Environmental influences on living marine stromatolites: insights from benthic microalgal communities

Abstract

Extant marine stromatolites act as partial analogues of their Achaean counterparts, but are rare due to depleted ocean calcium carbonate levels and suppression by eukaryotic organisms. Unique, peritidal tufa stromatolites at the interface between marine and freshwater inputs were discovered in South Africa in the past decade. Our aim was to investigate the benthic microalgal community (green algae, diatoms and cyanobacteria) of these stromatolites to assess succession and dominance patterns using real-time, in situ measurements of algal concentrations and composition. These biological measurements were modelled using generalized linear modelling (GLM) multivariate statistics against water physical and chemical parameters measured at regular monthly intervals, from January to December 2014. Salinity peaked and temperature dipped in winter, with both correlated to microalgal community change (GLM: P < 0.01). Diatoms and cyanobacteria, which construct the stromatolites, were consistently the dominant groups within the algal community, with minimal green algae present throughout the year. Importantly, this demonstrates a unique, relatively stable microalgal stromatolite community as opposed to those of other marine stromatolites, which likely require seasonal and stochastic disturbance to persist. This has implications in terms of interpreting community succession and differential layering in modern and fossilized stromatolites respectively.

Coexisting living stromatolites and infaunal metazoans

“…Tufa stromatolites occurring in South Africa form at the interface of groundwater seepage and the ocean high-water mark (Perissinotto et al 2014;Rishworth et al 2016), constructing rimstone dams through the layered precipitation of inflowing calcium carbonate and the trapping of sediment (Smith et al 2011(Smith et al , 2005. Within the clearly layered, growing stromatolite matrix (Fig.…”

“…1 (Perissinotto et al 2014;Rishworth et al 2016). Each site is characterized by a freshwater inlet stream which flows into a 'barrage' tufa stromatolite pool or series of pools in the upper intertidal zone.…”

“…Each site is characterized by a freshwater inlet stream which flows into a 'barrage' tufa stromatolite pool or series of pools in the upper intertidal zone. Dominant mat-building microalgae include diatoms and cyanobacteria (Rishworth et al 2016), specifically genera such as Lyngbya, Plectonema, Leptolyngbya and Schizothrix (Perissinotto et al 2014). The stromatolites accrete at an optimal rate of 2.1-3.6 mm year −1 , forming sequential primary and successional layers ( Fig.…”

“…Samples were collected monthly from each of the three pools at each site during spring low tide from January to July 2014. In addition, physico-chemical properties of the water in the lower, barrage, and upper stromatolite pools were recorded using a YSI 6600 multi-parameter meter (YSI Incorporated, Yellow Springs, OH, USA), the details of which are presented elsewhere (Rishworth et al 2016). These included temperature, salinity, and dissolved oxygen (mg L −1 ).…”

“…Consequently, most modern stromatolites are restricted to marginal, harsh environments where metazoans are largely excluded (Grotzinger and Knoll 1999;Riding 2011), whereas in the presence of metazoans, thrombolites (clotted microbialites) rather than laminated stromatolites may form (Feldmann and McKenzie 1998;Konishi et al 2001;Planavsky and Ginsburg 2009;Walter and Heys 1985). Recently-discovered peritidal stromatolites along the southern African coastline (Perissinotto et al 2014;Rishworth et al 2016;Smith et al 2011) were used in this study to further assess metazoan coexistence and lamination in comparison to other modern stromatolites (Edgcomb et al 2014;Garcia-Pichel et al 2004;Tarhan et al 2013) in light of the ecologically and evolutionarily relevant metazoanstromatolite exclusion hypothesis (Garrett 1970;Mata and Bottjer 2012;Riding 2000;Walter and Heys 1985).…”