Microbial interactions – definition focus: Syntrophism in environmental microbiology involves which of the following interaction patterns between two different species?

Introduction / Context:
Syntrophism (syntrophy) underpins many anaerobic processes, including methanogenic degradation where one organism consumes a product that would otherwise accumulate and inhibit its partner. Understanding the precise definition is crucial for interpreting consortia in digesters and sediments.

Given Data / Assumptions:

• Two different species cooperate metabolically.
• Cross-feeding often involves hydrogen, formate, acetate, or short-chain fatty acids.
• The relationship may be obligate under given conditions.

Concept / Approach:
In syntrophy, species exchange metabolites so that energetically unfavorable reactions become favorable when a partner removes inhibitory products (e.g., H2). This is distinct from general community exchange, from neutralism (no interaction), and from parasitism (unilateral exploitation).

Step-by-Step Solution:
Clarify that syntrophy is a two-partner metabolic cooperation. Identify exchanged metabolites (e.g., H2 from fermenter to methanogen). Recognize energetic coupling that enables both partners to grow. Select the option explicitly describing nutrient/metabolite exchange between two species.

Verification / Alternative check:
Classical pairings include Syntrophomonas (butyrate-oxidizer) with Methanospirillum (H2-consuming methanogen), demonstrating interspecies hydrogen transfer.

Why Other Options Are Wrong:
Community-wide exchange – too broad; syntrophy is specifically pairwise.

No exchange – contradicts definition.

Parasitism – net benefit only to one partner, unlike syntrophic mutualism.

Common Pitfalls:
Confusing syntrophy with commensalism; in syntrophy both partners benefit through shared metabolism.

Final Answer:
Exchange of nutrients and metabolites between two species (obligate or facultative cross-feeding).