Coexisting living stromatolites and infaunal metazoans

Abstract

Microbialites, bioaccretionary structures formed during the growth and metabolism of microorganisms (principally cyanobacteria) were the dominant lifeform in shallow late-Archean and Proterozoic oceans. During the Cambrian radiation of metazoan life, which began ~540 Mya, microbialite abundance and diversity further declined following a peak in the Mesoproterozoic. Notwithstanding contention, grazing and bioturbation effects of metazoans have been hypothesized as the dominant driver of modern microbialite scarcity. However, this metazoan-microbialite exclusion has not been fully explored in the few extant microbialites. Here we provide further evidence showing that living marine layered microbialites (stromatolites) coexist with a persistent assemblage of benthic macro-invertebrates, as has previously been demonstrated in some thrombolitic (clotted) microbialites. Surprisingly, these metazoans have active habits, such as burrowing, which should be expected to disrupt the layered matrix. As other studies have shown, through a network of burrows, metazoans can exploit local diurnal oxygen refugia within microbialites as well as escape predation. Our results, therefore, add novel evidence in support of the hypotheses that geologically, metazoans are not always incompatible with stromatolites, while ecologically, microbialites may act as micro-refugia for modern metazoans and historically have performed a similar inferred role in past ecosystems.

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

“…Stromatolites occur along the southern African coastline at locations where groundwater dune seeps meet regular saline incursion during tidal and storm surges in the peritidal zone, currently known from 540 sites along a 200 km stretch (Perissinotto et al ). Three representative locations have been investigated as part of previous studies (Perissinotto et al ; Miranda et al ; Rishworth et al 2016 a , b , 2017 b ) (Fig. ), these occurring at Cape Recife (site A; 34°02′42.13′S, 25°34′07.50″E), Schoenmakerskop (site B; 34°02′28.23″S, 25°32′18.60″E), and Seaview (site C; 34°01′03.16″S, 25°21′56.48″E).…”

“…Rare, extant examples are often restricted to environments in which competitors (such as macroalgae) or metazoans are restricted and scarce, such as under hypersaline (Suosaari et al ) or erosive (Bowlin et al ) conditions, or where calcium carbonate levels are high and therefore enable microbialite growth rates beyond the level of the effect of metazoan bioerosion (Garcia‐Pichel et al ). However, metazoans and microbialites do coexist in some modern environments in regions such as Mexico (Garcia‐Pichel et al ; Dinger et al ), Australia (Konishi et al ; Allen et al ; Edgcomb et al ), and the Bahamas (Garrett ), including in habitats with apparently “normal” conditions (Tarhan et al ), this relationship having recently been synthesized by Rishworth et al (2016 a ).…”

“…Along the southern coastline of South Africa, actively accreting stromatolites occurring at the interface of groundwater seepage points and marine tidal‐ or storm‐induced incursion have been discovered within the past two decades (Smith and Uken ; Perissinotto et al ). These systems were initially highlighted as supporting a metazoan assemblage (Perissinotto et al ) and further preliminary investigations have shown that several species do consistently occur within this habitat and, importantly, the metazoans do not appear to disrupt the clear stromatolite layering (Rishworth et al 2016 a ). Rather than disrupt the matrix, metazoans may use some microbialite habitats, both in past and modern ecosystems, as a refuge from unfavorable conditions such as hypoxia, exposure or predation, which is a hypothesis that is developing (Dinger et al ; Gingras et al ; Tarhan et al ; Rishworth et al 2016 a ).…”

“…These systems were initially highlighted as supporting a metazoan assemblage (Perissinotto et al ) and further preliminary investigations have shown that several species do consistently occur within this habitat and, importantly, the metazoans do not appear to disrupt the clear stromatolite layering (Rishworth et al 2016 a ). Rather than disrupt the matrix, metazoans may use some microbialite habitats, both in past and modern ecosystems, as a refuge from unfavorable conditions such as hypoxia, exposure or predation, which is a hypothesis that is developing (Dinger et al ; Gingras et al ; Tarhan et al ; Rishworth et al 2016 a ). The South African stromatolite sites also appear to support interesting species such as the first local record of a halophilic centipede, Tuoba cf.…”

“…The South African stromatolite sites also appear to support interesting species such as the first local record of a halophilic centipede, Tuoba cf. poseidonis (Rishworth et al 2016 a ), and invasive species such as the polychaete, Ficopomatus enigmaticus (Miranda et al ).…”