Wednesday, May 6, 2009
Biological activity in the world open ocean’s surface is characterized by autotrophic and by heterotrophic processes. Phytoplankton organisms take up dissolved CO2 (dissolved inorganic carbon, DIC) and together with other inorganic nutrients and light they produce biomass (particulate organic carbon, POC) and dissolved organic carbon (DOC). By these processes marine phytoplankton is responsible for approximately half of the worlds primary production. These two carbon compounds (POC and DOC) either sink down to the deep ocean (which is basically the biological carbon pump) or they are consumed by other trophic levels. One important part of the planktonic food web is the microbial community which consists of bacteria (smaller than 3 µm), auto- and heterotrophic flagellates and other protists (larger than three µm). This community takes up both POC and DOC and by respiration recycles these carbon compounds back into DIC. Thus in terms of carbon flux the microbial community potentially competes with the biological carbon pump.
In a mesocosm experiment with natural marine plankton Julia Wohlers and her colleagues manipulated future ocean surface warming and measured the carbon flux during the plankton bloom peak. Whereas in this experiment phytoplankton biomass production (POC of autotrophs) was not affected by warming the authors found that respiration by the microbial community, in particular by organism larger than 3 µm, significantly increased. This increase in respiration led to a significant decrease in net DIC reduction in the whole planktonic foodweb. The results are a potential sign for future declining carbon sequestration by biological processes in the world oceans.
Julia Wohlers, Anja Engel, Eckart Zöllner, Petra Breithaupt, Klaus Jürgens, Hans-Georg Hoppe, Ulrich Sommer and Ulf Riebesell (2009). Changes in biogenic carbon flow in response to sea surface warming. Proceedings of the National Academy of Sciences. DOI:10.1073/pnas.0812743106
Wednesday, February 18, 2009
After attending the presentation by Elena Litchman at the ASLO Aquatic Science Meeting in Nice three weeks ago I came across this paper. Although it was published already two years ago, this works need to be highlighted! Marine phytoplankton is important. It contributes approximately 50% to world primary productivity. Among other factors phytoplankton communities are structured by competition for limiting nutrients (mainly for nitrate and ammonia) in the ocean. Litchman et al. base their paper on the presumption that phytoplankton organisms can achieve higher competitive ability (Tilman’s R*) by different strategies. That is, the organisms can either increase their maximum nutrient uptake and/or growth rate or they decrease the minimum cell quota, the half saturation constant for nutrient uptake and/or their mortality. Litchman et al. tested if they can find constraints and trade-offs on the evolution of better competitive abilities (lower R*) in major phytoplankton groups. Specifically they asked if there is a positive relationship between maximum growth rate and R* which would show a gleaner-opportunist trade-off.
The authors show positive relationships between measurements for growth and nitrate uptake which can constrain the evolution on competitive ability. Indeed major groups of phytoplankton group along these trade-off curves. Whereas coccolithophores e.g. show low nitrate uptake rates and low half-saturation constants, diatoms and dinoflagelates show the opposite nitrate uptake strategy with high uptake rates and high half-saturation constants. A gleaner-opportunist trade-off, i.e. a positive correlation between maximum growth rates and R*which would result in a super species, could not be found across major groups but within the diatoms. The paper gives more results about trait differences among taxonomic groups and allometric scaling relationships. Trade-offs and different strategies in nutrient uptake are discussed in a very concise way either from a mechanistic physiological view as well as from the evolutionary history perspective.
Elena Litchman, Christopher A. Klausmeier, Oscar M. Schofield and Paul G. Falkowski (2009) The role of functional traits and trade-offs in structuring phytoplankton communities: scaling from cellular to ecosystem level. Ecology Letters. DOI: 10.1111/j.1461-0248.2007.01117.x