Compound-specific radiocarbon analysis for ecological research: A case study using 14C composition of chlorophyll a from stream periphyton

Background/Question/Methods

Tracing carbon in natural ecosystems is a major challenge that provides both biogeochemical (e.g., materials cycling) and ecological (e.g., food webs) implications. Recent studies have found that radiocarbon natural abundance (Δ¹⁴C) is a powerful tool to estimate the source and turnover of carbon in various environments. ¹⁴C is a naturally occurring radioisotope that is constantly made from ¹⁴N in the stratosphere, and it has a half-life of 5,730 years. Carbon storage reservoirs in watersheds have distinctive Δ¹⁴C values (e.g., ancient carbonate rocks: Δ¹⁴C ~ –1000‰) from modern atmospheric CO₂ (Δ¹⁴C ~ +40‰). Our previous studies determined Δ¹⁴C values of invertebrates, fishes, and their potential food sources, collected from several streams in Lake Biwa basin (central Japan), showing that stream food webs are composed of carbon originating from various sources of different ¹⁴C ages. In stream food webs, periphytic algae attached to a substrate (periphyton) play an important role as benthic primary producer. However, few studies have traced algal signatures from periphyton matrix to animal consumers because periphyton is a mixture of algae, bacteria, and other particulate organic matters, all of which may have unique isotopic compositions.

Results/Conclusions

We determined stable carbon and nitrogen isotopic compositions (δ¹³C and δ¹⁵N) and Δ¹⁴C values of chlorophyll a that was purified from periphyton using a high performance liquid chromatography. Their isotopic compositions were compared with those of the bulk periphyton, carbon source of algal photosynthesis (dissolved inorganic carbon: DIC), and terrestrial plants. Chlorophyll a was used as a biomarker of photosynthetic autotrophs, including periphytic algae. The Δ¹⁴C values of chlorophyll a in periphyton (–255‰ to –190‰) were lower than the Δ¹⁴C values of bulk periphyton (–228‰ to –179‰), indicating that the bulk matrix of periphyton consists of 80% to 82% algal carbon (derived from ¹⁴C-depleted DIC) and 18% to 20% terrestrial organic matters (derived from ¹⁴C-enriched atmospheric CO₂). This study demonstrated that aged organic matters partly fuel carbon cycling in stream ecosystems via benthic primary production. The compound-specific radiocarbon analysis will be a promising tool for precise estimation of the carbon source in natural ecosystems where heterogeneous resources (e.g., aquatic and terrestrial organic matters) are mixed.