Complex interactions in microbial food webs: Stoichiometric and functional approaches

Abstract

The food web structure in some high mountain lakes deviates from the established tendency of high heterotrophic bacteria: phytoplankton biomass ratios in oligotrophic ecosystems. Thus, the microbial food web in La Caldera Lake is weakly developed, and bacteria constitute a minor component of the plankton community in terms of abundance, biomass and production. Autotrophic picoplankton is absent, and heterotrophic microbial food web is weakly developed compared to a grazing chain dominated by calanoid copepods and a phytoplankton community mainly composed of mixotrophic flagellates. In order to explain the singular food web structure of this lake, functional, stoichiometric and taxonomical approaches are followed to assess, on various temporal and spatial scales, the relevance of stressful abiotic factors (ultraviolet solar radiation and P-limitation) on the structure and functioning of this ecosystem. P-availability was the main factor controlling the algal biomass whereas bacterial P-limitation was a transient phenomenon. The algae-bacteria relationship was predominately commensalistic. In contrast to algae, full-sunlight radiation had no negative effect on bacterial growth but rather enhanced bacterial dependence on the carbon released by algae. The prevalence of the commensalistic-mutualistic relationship and the development of a more complex microbial food web were related to the stoichiometry of algae and bacteria (N:P ratios). The microbial food web only developed at balanced algal and bacterial N:P ratios, with the appearance of ciliates after a nutrient pulse. However, mixotrophic algae dominated the planktonic community under P-deficit conditions, and they were the main factor controlling bacterioplankton. Their regulatory effect has a dual nature: (i) a resource-based control, where bacteria depend on the photosynthetic carbon released by algae, i.e., a commensalistic interaction ("without you I cannot live"); and (ii) a predatory control, where bacteria is a prey for mixotrophs ("with you I die"). Hence, the niche of microheterotrophs (nanoflagellates and ciliates) is occupied by mixotrophs, and there is a resulting simplification of the planktonic structure. With respect to the carbon cycle, mixotrophic bacterivory constitutes a "by-pass" for the flux of C towards the grazing chain, precluding the development of a complex heterotrophic microbial food web. Mixotrophs thereby improve the energetic transfer efficiency in high mountain lakes through a reduction in the number of trophic levels. Antagonistic effects of UVR x P interactions on the algae-bacteria relationship were caused by an enhancement of dual (resource and predation) control. Based on these results, an alternative model for the flux of C in autotrophic high mountain lakes has been proposed.

Effects of ultraviolet radiation and nutrients on the structure-function of phytoplankton in a high mountain lake

“…A highly characteristic trait of the pelagic structure is an absence of autotrophic picoplankton. 7,[30][31][32] Fish are absent and cladocerans (Daphnia) are scarce. The calanoid copepod Mixodiaptomus laciniatus is the dominant crustacean zooplankton in the lake.…”

Application of DNA Metabarcoding for Identifying the Diet of Asian Clam (Corbicula fluminea, Müller, 1774)

“…In aquatic ecosystems, producers, consumers, and decomposers are systematically linked at each trophic level to form food chains, which are intertwined like a net to form a food web [4]. It is important to understand the role and function of interactions in the food web of aquatic ecosystems [5][6][7][8].…”

“…As ecosystem engineers, filter-feeding bivalves exert profound effects on their environment via feeding, metabolic activity, and movement, which can cause bioturbation effects [9,10]. In general, bivalves exhibit a high feeding capacity for phytoplankton as their main potential food [2], but recently other taxonomic groups, such as zooplankton and bacteria, have also been found to be important potential foods [3][4][5]. This selective feeding of bivalves allows them to highly modify plankton community structure [9,11].…”