Microscale measurements reveal contrasting effects of photosynthesis and epiphytes on frictional drag on the surfaces of filamentous algae

Abstract

Summary Filamentous algae are widespread in freshwater ecosystems worldwide with a significant presence in streams, rivers and lakes with sufficient light and nutrients. Although typically not a preferred food source for grazers, dense filamentous mats provide surfaces for epiphytic microorganisms that are more palatable, thus adding to stream productivity. We tested the hypothesis that epiphytes change velocity gradients, and consequently shear stress and skin friction drag, near the surface of algal filaments. Using both digital holography and particle image velocimetry to measure micrometre‐scale velocity fields, we found that the surface shear stress on filamentous algae was much greater when the algae were actively photosynthesising. The presence of attached diatoms significantly reduced surface shear stress, while those filaments were photosynthesising, compared with bare filaments. A nutrient flux model, based on boundary layer thickness and surface shear stress, predicts that nutrient flux to the surface of a photosynthesising filament under measured flow conditions will be 1.5 times greater than for a preserved (i.e. dead) filament. Modelled nutrient flux to filaments with epiphytic assemblages dominated by diatoms is 75% of the flux to bare filaments under similar flow conditions. The proposed positive feedback between photosynthesis, surface shear stress and nutrient flux could be an important biophysical mechanism that overcomes diffusion limited nutrient supply within dense algal mats, enhancing algal survival through increased nutrient flux to actively photosynthesising filaments and decreased sloughing risk for filaments with lower rates of photosynthesis (due to epiphyte coverage or other light‐limiting factors).

Sinking of Four Species of Living Diatom Cells Directly Observed by a “Tumbled” Optical Microscope

“…The specific weight of diatoms has been studied by several authors (Hansen et al, 2014; Aitken et al, 2016; Luo & Greer, 2018). The value is slightly larger than that of water (Hansen et al, 2014; Aitken et al, 2016; Luo & Greer, 2018).…”

“…The specific weight of diatoms has been studied by several authors (Hansen et al, 2014; Aitken et al, 2016; Luo & Greer, 2018). The value is slightly larger than that of water (Hansen et al, 2014; Aitken et al, 2016; Luo & Greer, 2018). The density of seawater is 1,023 kg/m 3 at 20°C and 33 g/L salinity, and diatom cells are 1,030−1,100 kg/m 3 in the cytoplasm and 1,400−2,200 kg/m 3 in the shell called the frustule (Miklasz & Denny, 2010).…”

Periphyton control on stream invertebrate diversity: is periphyton architecture more important than biomass?

“…One potential explanation for this is that some filamentous algae can host other palatable epiphytic algae such as diatoms, which in turn provides a food source for grazers. Recent research has shown that epiphytic diatoms can lead to reduced shear stress on photosynthesising filamentous algae (Hansen et al 2014), which could in fact alter the hydrodynamic environment for grazers. Filamentous algae may also provide a greater retention of organic detritus that in turn would support more taxa from other functional feeding groups such as shredders.…”

Imaging bacterial 3D motion using digital in-line holographic microscopy and correlation-based de-noising algorithm

“…Inline Digital Holography Microscopy (DHM) has recently been successfully used in providing 3D image of micro-organisms with high spatial resolution [41, 46,55], tracking their 3-D movement and behavior within a dense suspension [7,39,40,49,56], and measuring flow around the free swimming organisms [41, 50,57]. Here an in-line approach for DHM is used, in which the sample is illuminated by a collimated laser beam and the hologram formed outside of the sample volume is recorded.…”