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Interactions between marine algae and bacteria

Final Report Summary - ALGBACT (Interactions between marine algae and bacteria)

Trophic interactions between bacteria and phytoplankton in aquatic systems are well documented in the literature, as the microbial loop plays a critical role in important processes such as carbon flux and nutrient regeneration. In contrast, chemical interactions between bacteria and harmful/toxic phytoplankton and their potential impact on the production of biotoxins and population dynamics have received increasingly, yet still relatively little attention. While bacteria can play a role in the production of algal biotoxins or bloom termination, phytoplankton can produce biologically active compounds, i.e. allelochemicals that affect bacterial diversity.
The goal of the project is to identify specific mechanisms of action involving allelochemicals in bacterial-algal interactions to understand their role in harmful algal bloom (HAB) ecology.
The overall objective of the project is to explore whether or not harmful algal species are able to selectively affect the growth of bacteria, through the production of allelochemicals.

In this project, we used the neurotoxic dinoflagellate Alexandrium tamarense as a model organism producing allelochemicals. The approach combined microbiological, molecular-based and chemical approaches to efficiently meet the objectives. Bulk- and single-cell techniques were applied to quantify chemical interactions both at the community and the cellular levels.
Recent evidence showed that allelochemicals produced by Alexandrium altered the composition of natural bacterioplankton after the spring bloom in the North Sea, but did not inhibit growth or production of marine bacteria. This trend could be explained by both a reduction of bacterivory and a potential increase in dissolved organic carbon due to plankton cell lysis after exposure to algal allelochemicals. The impact of algal allelochemicals on microbial communities depends greatly on the trophic organization and the composition of these communities. These findings raise questions of whether or not these phylogenetic and phenotypic changes in bacterial communities are directly caused by the antibiotic properties of the allelochemicals, or by antagonistic interactions between bacterial taxa.
To quantify the antibiotic properties of algal allelochemicals, their effect was tested on the physiological response of 20 bacterial isolates from the field or algal cultures. In general, bacteria exhibiting high growth rate (> 4 d-1) were more affected by allelochemicals than slow-growing bacteria (< 1d-1). Among the fast-growing bacteria, many were able to produce biocidal compounds. Bacteria exposed to allelochemicals also demonstrated a change in their functional response. The efficiency of using different carbon sources varies as a function of exposure to allelochemicals.

Our results showed that algal-bacteria interactions mediated by algal allelochemicals would directly affect carbon cycling and thus indirectly phytoplankton, e.g. Harmful Algal Blooms dynamics. Our findings contribute to an integral understanding of complex microbiological and chemical processes, essential to predict fundamental processes in marine systems, specifically primary and microbial production and decomposition of organic matter.

Socio-economic relevance of the project
Harmful algal blooms (HABs) have been reported in almost every European coastal marine waters, and their occurrence appears to be increasing. At present, HABs in European coastal water cause about €50-100 million in economic losses to the seafood, restaurant and tourism industries each year, and there are other economic and social costs associated with toxin monitoring, deleterious effects on the aquaculture and fisheries industries, and risks to public health.