A Lab Notebook

 
Lab Meeting and more
I started my presentation to the lab with what can be considered the equivalent of the kiss of death in our lab. I showed the famous Behrenfeld & Falkowski [1997] image of global oceanic primary production. Primary production is the synthesis of plant material (phytoplankton in this case) from the photosynthetic reactions). What made the presentation of this map of global oceanic primary production so treacherous is that Falkowski can be a bit intimidating, but especially so when the work is his own. Anyways, I survived the meeting, a full hour of standing in front of my closest colleagues answering questions about the data I presented, and graphs I made from compiled datasets published by other scientists in the field. Below are more notes for myself than for other, so much of what is below should make little no sense to anyone else. The bottom of this post contains my running log for yesterday an today.

Exudate is both a sink for organic carbon (not observable by satellites for example), and is a cause of increased export rates (due to aggregation/coagulation) of carbon from the surface ocean
and does contribute to heterotrophic bacterial production. There is positively correlated relationship between photosynthetic rate and extracellular release. Indeed this is the most agreed upon fact of the field of study => fig.4 from Mague et al. [1980], Nalewajko [1976]. However, exudate is not merely a portion of the intact cells' contents based on different %mol (a normalized measure of mass) compositions between extracellular and intracellular material (Mague et al. [1980])
What are considered the main hypotheses for the explanation of exudation? It can best be explained by an imbalance between light intensity and nutrient availability. 1) light saturation + normal nutrient levels 2) sub saturating light + nutrient limitation. Both instances provide too much photosynthate compared to the availability of nutrients for biosynthesis of cellular components. Evidence points to an intracelllular pool of carbohydrates (Henriques Viera & myklstad [1986], Lancelot [1984]) as deduced by the observation of continued exudation for up to 24 hours after no light, and delayed resumption of exudation after the light was turned back on. This also is further evidence of exudation of carbohydrate as a result of active photosynthesis. However, these do not seem to cover all of the observed mechanisms for the induction of exudation (I have this chart from my presentation today) One of the more recently focused upon "triggers" of exudation is atmospheric carbon dioxide concentrations, for obvious global warming implications. The Smith and Wiebe [1976] data contradict that from Engel et al. [2001] where both studies examined the affects of concentration of DIC on exudate production. The quantity of material released by the cell can be quite variable. Original extracellular release rates ranged from 5% - 75% throughout the 1970's (Mague et al. [1980]). The most recent estimates are 20+-1% (Marañon et al [2002]). There are two phases of excretion (Nalewajko et al. [1976), and a Kiørboe 1990's paper] of the photosynthate. The first phase sticks to the cell, while the second phase is released to the medium. Lancelot [1984] found small metabolites (<500d, d = daltons, metabolites are substances produced by metabolism such as sugars are metabolites from carbon fixation) are immediately released and after a lag phase large metabolites are released (>500d). Since larger metabolites are less directly usable by bacteria, and as such, the larger molecules have longer residence times in the medium. This allows them more time to interact with other particles compared to small molecular weight metabolites (Lancelot [1984]). Current hypotheses posit that the large molecular weight material coagulates and aggregates with other similarly sized (and compositionally similar - ie acidic polysaccharides derived from extracellular release) particles. The results of such interactions are not entirely clear but it is assumed that fibrils of size >5kd (Leppard [1995]) are formed. Of the initial molecules to be exuded, galactose is the most prominent monosaccharide. Mannos glucose, xylose and arabinose are all usually but not necessarily present (Henriques Viera and Myklestad [1986]). There is also speculation as to the effect of cellular exudates on reducing settling velocity compared to aggregates or particles without increased extracellular material. This was considered a while ago by Alldredge and Gotschalk [1989] in Santa Barbara who deduced that "differential settling" of diatom chains to be the primary mechanism of aggregate formation. And to throw more shit on any notion of a universal relationship between nutrient stress and exudation, Alldredge and Gotschalk [1989] also observed nitrate replete conditions during floc formation off California coast.

Yesterday, I ran for 40 minutes easy at around 7am. The sun was bright and the air was briskly in the 30s. Perfect shorts and long sleeve shirt weather. I stayed up to the wee hours and rose early on thursday to continue preparations for my presentation. As a result, I ran this evening with Elizabeth in Central Park. Temps were in the 40s and the breeze was light. Zib, felt like ass, and wanted to puke. No puking and a very enjoyable 57 minute jaunt in the park tonight. I can promise a good tale or two will come out of my planned adventures for the next few days.
- posted by alex @ 22:47