Phototrophic pigment diversity and picophytoplankton in permafrost thawlakes

[摘要] Permafrost thaw lakes (thermokarst lakes) are widely distributedacross the northern landscape, and are known to be biogeochemically activesites that emit large amounts of carbon to the atmosphere as CH₄ andCO₂. However, the abundance and composition of the photosyntheticcommunities that fix CO₂ have been little explored in thisecosystem type. In order to identify the major groups of phototrophicorganisms and their controlling variables, we sampled 12 permafrost thawlakes along a permafrost degradation gradient in northern Québec, Canada.Additional samples were taken from five rock-basin reference lakes in the regionto determine if the thaw lakes differed in limnological properties andphototrophs. Phytoplankton community structure was determined by high-performance liquid chromatography analysis of their photoprotective andphotosynthetic pigments, and autotrophic picoplankton concentrations wereassessed by flow cytometry. One of the black-colored lakes located in alandscape of rapidly degrading palsas (permafrost mounds) was selected forhigh-throughput 18S rRNA sequencing to complement conclusions based on thepigment and cytometry analyses. The results showed that the limnologicalproperties of the thaw lakes differed significantly from the reference lakes,and were more highly stratified. However, both waterbody types containedsimilarly diverse phytoplankton groups, with dominance of the pigmentassemblages by fucoxanthin-containing taxa, as well as chlorophytes,cryptophytes and cyanobacteria. Chlorophyll a concentrations (Chl a) werecorrelated with total phosphorus (TP), and both were significantly higher inthe thaw lakes (overall means of 3.3 µg Chl a L⁻¹ and 34 µg TP L⁻¹) relative to the reference lakes(2.0 µg Chl a L⁻¹ and 8.2 µg TP L⁻¹). Stepwise multiple regressionof Chl a against the other algal pigments showed that it was largely afunction of alloxanthin, fucoxanthin and Chl b (R² =  0.85). The bottomwaters of two of the thaw lakes also contained high concentrations ofbacteriochlorophyll d, showing the presence of green photosynthetic sulphurbacteria. The molecular analyses indicated a relatively minor contribution ofdiatoms, while chrysophytes, dinoflagellates and chlorophytes were wellrepresented; the heterotrophic eukaryote fraction was dominated by numerousciliate taxa, and also included Heliozoa, Rhizaria, chytrids and flagellates.Autotrophic picoplankton occurred in biovolume concentrations up to 3.1  ×  10⁵ µm³ picocyanobacteria  mL⁻¹ and1.9  × 10⁶ µm³ picoeukaryotes mL⁻¹, with large variations among lakes. Both groups of picophytoplankton were positivelycorrelated with total phytoplankton abundance, as measured by Chl a;picocyanobacteria were inversely correlated with dissolved organic carbon,while picoeukaryotes were inversely correlated with conductivity. Despitetheir net heterotrophic character, subarctic thaw lakes are rich habitats fordiverse phototrophic communities.

[学科分类] 地球化学与岩石