September and October were dense with work and preparation. In late August, I switched gears with respect to scheduled research due to systemic equipment malfunction in the growth flumes, which can only be explained by demonic intrusion (Hurlbert 1972). So instead of investigating the influence of water flow + drag forces on the C and N metabolism of kelps, my current research in the Martone Lab now targets the identification of patterns in the activity of carbonic anhydrase (an enzyme that interconverts dissolved CO2 and HCO3-) across multiple macroalgal species within a single community. This includes investigating what might help explain these patterns in activity. Better understanding the expression and activity of carbonic anhydrase (CA) will allow more informed exploration of carbon use and partitioning within algal communities, as well as help refine the ecological function (with respect to carbon sequestration) of specific species or even species interactions.
How am I tackling this at the moment? I collected replicates of every species possible at one field site (Ogden Pt, Victoria, BC) and transferred them back to the lab at UBC in Vancouver. Upon returning to the lab, I learned that the collection included 39 species of macroalgae (2 green, 27 reds, 10 browns)! These were acclimated in the lab, each species exposed to the same light, nutrients, and temperature. After a day of acclimation, I sampled tissues for the following analyses: carbonic anhydrase activity, nitrate reductase activity, pigments, and oxygen evolution in seawater with low CO2 concentrations (this helps determine the importance of dissolved CO2, opposed to HCO3-, as a carbon source for photosynthesis).
What have I found so far? Here is a snapshot of the CA activity for each species:
What patterns can we identify from these data?
(1) Taxonomy and phylogeny: Can patterns in CA activity be explained by taxonomy/relatedness? Many assume that physiology/metabolism is conserved within closely related species; i.e. members of the same genus are more likely to have similar physiological function compared to more distantly related species. I am still waiting for molecular IDs on many of the algae that I collected (since reds tend to be cryptic!) so that I can build an appropriate phylogenetic tree that enables me to directly test the influence of relatedness, but I can at least comment on discrete taxonomy (refined to the within-genus level). So far, it appears that taxonomy does not explain the patterns
(2) Epiphyte-host associations: What if ecological function/associations are considered instead of taxonomy? There appears to be a pattern; an average, epiphytes have significantly higher CA activity than both host algae and other species within their sister species (within in the same genus in all cases except one). Hosts have significantly lower CA activity than their sister species.
(3) Morphological complexity / branching density: Macroalgae can also utilize external CA (eCA), which is CA that is located at the interface between the bulk seawater and the surface of algal tissue, allowing for the conversion of HCO3- to CO2 before carbon uptake. It appears that activity of eCA is positively correlated with the branching density of the species, that is to say, that species with higher branching density are more likely to have higher eCA activity. Why? There are two reasons that I consider most likely. First, a purely physical driver: branching density often increases the thickness of the diffusion boundary layer surrounding the algal individual (due to turbulence), higher branching density might reduce the risk that eCA is "swept away" as water flows past. Second, a mix of physical & chemical drivers: research by Cornwall et al. 2013 suggests that because pH at the surface of the algae is high under slow flow, CO2 would be reduced but instead HCO3- increased because of the equilibrium 'rules' between CO2 and HCO3- under high pH. So, its possible that highly branched algae have higher eCA activity because the theoretical thicker boundary layers around their thallus are likely to have more HCO3- than CO2 compared to algae with inherently thinner boundary layers under the same flow (like expected for less complex algae).
This work was recently shared at the annual meeting for the Western Society of Naturalists in Monterey, California! You can download the Powerpoint for the oral presentation, if you're excited enough. :)