How algae and fungi team up to nourish aquatic invertebrates

In this blog post, Alexander Feckler and his colleagues explore the role of autochthonous primary production for the quality of microbially-colonized leaf material as food in heterotrophic food web in headwater streams. Despite reduced autochthonous primary production in shaded headwater streams, this study indicated a potential for enhanced secondary production and energy transfer to higher trophic levels within the aquatic ecosystem due to so-called “algal priming”.

Introduction

Nutritious food is essential for headwater invertebrates, such as detritivores, that feed on autumn-shed leaves from riparian vegetation. Since the leaves themselves are not highly nutritious, detritivores depend on microbial colonization, particularly fungi, to enhance nutrient and lipid availability, making the leaves a more palatable food source. Aquatic fungi are often called the “peanut butter on the cracker” for this reason. However, the role of algae in this process has been overlooked. Algae excrete labile organic carbon, which fungi can more easily utilize than leaf carbon, promoting fungal growth in a process known as “algal priming.”

The study

To test this hypothesis, we conducted a 40-day microcosm experiment, feeding two size classes of the detritivore species Gammarus fossarum (Crustacea; Amphipoda) leaf material of varying quality: pure leaves (low quality), leaves colonized by fungi (intermediate quality), and leaves colonized by fungi with a diatom (high quality). We assessed the impact of algal priming on energy turnover in streams based on energy turnover, growth, and fatty acid (FA) composition.

Results

Figure 1 shows the means of (A) ingestion rate and (B) biomass gain by small and large Gammarus fossarum when feeding on low-, intermediate-, and high-quality leaf material for 40 days
Figure 1: Means (±95% credible intervals) of (A) ingestion rate (mg d-1) and (B) biomass gain (μg d-1) by small (white) and large (black) Gammarus fossarum when feeding on low-, intermediate-, and high-quality leaf material for 40 d.

The plot (Fig. 1) shows that Gammarus ingestion rates increased (55–164%) with food quality, spurring accelerated growth (4–14%), irrespective of the size class. Furthermore, Gammarus’ overall FA quantity tended to rise with higher-quality food (12–318%), with the FA profile exhibiting increased proportions of specific polyunsaturated FAs that are essential for detritivores. These observations can likely be attributed to leaf-associated fungi, which are more readily assimilated than the leaves and are known as a source of FA. This enhancing effect by fungi was further amplified in the presence of algae, presumably through the positive effect of algal-derived labile organic carbon, which supports fungal growth. Therefore, the findings confirm the significance of leaf-associated fungi in enhancing the quality of this food item for detritivores and furthermore confirm that the presence of algae during microbial leaf colonization can indirectly enhance secondary production.

More information

The publication entitled “Detritivore physiology and growth benefit from algal presence during microbial leaf colonization” was authored by Alexander Feckler, Sebastian Pietz, Sara Gonçalves, Verena Gerstle, Ute Risse-Buhl, and Mirco Bundschuh and published in Limnology and Oceanography (doi: 10.1002/lno.12530)