Answer: Both!
For this week’s LDEO biological oceanography journal club we discussed a very interesting paper recently published in Nature Communications by Pablo Serret et al., titled Both respiration and photosynthesis determine the scaling of plankon metabolism in the oligotrophic ocean. The paper was suggested by graduate student Hyewon Kim who did an excellent job leading the discussion.
Given the scale of the global ocean, whether it serves as an overall source or sink of CO2 is a big question, with major implications for the flux of CO2 to the atmosphere. The primary processes controlling this are photosynthesis (autotrophy), a sink for CO2, and respiration (heterotrophy), a source of CO2. The difference between these (photosynthesis – respiration) is called net community production (NCP). When NCP is positive, CO2 is fluxing into the surface ocean, when negative, CO2 is fluxing out. The dynamics that determine exactly how much photosynthesis and respiration is going on are extremely complex and our understanding of both is definitely incomplete. Clearly light, temperature, and nutrients have something to do with the efficiency of these processes, but so does community composition (controlled in turn by light, nutrients, water column structure), grazing pressure, and a host of other variables. Predicting the sign of NCP mechanistically, although highly desirable, is beyond our current abilities. In place of this we rely on estimates of NCP from direct measurements of photosynthesis and respiration.
It is reasonable to think that direct measurements should be better than a mechanistic model anyway, however, without a model there is no way to confirm that the measurements are actually right. This is particularly important for NCP estimates, as the measurements of both photosynthesis and respiration are all over the board and are known to be fundamentally flawed, as shown in this figure from Williams et al., 2013.
As categorized in the above figure, measurements of NCP come in two flavors, in vitro and in situ. In vitro measurements are typically bottle incubations, conducted on the deck of a ship in a water bath (to maintain in situ temperature) under both light and dark conditions. The amount of oxygen consumed in the dark provides an estimate of nighttime respiration. The amount of oxygen produced in the light (slightly shaded to represent in situ light conditions) provides an estimate of daytime NCP. Subtracting nighttime respiration from daytime NCP gives an estimate of NCP over an entire 24 hour period. There is lot, however, that can go wrong with this.
One of the biggest issues is “bottle effect”, the influence the incubation conditions have on the microbial community. Biological oceanographers have known about “bottle effects” for a long time. We try to minimize these effects, for example by acid washing glassware to eliminate trace metals, but there is no way that a bottle incubation will ever accurately reflect what happens in the ocean. Even slightly inhibiting or aiding photosynthesis or respiration in these incubations can tip the NCP balance in one direction of the other.
In situ observations are probably more accurate – at least they look a lot cleaner in the Williams et al. figure – but they have their own set of problems. In situ values are calculated from the concentration of O2 and argon (Ar) in the water. Ar is an inert gas and helps constrain the amount of O2 delivered to the water by physical processes. It is non-trivial (and expensive) to make accurate measurements of these gases in seawater, and the estimate of NCP can be thrown off by unaccounted for physical processes in the water column.
From Serret et al. 2015. Blue line/closed squares are for the North Atlantic subtropical gyre. Red line/open squares are for the South Atlantic subtropical gyre. Values above the 1:1 line indicate a negative NCP, values below indicate positive NCP.
Back to the paper. In this paper Serret et al. undertook perhaps the most comprehensive in vitro analysis of NCP to date on a series of cruises following a north/south transect in the Atlantic Ocean. In addition to the broad geographical area, what’s special about the dataset they collected is not that it’s necessarily right – they are still relying on bottle incubations, after all – but, because the data was collected by the same personnel using the same methods, it might at least be consistently wrong. That’s a bigger improvement than it sounds. Previously NCP estimates have assumed that the subtropical gyres (the vast, low productivity regions in the center of each ocean basin) are more or less the same. Serret et al. conclusively demonstrated that the North Atlantic and South Atlantic gyres have different NCP values; negative in the north (more respiration than photosynthesis) and positive in the south (more photosynthesis than respiration).
In this day and age the idea that the ocean is the same all over is pretty quaint. We understand that there is tremendous spatial and temporal variability, even when we average across pretty large areas and timescales. Direct observations of this variability are lacking however, given the size of the ocean and the limited number of oceanographic cruises tasked to studying it. There are some issues with this study in terms of possible seasonal and geographic bias (one member of our discussion group noted that the northern cruise track veers strongly east), and sample size, but it’s a tremendous improvement over what’s out there.
