analysis indicates that K. brevis impacts grazer populations via multiple synergistic mechanisms: (1) decreased ingestion rates, (2) decreased egg production, and (3) increased mortality of copepods through a combination of toxicity and nutritional inadequacy.

Mathematical models are a useful tool for predicting the responses of marine phytoplankton to changes in nutrient inputs and other environmental factors. Two modeling approachesMonod and Droop -have been traditionally used. These 2 model types were fitted to empirical data for specific growth rate, cellular N:C ratio, cellular ammonium uptake rate, and ammonium concentration measured in N-limited cyclostats at different dilution rates and in nutrient-saturated batch cultures. The modeled data were for a small, fast-growing coastal diatom Thalassiosira pseudonana (~4.5 μm diameter) and for a larger, slower growing diatom T. weissflogii (~11 μm diameter) cultured in seawater medium at 20°C and 14 h d -1 of light. The observed data did not conform well to the classic Monod equation, but could be fit to a modification of this equation in which the maximum growth rate was assigned a value higher than the observed maximum rate. Likewise, data for cellular N uptake rate versus ammonium concentration did not conform well to the standard saturation equation, but could be fit to a modification of the equation in which the maximum uptake rate was set above the empirically measured value and the x-intercept was shifted from the origin to a finite positive value. Both modified models accurately fit the observed steady-state relationships between ammonium concentrations and specific growth rates of the 2 species. However, the 2 models showed different transient dynamics in response to a change in the concentration of inflowing nutrients in time-course simulations. Such differences suggest that the choice between Droop and Monod approaches, when used as part of larger food web models, could lead to widely divergent predictions of algal blooms and other nonequilibrium dynamics of ecosystems.

Adhesiveness of washed platelets resuspended in citrated plasma, serum, or several different media has been investigated. A method specific for quantitation of adhesion was used. Platelets suspended in saline or Tyrode’s solution were found to be highly adhesive to glass, polyethylene, polyvinyl chloride, or Cuprophane. This adhesiveness of platelets to test surfaces decreased by nearly 50% when plasma was the suspension medium. When the suspension medium was serum, the decrease in adhesion was nearly 75 %. Cohn fraction V also decreased the adhesiveness of platelets significantly, but highly purified albumin had only a small effect. Several pharmacologic agents decreased platelet adhesiveness when added to platelets suspended in plasma or serum, but had negligible effect on the adhesiveness of platelets suspended in artificial media devoid of proteins. Normal washed platelets, when suspended in citrated plasma obtained from an afibrino-genemic donor or in normal serum, showed a significant decrease in adhesion compared to the same platelets suspended in normal citrated plasma. Addition of fibrinogen to afibrinogenemic plasma or normal serum restored the adhesiveness of platelets to normal levels. Normal platelets resuspended in plasma obtained from a thrombasthenic donor exhibited normal adhesiveness. These observations suggested that while fibrinogen promotes platelet adhesion, plasma or serum possess also an adhesion-inhibiting activity.

No abstract