Photosynthesis slows the flow

Posted on March 23, 2015

Mats of filamentous freshwater algae play an important ecological role by helping to cycle nutrients, by forming complex three-dimensional structures that can be colonised by diatoms, bacteria and microalgae, and by providing refuge for small animals. The productivity of algal mats is limited mainly by light, nutrients and grazing activity, all of which are affected by water movement. To examine alga – flow interactions in detail, American researchers used laser and holographic techniques to study velocity profiles in a Cladophora mat in the field and flow patterns around Cladophora and Pithophora filaments in the laboratory. Unexpectedly, they found that velocities close to the surface of algal filaments were higher in the dark than in the light, which suggested that the process of photosynthesis was somehow increasing the drag on the filament surface. More work is needed to find out exactly how this happened, but it’s likely that it was due to bubble formation caused by oxygen production at the algal surface. The resulting increase in skin drag reduced the thickness of the boundary layer, which led to a 50% increase in the rate at which nutrients reached the algal surface. Therefore, photosynthesis, nutrient supply and algal growth formed a positive feedback loop, which helps to explain how algae manage to thrive in dense mats. The colonisation of algal filaments by diatoms inhibited this process, seemingly by impeding light penetration and gas exchange, and therefore photosynthesis.

Reference: Hansen, A.T. et al. 2014. Microscale measurements reveal contrasting effects of photosynthesis and epiphytes on frictional drag on the surfaces of filamentous algae. Freshwater Biology 59, 312–324.