Frederico Meirelles-Pereira

Tropical freshwater ecosystems have lower bacterial growth efficiency than temperate ones.

Current models and observations indicate that bacterial respiration should increase and growth efficiency (BGE) should decrease with increasing temperatures. However, these models and observations are mostly derived from data collected in temperate regions, and the tropics are under-represented. The aim of this work was to compare bacterial metabolism, namely bacterial production (BP) and respiration (BR), bacterial growth efficiency (BGE) and bacterial carbon demand (BCD) between tropical and temperate ecosystems via a literature review and using unpublished data. We hypothesized that (1) tropical ecosystems have higher metabolism than temperate ones and, (2) that BGE is lower in tropical relative to temperate ecosystems. We collected a total of 498 coupled BP and BR observations (Ntotal = 498; Ntemperate = 301; Ntropical = 197), calculated BGE (BP/(BP+BR)) and BCD (BP+BR) for each case and examined patterns using a model II regression analysis and compared each parameter between the two regions using non-parametric Mann–Whitney U test. We observed a significant positive linear regression between BR and BP for the whole dataset, and also for tropical and temperate data separately. We found that BP, BR and BCD were higher in the tropics, but BGE was lower compared to temperate regions. Also, BR rates per BP unit were at least two fold higher in the tropics than in temperate ecosystems. We argue that higher temperature, nutrient limitation, and light exposure all contribute to lower BGE in the tropics, mediated through effects on thermodynamics, substrate stoichiometry, nutrient availability and interactions with photochemically produced compounds. More efforts are needed in this study area in the tropics, but our work indicates that bottom-up (nutrient availability and resource stoichiometry) and top-down (grazer pressure) processes, coupled with thermodynamic constraints, might contribute to the lower BGE in the tropics relative to temperate regions.

The Combination of Different Carbon Sources Enhances Bacterial Growth Efficiency in Aquatic Ecosystems

The dissolved organic carbon (DOC) pool is composed of several organic carbon compounds from different carbon sources. Each of these sources may support different bacterial growth rates, but few studies have specifically analyzed the effects of the combination of different carbon sources on bacterial metabolism. In this study, we evaluated the response of several metabolic parameters, including bacterial biomass production (BP), bacterial respiration (BR), bacterial growth efficiency (BGE), and bacterial community structure, on the presence of three DOC sources alone and in combination. We hypothesized that the mixture of different DOC sources would increase the efficiency of carbon use by bacteria (BGE). We established a full-factorial substitutive design (seven treatments) in which the effects of the number and identity of DOC sources on bacterial metabolism were evaluated. We calculated the expected metabolic rates of the combined DOC treatments based on the single-DOC treatments and observed a positive interaction on BP, a negative interaction on BR, and, consequently, a positive interaction on BGE for the combinations. The bacterial community composition appeared to have a minor impact on differences in bacterial metabolism among the treatments. Our data indicate that mixtures of DOC sources result in a more efficient biological use of carbon. This study provides strong evidence that the mixture of different DOC sources is a key factor affecting the role of bacteria in the carbon flux of aquatic ecosystems.

Long-Term CO2 Variability in Two Shallow Tropical Lakes Experiencing Episodic Eutrophication and Acidification Events

Here we report the long-term (13-year) dynamics of surface pCO2 and its response to episodic eutrophication and acidification events in two contrasting tropical coastal lakes, one clear-water and the other humic. A short-term nutrient addition experiment was also conducted in mesocosms in the humic lake where in situ eutrophication was moderate. Our objective was to elucidate the response of pCO2 to interannual changes in key limnological conditions, such as nutrient concentrations and pH. The humic waters showed a median pCO2 almost ninefold higher across the 13-year study than the clear waters, supporting pCO2 values about tenfold above atmospheric equilibrium. Eutrophication of the clear-water lake resulted in a decrease in pCO2 to median values below atmospheric equilibrium, producing a strong sink for atmospheric CO2. In contrast, pCO2 increased by over tenfold in both lakes during the acidification phase, resulting in very large CO2 emissions to the atmosphere. Experimental nutrient additions in the humic lake showed a strong persistence of high pCO2. The extreme variability in pCO2 observed here might be a characteristic of tropical lakes and may have important consequences for regional carbon budgets.

Effects of food web structure and resource subsidies on the patterns and mechanisms of temporal coherence in a tropical coastal lagoon: an experimental mesocosm approach

OBJETIVOS: Estudos sobre os padroes e mecanismos de coerencia temporal de variaveis ecologicas entre lagos tem se tornado um tema importante na limnologia. Ate o momento, nenhum estudo testou se e como a oferta de recursos e a configuracao da teia trofica afetam os padroes e mecanismos da coerencia temporal de variaveis limnologicas. Nos conduzimos um experimento de mesocosmos em campo durante 11 semanas para testar as seguintes hipoteses: (i) a adicao de nutrientes reduz a coerencia temporal de variaveis ecossistemicas; (ii) a predacao por peixes potencializa a coerencia temporal de variaveis ecossistemicas e (iii) a coerencia temporal e mais forte para variaveis fisicas (transparencia da agua), intermediaria para variaveis quimicas (concentracao de oxigenio dissolvido [OD]) e fraca para variaveis biologicas (biomassa zooplanctonica total). METODOS: Nos manipulamos a presenca de peixe e a adicao de nutrientes inorgânicos (N e P) em um desenho fatorial 2 × 2 em dezesseis mesocosmos instalados em uma lagoa costeira tropical. A coerencia foi estimada por correlacoes de Pearson par-a-par das trajetorias temporais de cada variavel resposta entre os mesocosmos de um mesmo tratamento. RESULTADOS: A presenca de peixes aumentou significativamente apenas a coerencia temporal da biomassa zooplanctonica, e, contrario as nossas expectativas, a adicao de nutrientes aumentou a coerencia temporal da [OD]. A intensidade dos efeitos da presenca de peixe e da adicao de nutrientes sobre a coerencia temporal foi afetada pela identidade da variavel monitorada, mas nao em um padrao consistente. No entanto, a interacao da presenca de peixe e adicao de nutrientes nao afetaram a coerencia temporal de nenhuma variavel monitorada. CONCLUSOES: Nossos resultados indicam que a predacao de peixes e a disponibilidade de recursos podem afetar significativamente padroes de coerencia temporal, mas tais efeitos dependerao mais de efeitos diretos do fator local sobre a variavel do que da identidade da propria variavel. Concluimos que a eutrofizacao e a sobrepesca podem interferir no acoplamento da dinâmica espaco-temporal de algumas variaveis limnologicas.