Grazer responses to variable macroalgal resource conditions facilitate habitat structuring

Abstract

Consumer responses to altered resource conditions can vary depending on dietary preference, resource characteristics and secondary resource features such as shelter. These can have cascading effects, especially if the consumed resource impacts on overall ecological functioning. In this study, we assessed the dietary composition of grazer communities following seasonal changes in the characteristics of their staple food-source (macroalgae). This was conducted in the living stromatolite pools growing along the coast of South Africa. Stable isotope mixing models suggested that following macroalgal bleaching in summer, metazoan consumers shifted their diet from predominantly macroalgae to a generalist composition. This has important implications for the integrity of the stromatolite matrix and its layered deposition. Where previously in winter stromatolite microalgae comprised a minor component of metazoan consumer diets, in summer, following a change in the resource conditions of macroalgae, microalgae featured more prominently in grazer diets. This seasonal grazing pressure on stromatolite-related resources probably promotes the pattern of annual layering observed in the stromatolite accretion. It also demonstrates a mechanism whereby grazer dietary shifts following a change in their preferred food resource can affect the ecosystem structure of their environment, specifically the stromatolite layering process which responds to microalgal growth or grazing conditions.

Conserved bacterial genomes from two geographically isolated peritidal stromatolite formations shed light on potential functional guilds

“…S2). The stromatolites at Cape Recife and Schoenmakerskop experience limited inorganic phosphate availability (Rishworth et al, 2016(Rishworth et al, , 2017a(Rishworth et al, , 2018Dodd et al, 2018) and the high abundance of genes encoding phosphate-metabolizing enzymes may be indicative of how stromatolite communities cope with low dissolved inorganic phosphorus in their environment as suggested for microbialite assemblages in Lake Alchichica, Mexico where a similar over-representation of phosphateassociated genes was observed (Breitbart et al, 2009;Valdespino-Castillo et al, 2014). These overrepresented genes were hypothesized to play an important role in controlling carbonate precipitation through the production of stored polyphosphate, which chelates metals, such as calcium (Breitbart et al, 2009;Valdespino-Castillo et al, 2014).…”

Conserved bacterial genomes from two geographically distinct peritidal stromatolite formations shed light on potential functional guilds

“…There may be several other genes, or potentially divergent orthologs of phoR in these genomes [64, 65], which would account for the apparent absence of phoR , but the widespread absence of this gene across taxonomically diverse bacteria at Cape Recife and Schoenmakerskop, suggests that this seemingly important regulatory protein is not advantageous to this system. It is hypothesized that stromatolites of Cape Recife and Schoenmakerskop largely receive phosphorous from the oceans and have consistently been shown to experience limited inorganic phosphate availability [15, 66–68] and this consistent struggle for phosphate may have resulted in the loss of phoR as its role may have become redundant. Loss of PhoR has also resulted in increased alkaline phosphatase activity [69] and consequently, it is possible that the combined effect of extracellular alkaline phosphatase abundance and loss of phoR , could result in a relative excess of local bioavailable phosphate in the extracellular stromatolite environment.…”

Metazoan activity facilitates passive sediment trapping in modern supratidal microbialites: Revealed using µ‐CT‐scanning and microscopy

“…Figure 7a) in the lateral growth are presumably rather due to the decomposition of macroalgae, likely Ulva spp. (Rishworth et al, 2018), which were covered by or incorporated within the stromatolitic growth. It is doubtful whether the possible incorporation and later decomposition of macroalgae on the subaerial layers would account for the larger cavities seen on the top of the front phytoherms, since the growth of a microbialite layer is likely slower than the lifecycle of the macroalgae.…”

“…can reach maturity in less than four months (Oza & Sreenivasa Rao, 1977) and have been observed to bleach during warmer summer months in the microbialite pools (e.g. Rishworth et al, 2018).…”

“…Although persistent grazer species inhabit the microbialite pools and could potentially consume the microbialite-forming microalgae (e.g. bacillariophytes) or the microbialite material itself, their diets consist primarily of macroalgae (Rishworth et al, , 2018. Although no evidence exists for the trapping of larger grains by macroalgae (Frantz et al, 2015), cavities created by the decomposition/ consumption of the macroalgae within the microbialite growth will provide additional pore space for the periodic accumulation of sediment.…”