Adaptation of Coccomyxa sp. to Extremely Low Light Conditions Causes Deep Chlorophyll and Oxygen Maxima in Acidic Pit Lakes.

Javier Sánchez-España,Diana Ayala,Katrin Wendt-Potthoff,Carmen Falagán

doi:10.3390/microorganisms8081218

Abstract

Deep chlorophyll maxima (DCM) and metalimnetic oxygen maxima (MOM) are outstanding biogeochemical features of acidic pit lakes (APL). However, knowledge of the eukaryotic phototrophs responsible for their formation is limited. We aimed at linking the dynamics of phototrophic communities inhabiting meromictic APL in Spain with the formation of these characteristic layers. Firstly, the dynamics of DCM and MOM and their relation to physico-chemical parameters (photosynthetically active radiation (PAR), pH, dissolved ferric iron concentration, temperature), pigments and nutrient distribution is described; secondly, the phototrophic community composition is studied through a combination of microscopy, biomolecular and “omics” tools. Phototrophic communities of the studied APL show a low diversity dominated by green microalgae, specifically Coccomyxa sp., which have been successfully adapted to the chemically harsh conditions. DCM and MOM are usually non-coincident. DCM correspond to layers where phototrophs have higher chlorophyll content per cell to cope with extremely low PAR (<1 µmol m−2 s−1), but where photosynthetic oxygen production is limited. MOM correspond to shallower waters with more light, higher phytoplankton biomass and intense photosynthetic activity, which affects both oxygen concentration and water temperature. The main drivers of DCM formation in these APL are likely the need for nutrient uptake and photo-acclimation.

Highlights

Deep chlorophyll maxima (DCM) are subsurface layers enriched in chlorophyll and located at certain depths below the surface of thermally stratified water bodies such as lakes or the open ocean [1,2,3,4,5]
Leach et al (2017) [3] found that DCM are usually dominated by diatoms, dinoflagellates, chrysophytes, cryptophytes, and less frequently, cyanobacteria, as all these groups can maintain their vertical position in the water column through active swimming or buoyancy regulation, by decreased settling rate under nutrient-depleted conditions, or by increased growth rate under low light conditions due to the presence of phycoerythrin [3]
Based on the data presented in this study, we propose that the combination of mechanisms 1 and 5 can probably explain the formation of DCM in the studied pit lakes

Summary

Introduction

Deep chlorophyll maxima (DCM) are subsurface layers enriched in chlorophyll and located at certain depths below the surface of thermally stratified water bodies such as lakes or the open ocean [1,2,3,4,5]. In a comprehensive study on the patterns and drivers of DCM in 100 seasonally stratified lakes around the world, Leach et al (2017) [3] have recently concluded that the light attenuation (e.g., attenuation coefficient or 1% photosynthetically active radiation (PAR) depth, both depending mostly on dissolved organic carbon concentration) was a more reliable predictor of DCM depth than thermal stratification (e.g., thermocline slope and depth). A detailed revision on the occurrence and ecology of DCM has been provided for Spanish lakes [5], though this study focused on freshwater lakes neutral to alkaline in mountain regions or karstic environments (saline to hypersaline) where cryptophytes and/or purple sulfur bacteria, cyanobacteria, and anoxygenic phototrophs are usually the dominating microorganisms forming the DCM.

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