Non-reliance of metazoans on stromatolite-forming microbial mats as a food resource

Abstract

Grazing and burrowing organisms usually homogenise microalgal mats that form on benthic sediments of many aquatic ecosystems. In the absence of this disruption, microalgal mats can accrete laminated deposits (stromatolites). Stromatolites are rare in modern coastal ecosystems, but persist at locations where metazoans are largely excluded. This study aimed to assess the trophic structure at stromatolite locations where metazoans co-occur, to determine the grazing influence exerted by the metazoans on the stromatolite-forming microalgae (cyanobacteria and diatoms). Stable isotope signatures (δ13C and δ15N) were used as food-web tracers and dietary composition of consumers was calculated using source mixing models. Results clearly demonstrate that the dominant macrofaunal grazers do not utilise stromatolite material as a food resource, but rather subsist on autochthonous macroalgae. For instance, the mean (±SD) dietary composition of two of the most abundant grazers, Melita zeylanica (Amphipoda) and Composetia cf. keiskama (Polychaeta), consisted of 80 ± 11% and 91 ± 7% macroalgae, respectively. This suggests that the stromatolite-forming benthic microalgae are not disrupted significantly by grazing pressures, allowing for the layered mineralisation process to perpetuate. Additionally, grazers likely have a restrictive influence on pool macroalgae, maintaining the competitive balance between micro- and macroalgal groups.

Patterns and drivers of benthic macrofaunal communities dwelling within extant peritidal stromatolites

“…The implications of this are instructive from a stromatolite persistence perspective. Our results suggest that the benthic invertebrates might be consuming macroalgae while comparatively ignoring the stromatolite benthic microalgae (see also Rishworth et al 2017 a for further evidence), as no association was observed between macrofauna and benthic microalgae, the latter forming the stromatolite structure inhabited by the infauna. This apparently relaxed grazing pressure on the microalgae would contribute toward explaining why the stromatolites at these sites are able to coexist with metazoans that live in a manner potentially destructive to stromatolite layering (Rishworth et al 2016 a ).…”

“…This apparently relaxed grazing pressure on the microalgae would contribute toward explaining why the stromatolites at these sites are able to coexist with metazoans that live in a manner potentially destructive to stromatolite layering (Rishworth et al 2016 a ). Dietary observations, specifically focused on the reliance of metazoans on stromatolite microalgae compared to other primary producer groups, have confirmed this observation using stable isotopes (Rishworth et al 2017 a ).…”

The extant shore platform stromatolite (SPS) facies association: a glimpse into the Archean?

“…No attempt has been made to institute a scientific survey due to the geographical distances involved and limited manpower. Stromatolite (SPS in this paper) morphology, microstructure, and ecology is discussed in detail elsewhere (Smith et al, 2011;Perissinotto et al, 2014;Rishworth et al, 2016Rishworth et al, , 2017Edwards et al, 2017). New localities are discussed and compared with known; such review is required to establish similarities and differences.…”

Grazer responses to variable macroalgal resource conditions facilitate habitat structuring

“…However, they prevail when the impact of metazoan bioturbation is less than the microbialite growth rate [ 24 ] or when selective forces act against metazoan destruction, due to the refugia benefit provided by the microbialite matrix in terms of predator avoidance, ambient buffering or oxygen supply [ 27 ]. Recent evidence suggests that metazoan–microbialite coexistence might also be facilitated when there is an alternative food source for grazers, rather than the microbialite microalgae themselves [ 28 ]. This restricts the grazing impact on the microbialite matrix, therefore, enabling unimpeded layering, and provides grazer control on other primary producers (macroalgae particularly) that might otherwise outcompete the microbialite microalgae [ 29 ].…”

“…), with arrows indicating bleached areas. Photographs were taken in winter (August 2015: [ 28 ]) and summer (January 2016: this study) by Ross-Lynne Weston and Lynette Clennell, respectively. Scale bars (0.5 m) are reflective of foreground objects.…”