Algae living in extreme acidic conditions in Río Agrio - Lago Caviahue system

DIAZ, M.; BAFFICO, G.; BEAMUD, G.

Caviahue, Argentina

Workshop; 6th Meeting of the IAVCEI Committee on volcanic lakes (CVL); 2004

In volcanic regions, dissolution of gases and the interaction between the native bedrock and water at high temperature can produce extreme water chemistry, including high electrical conductivity, high concentrations of sulphate, chloride and heavy metals and very high concentrations of H+ and consequent low pH. The waters from major springs near the summit of the Copahue volcano, (37°53’S, 71°02’W), in the Andean area of Northern Patagonia, Argentina, provide one of the most striking natural acidic environments.  The algae of the present study were sampled in this system, which includes the Copahue Volcano, the Upper Rio Agrio, the lake Caviahue and the Lower Rio Agrio. The in situ measurements in Rio Agrio shown a high daily variability not only in temperature, but also in all of the environmental variables measured (pH= 1.78; acidity: > 20 mmol H+ l-1). Consequently, environmental conditions for algae are highly variable, not only in the longitudinal direction of the river, but also daily and seasonally. Two algal species dominated the periphyton of the Upper Agrio River: Euglena mutabilis (Euglenophyceae) and Gloeochrysis sp Pascher (Chrysophyceae). E. mutabilis was present mainly on the sediments. They grow as aggregates, which probably is a living strategy. These biofilms on the surface of the stones in the Upper Agrio River were dominated by the chrysophyte Gloeochrysis sp. that it was observed only as brown mucilaginous films on stone surfaces. The phytoplankton community of Lake Caviahue (pH= 2.56; acidity: >5 mmol H+ l-1); is characterized by the dominance of one green alga, Keratococcus raphidioides (90% of the total abundance and biomass), others green coccal  (Chloridella sp, Viridiella sp) and three flagellate (Palmellopsis sp, Chlamydomonas sp and Euglena mirabilis). The vertical distribution of the total phytoplankton biomass ranges from less 2 and 18 mg L-1 fresh weight with a characteristic maximum phytoplankton biomass was observed under the thermocline in all the samplings. These conditions not only pose unique physiological problems for osmoregulation and pH homeostasis for algae but also alter the availability of resources. For example, the low pH controls the equilibrium of certain essential resources: phosphorus is present as phosphoric acid instead of phosphate and inorganic carbon is present as CO2 with virtually no bicarbonate (HCO3-) ions. At low pH, the concentration of inorganic carbon will approximate the air-equilibrium concentration, which at 20 ºC and a CO2 partial pressure of 360 ppm, is about 0.014 mmol L-1. This low carbon concentration has led a number of authors to suggest that inorganic carbon may limit algal production in acid waters. Besides, the very low N:P ratio suggested that the phototrophs would be nitrogen limited in Lake Caviahue and Upper Agrio River. This presentation provides an overview of the ongoing research. Taking into account the few existing data on acid lakes and rivers of volcanic origin and the limited knowledge of the response of acidophilus algae growing at pH 2 to nutrient input such as Carbon, Nitrogen and Phosphorus, the present study provides basic information to improve our understanding of the functioning of naturally acidic lakes and rivers.