Effects of the submarine volcanic eruption of El Hierro (Canary Islands) on the bacterioplankton communities of the surrounding

Abstract

Direct observations of submarine eruptions are scarce because they usually occur unexpectedly in remote locations. The submarine volcano eruption of El Hierro (Canary Islands) occurring in October 2011 pro- vided a unique opportunity to determine the effects of volcanic eruptions on the microbial populations of the surrounding waters. The birth of a new underwater volcano 1.8 km south of El Hierro produced a large plume of vent material detectable from satellite that lead to abrupt changes in the physico-chemical properties of the water column. In order to test whether these changes resulted in changes in bacterioplankton communities, we combined flow cytometry and 454-pyrosequencing of 16S rRNA gene amplicons to monitor the area around the volcano through the eruptive and post-eruptive phases (November 2011 to April 2012). Flow cytometric analyses revealed the presence of large amounts of inorganic particles likely associated to the discharge of vent material. Likewise, a population of cells attached to these particles and an increase in the proportion of cells with high DNA content was detected during the eruptive phase. Changes observed in populations detectable by flow-cytometry were more evident at the depths closer to the volcano (75-125 m) coinciding also with oxygen-depleted areas. Alpha-diversity analyses unveiled that species richness (Chao1 index) decreased during the eruptive phase. However, no dramatic changes in community composition were observed. The most abundant taxa during the eruptive phase were similar to those in the post-eruptive stages and to those typically prevalent in oceanic bacterioplankton communities (i.e., the alphaproteobacterial SAR11 group, the Flavobacteria class of the Bacteroidetes, and certain groups of Gammaproteobacteria). Yet, we also detected the presence of taxa not typically found in bacterioplankton communities such as the Epsilon- and Zetaproteobacteria and members of the candidate division ZB3, particularly apparent in the eruptive stage. These groups are often associated with deep-sea hydrothermal vents, seamounts or sulfurrich springs. Both cytometric and sequence analyses showed that once the eruption ceased, bacterial populations returned to typical patterns and no evidences of a volcano-induced fertilization could be observed.